M A S T E R ' S T H E S I S
The Utilization of Amendments in Soil Remediation
Roger Hamberg
Luleå University of Technology D Master thesis
Chemistry
Department of Civil and Environmental Engineering Division of Waste Science and Technology
remediation.
Roger Hamberg
2010-08-27
Preface
This master thesis was performed at the Division of Waste Science and Technology at the Luleå University of Technology. I would like to thank my supervisors Désireé Nordmark and Jurate Kumpiene for all their help and engagement. I also would like to thank Ulla-Britt Uvemo for her help with analysis.
This study was financially supported by the European Union Structural Funds and Objective 2, North Sweden Soil Remediation Center and by the research trainee program at the Luleå University of Technology.
Summary
Soil originating from old wood preservation industries is often contaminated by a mixture of polycyclic aromatic hydrocarbons (PAH) and heavy metals, from the simultaneous utilization of creosote and CCA salts (copper, chromium, arsenic), respectively. Multi-element
contaminated sites are usual and pose large difficulties for remedy selection, most techniques were not developed for these situations and stepwise or combined techniques are often required. Choice of a remediation technique depends primarily on target contaminant/s and soil properties but site specific conditions and economics are always to be considered.
Thermal treatment is effective for organic contaminants but tend to increase leaching of chromium (Cr) and arsenic (As), while the leaching of copper (Cu) decreases. Soil washing is primarily most effective for solely inorganics or organic contaminants. Soil washing
applications efficient for the simultaneous removal of inorganic and organic contaminants are rare. In these applications the soil washing extractant solutions could be recalcitrant and toxic in nature. Whenever treatment goals are not fulfilled after these treatments chemical
stabilisation could be an option for contaminant immobilization. Choice of an appropriate amendment for chemical stabilisation could be based on the affinity of As, Cr and Cu for different soil fractions such as metal oxides and sulphides.
Aim for this study was to develop an extractant in a soil washing application efficient for simultaneous removal of inorganic and organic contaminants with environmental concern taken. For the stabilisation of remaining contaminants this study also aimed to evaluate how the addition of certain products (iron compounds, fly ash, gypsum and zeolite) prior to thermal treatment could effect leaching of As, Cu and Cr. Leaching was evaluated in oxidizing or anoxic conditions. The addition of these products was also evaluated with no prior thermal treatment. Results were compared with EU landfill regulations and guideline values for contaminated soil.
Leaching of As, Cu and Cr was reduced by all amendments after thermal treatment, most prominent exception is a raised leaching of Cr due to an addition of fly ash which raised pH in soil. The most significant leaching element in anoxic conditions was As. Gypsum and metallic iron were chosen based on their inherent content of Ca, S and Fe which could bind As in certain conditions. A mixture of gypsum and iron reduced leaching most extensively in anoxic conditions. Gypsum decreased leaching of As at a higher extent in anoxic than oxidized conditions.
Leaching of As was most extensively reduced by an addition of iron compounds and increased in a fly ash mixture in soil with no prior thermal treatment. All amendments decreased leaching of Cu and Cr except for fly ash which increased leaching of Cr. Leaching of soil with no prior thermal treatment was evaluated solely in oxidized conditions.
A two-step soil washing test was performed for the evaluation of a humic acid (HA) extract in remediating soil contaminated with high levels of trace metals and polycyclic aromatic
hydrocarbons (PAH). pH was initially set to 11 in the first step and 7 in the second step.
Heated soil washing solutions of a humic acid extract, pH-adjusted water and solely water were compared. Humic acid extract significantly enhanced the extraction of PAH and Cu, Cr and As in comparison with other solutions tested but solely to a minor extent. The
enhancement was most prominent concerning high molecular weight PAH. PAH was removed at a higher extent than trace metals using extractants containing HA.
Table of content
Preface ... 2
Summary ... 3
1. Introduction ... 6
2. Materials and methods ... 7
2.1 Soils ... 7
2.2 Soil amendments and extracting solution ... 7
2.3 Thermal treatment ... 8
2.4 Soil washing ... 8
2.4.1 Experimental design ... 8
2.5 Evaluation methods ... 8
2.5.1 Leaching at oxidized conditions ... 8
2.5.2 Leaching at anoxic conditions ... 9
2.5.3 Total element concentration ... 9
2.5.4 Column test ... 9
2.5.5 Chemical analyses ... 9
3. Results ... 10
3.1 Soil T ... 10
3.1.1 Soil properties ... 10
3.1.2 Leaching in oxidized conditions ... 11
Thermally treated soil ... 11
Untreated soil ... 11
3.1.3 Column test ... 12
3.1.4 Leaching in anoxic conditions ... 14
3.2 Soil W ... 15
3.2.1 Soil properties ... 15
3.2.2 Peat properties ... 16
4. Discussion ... 19
4. 1 The mobility of Cu, Cr and As in thermally treated soil with and without amendments ... 19
Leaching in oxidized conditions ... 19
Leaching in anoxic conditions and column test... 20
4.2 The mobility of Cu, Cr and As with and without amendments with no prior thermal treatment ... 20
4.3 The mobility of Cu, Cr, As and PAH affected by an addition of HA ... 21
5. Conclusions ... 22
Appendix A: Soil washing design ... 23
Appendix B: ... 24
Review: Soil remediation studies at the Division of Waste Science and Technology at the Luleå University of Technology ... 24
6. References ... 46
1. Introduction
Thermal treatment and soil washing are two conventional soil remediation techniques effective for removal of organic and inorganic contaminants, respectively. Soil washing is primarily done when soils are contaminated with either solely inorganic or organic
contaminants, while thermal treatment is used for organic contaminants. Thermal treatment destroys organic contaminants but tends to increase leaching of chromium (Cr) and arsenic (As) while leaching of copper (Cu) decreases (Nordmark, 2008). The efficiency of a soil washing and separation of contaminants is very dependent on soil washing chemicals used and the clay content of the soil. Old wood preservation sites are often contaminated with polycyclic aromatic hydrocarbons (PAH) and trace elements such as Cr, Cu and As. Soils contaminated with these constituents pose potential risks to humans and to the environment.
There are few techniques developed for simultaneous remediation of PAH and arsenic.
Evidence of successful remediation of PAH and trace metals is rare in literature. When treatment goals are met soil is preferably placed back into the environment with a possibility for revegetation. Successful remediation of these constituents is sometimes questionable because the chemicals used could endanger soil revegetation or be of great economical
burden. Although chemical surfactants could be efficient they may also introduce toxicity and be recalcitrant in nature (Liang et al., 2007). Humic acids (HA) belong to a group of natural surfactants and are the main constituent in organic matter by 40 - 90 % content and have been used as an extractant for both PAH (Conte et al., 2004) and arsenic (Wang and Sullivan, 2005), but not simultaneously. The remediation of PAH and As has some similarities where both have enhanced solubility at high pH and elevated temperatures, especially when attached to organic matter (Rastas-Amofah et al., 2010, Liang et al., 2007). Costs for soil washing and thermal treatment are highly dependent on levels of contamination and amount of soil in need for treatment, which in turn defines e.g. treatment temperature, choice and amount of
chemicals needed. A technical grade HA is however much expensive but HA could be extracted from peat or soil organic matter to a relatively low cost.
When extraction of trace metals is not a possibility or thermal treatment has preceded, stabilisation is needed for immobilisation of contaminants. Chemical stabilisation is a technique used to immobilize contaminants by adding soil amendments e.g. iron oxides, alkaline materials or organic matter etc. Trace metals are immobilized but still present and must therefore be monitored (Kumpiene et al., 2008). Products such as iron compounds, gypsum boards, fly ash and zeolites are primarily used in other applications. Due to their content of calcium (Ca), sulphur (S) and iron (Fe) these products could be interesting in regard to chemical stabilisation. Remediation of soil often means severe economical considerations.
The purpose of this study is to evaluate if the addition of amendments (iron compounds, gypsum, fly ash, an extract from peat containing HA and zeolite) could enhance soil remediation efficiency. Research questions to be answered are:
1. How does an addition of iron compounds, fly ash, gypsum and zeolite prior to thermal treatment affect the leaching of trace elements?
2. How does an addition of iron compounds, fly ash, gypsum and zeolite affect the leaching of trace metals in soil with no prior thermal treatment?
3. How could a heated solution containing HA extracted from peat affect the extraction of primarily As and PAH compared to heated solutions with high pH?
Evaluation of stabilisation is done in oxidized and anoxic conditions in order to resemble a development in a landfill. Leaching limit values (L/S 10) stipulated by the Council of European Union (EC, 2003) for classification of waste for disposal in landfills as inert, non- hazardous and hazardous has been considered and presented. Terms like less sensitive land- use (MKM, mindre känslig markanvändning) and sensitive land-use (KM, känslig markanvändning,) referring to guideline values where different future land use scenarios are also considered. MKM is applied at industrial areas while KM is applied in residential areas.
2. Materials and methods 2.1 Soils
Soil for thermal treatment (hereafter called “Soil T”) was taken from a former industrial wood impregnation site contaminated with primarily chromium, copper and arsenic. Seven containers (10 litres) of soil were sampled at different locations and depths with varying soil properties and contamination. Coning and quartering was applied to split the samples. Soil was tipped in the center of a tarp and divided into smaller sections; two soil sections were further divided by a riffle splitter before blending the soil with industrial by-products.
Unamended soil was dry sieved (0.075 – 8 mm) to determine the particle size distribution.
Soil samples for soil washing (hereafter called “Soil W”) was primarily contaminated with PAH and arsenic. The soil was obtained from a waste storage facility where it was stored prior to disposal at a landfill. The soil was divided by a riffle splitter and air-dried in 35 ºC before sieving. Soil was separated into fractions: < 0.2 and 0.2 – 2 mm. The fraction 0.2 – 2 mm was used in the soil washing experiment, remaining soil fractions was discarded.
2.2 Soil amendments and extracting solution
Amendments were mixed with soil T at 5 wt%, moistened at 50% of water holding capacity and left for 3 weeks to reach equilibrium before thermal treatment. Batches of 2329 ± 131 g soil were used. The amendments were: gypsum (waste of gypsum boards, CaSO4), oxygen scarfing granulates (OSG - steel processing residue with 70% Fe3O4 and 28% Fe0), steel abrasive (SA = 96.5% Fe0), fly ash (35.7% CaO) and natural zeolite (coarse grained clinoptilolite).
Humic material in peat was extracted following the procedure described by Anderson and Schoenau (1993) with some modifications: Air-dried peat (particles above 2 mm were withdrawn by sieving) was mixed and stirred with 0.5 M HCI for 1h and thereafter
centrifuged for 30 minutes in 3200 rpm. The supernatant was discarded after centrifugation with deionized water for 30 minutes in 3200 rpm. 0.5 M NaOH were thereafter mixed into the centrifugation bottles. Bottle head space was filled with oxygen-free N2 gas and placed in a rotating over and end shaker for 18h in 15 rpm. Solution from the centrifugation bottles was filtered through a 0.45 µm syringe filter, stored at +4 ºC before usage.
2.3 Thermal treatment
Soil mixtures and soil with no additives (Control) were treated at 800 °C and divided into a fine fraction < 0.125 mm and a coarse fraction > 0.125 mm after the treatment to simulate particle separation occurring in thermal treatment plants. Batch size was about 1 kg of bulk soil that was thermally treated in stagnant air for 20 min in a carbolite furnace CSF 1200.
2.4 Soil washing
A justification for the soil washing experimental design is presented in a separate chapter (appendix A).
2.4.1 Experimental design
A two step extraction test was used to compare extractant containing HA to a pH –adjusted solution and distilled water. HA and pH-adjusted solutions were adjusted to 11 in the initial extraction step and 7 in the second step of extraction. pH was adjusted by 0.5 M NaOH or 1 M HCI. Solutions were initially heated to 44.8 ± 0.4 º C and to 51.2 ± 1.0 º C in the second step. Soil washing tests are divided into two steps to favor soil revegetation (table 1). Soil fraction used in the extraction tests was 0.2 – 2 mm.
Table 1: Design of the soil washing experiment.
Factor Units First step Second step
1. Concentration DOC (mg/g soil) (%) 17 17
2. pH 11 7
3. Contact time h 24 24
4. Temperature ºC 45 50
Soil and extraction solution was centrifuged in 3500 rpm in 15 minutes to facilitate filtering after extraction.
2.5 Evaluation methods
2.5.1 Leaching at oxidized conditions
These soil mixtures were leached using a modified leaching test with reduced sample sizes test at L/S 10 and shaken for 24 h using a rotating device according to standard procedure (CEN, 2002). Mixtures evaluated:
1. Thermally treated soil with and without amendments
2. Soil with no prior thermal treatment with and without amendments
Leachates were filtered through a 0.45 µm syringe filter, stored at +4 ºC and analyzed for trace elements.
2.5.2 Leaching at anoxic conditions
Gypsum (CaSO4) and steel abrasive (97% Fe0) were chosen primarily in regard to their inherent properties and immobilizing effect for arsenic in oxidized conditions according to prior tests. 20 g of unamended thermally treated soil, soil mixtures of thermally treated soil and gypsum with and without addition of steel abrasive (table 2) were placed into 300 ml airtight bottles and filled with deionized water to reach L/S 10. The bottles were vacuumed and filled with a methane-carbon dioxide mixture (50:50 v/v) using a syringe.
Table 2: Soil mixtures used in the batch and column leaching tests at reduced conditions, thermally treated soil mixtures < 0.125 mm
Abbreviations Soil-Amendment mixtures
GFFE Soil +5 wt % gypsum + 1 wt % SA*
GFF Soil+5 wt % gypsum
REF Unamended soil
* = SA (steel abrasive) was added to soil after thermal treatment
Bottles were stored at room temperature for 25 weeks before the standard batch leaching test EN 12457-2 (CEN, 2002) was performed. Bottles were shaken 24 hours, filtered through a 0.45 µm syringe filter and stored at –18 ºC prior to analysis.
2.5.3 Total element concentration
For the determination of the total element concentration in untreated and thermally treated soil, soil samples of 1 g were digested in 10 ml of aqua regia (HCI:HNO3 = 3:1) using a microwave digester. Total element concentration in soil W was determined by an accredited laboratory.
2.5.4 Column test
A column test was made for the evaluation of amendments in anoxic conditions. Thermally treated soil mixtures (table 2) were packed into columns according to the standard (CEN, 2004). Reduced sample sizes were used due to a lack of thermally treated material (column size: 3×10 cm, 54 g material/column). Distilled water free from oxygen flushed with N2 was allowed to percolate in an upward flow through the columns at approximately 0.75 ml/min. to ensure conditions free from oxygen. Sampling of water was done according to the standard at L/S 2, 4, 10 and analyzed for electrical conductivity, pH, redox-potential and element
concentration.
2.5.5 Chemical analyses
Content of total solids (TS) and loss on ignition (LOI) on unamended, untreated bulk soil T and bulk soil W were determined according to Swedish standard SS 28113 (SIS 1981) at 105
ºC and 550 ºC respectively.
Electrical conductivity (EC), pH and redox potential (Eh) of fractions (< 0.125 mm and >
0.125 mm) of unamended, treated and untreated soil mixtures were determined in soil –
deionised water suspensions (1:2 by weight) after 30 minutes of equilibration. In soil W, EC, Eh and pH were measured immediately before and after soil extraction tests. Solutions with humic acids were centrifuged at 3200 rpm for 15 min before filtering through 0.45 µm syringe filters. In leaching tests at reduced conditions Eh was measured monthly and EC and pH immediately before element analysis.
Solutions were stored at +4 ºC prior to analysis by inductively coupled plasma optical
emission spectroscopy (ICP–OES, Perkin Elmer Optima 2000 DV). Dissolved organic carbon (DOC) of the washing solution for soil W was measured using a TOC analyzer (TOC-V CSH Shimadzu). Precipitates of organic matter in extracting solution (1 g) containing HA were dissolved in 10 ml concentrated HNO3 and 1 ml of 35 % H2O2 prior to ICP–OES analysis.
Water holding capacity was determined on bulk untreated, unamended soil. Soil was left to soak in water for 3 h and thereafter to drain on a saturated sand bed.
Sulphate analyses of the column test eluates were made by spectrophotometer QAAtro Bran+Luebbe.. Sulphide content was calculated by the difference between sulphate suphur (SO4 – S) and total sulphur.
Total PAH (PAH-16) was analysed by an accredited laboratory.
2.5.6 Statistical analysis
The two-sample t-test (p < 0.05) was used to determine statistically significant differences between the sample means.
3. Results 3.1 Soil T
3.1.1 Soil properties
A higher share of contaminants was concentrated in the fine soil fraction (< 0.125 mm) (table 3). The fine soil fraction was therefore evaluated in leaching tests (batch, anoxic and column).
Table 3: Main properties of Soil T (±SD, n=3)
< 0.125 mm (75.3 % of bulk) > 0.125 mm (24.7 % of bulk)
Dry weight (%) 83.8 ± 0.5*
pH 4.9 ± 0.1 5.1 ± 0.6
EC (µs/cm) 98.2 ± 5.3 116.2 ± 64.0
Redox potential (mV) 337.3 ± 4.1 331.9 ± 9.3
Loss on ignition (LOI) (%) 1.8 ± 0.1*
Water holding capacity (%) 57.4 ± 1.2*
Elements (mg/kg dw) n = 3 ± SD
As 1 801.0 ± 325.2 1 363.7 ± 192.9
Cr 334.5 ± 87.5 331.2 ± 69.6
Cu 169.9 ± 35.2 139.4 ± 43.4
* = Determined for bulk soil
3.1.2 Leaching in oxidized conditions Thermally treated soil
Batch leaching tests was made to predict the behavior of a material in environmental conditions and mainly addresses elements soluble in oxidizing conditions. Mixtures and Control soil with no prior thermal treatment was also tested. Thermal treatment increased soil pH, conductivity and leaching of As and Cr, while leaching of Cu decreased (table 4).
Leaching of Cu was below the limit for inert waste in all thermally treated mixtures. Results of this study showed that the addition of all amendments had a reductive effect on Cu and As leaching after thermal treatment in all mixtures compared to unamended soil (figure 1).
Leaching of chromium was reduced in all mixtures except fly ash which raised pH. Leaching of cupper was most extensively reduced by an addition of gypsum.
Table 4: Leaching limit values (L/S 10) stipulated by the council of European Union for accenptance of waste to landfills for inert, non-hazardous and hazardous waste (EC, 2003)
Element Unit Limit values
Inert Non-hazardous Hazardous
As mg (kg dw)-1 0.5 2 25
Cr “ ” 0.5 10 70
Cu “ ” 2 50 100
Figure 1: Concentrations of Cu, Cr, As, Fe and Ca in soil mixtures treated at 800°C, fraction
< 0.125 mm, (±SD, n=3) Untreated soil
OSG and SA significantly decreased leaching of As in thermally untreated soil while fly ash increased leaching of As. Other amendments had a minor effect on As. Leaching limit values for non-hazardous waste in a landfill were exceeded for As by a 2 – fold (table 4). Leaching of Cu was reduced by all amendments but most extensively by an addition of fly ash.
Leaching of chromium was reduced by all amendments (except fly ash), most extensively by gypsum and SA. Fly ash raised soil pH to 8.3. Leaching of Cu exceeded limit values for inert waste by 2 – 400% in all soil mixtures except those with fly ash, which reduced leaching of
Cu below the limit for inert waste (1.87 ± 0.18 mg/kg dw). Leaching of Fe increased in all mixtures except those with gypsum and fly ash where leaching decreased below the detection limit. Leaching of Ca decreased in a zeolite mixture, was unaffected in SA and OSG but increased by an addition of gypsum and fly ash.
Figure 2: Concentrations of Cu, Cr, As, Fe and Ca in untreated soil mixtures, fraction <
0.125 mm, (±SD, n=3).
Table 5: pH, redox potential and electrical conductivity in soil mixtures treated at 800 oC and untreated soil fraction < 0.125 mm (±SD, n=3)
Soil
mixtures pH
Redox potential (Eh, mV) Conductivity (mS/cm)
Untreated Treated Untreated Treated Untreated Treated Control 4.9 ±0.1 6.4 ± 0.1 337.3 ± 4.10 246.9 ± 1.0 98.2 ± 5.3 83.1 ± 1.0 Fly ash 8.3 ± 0.1 9.4 ± 0.1 343.3 ± 9.0 398.4 ± 12.1 341.3 ± 9.0 381.7 ± 4.0 OSG 5.5 ± 0.2 6.3 ± 0.1 385.7 ± 8.2 259.5 ± 34.4 388.7 ± 9.2 109.2 ± 50.1 SA 5.7 ± 0.2 7.2 ± 0.3 383.1 ± 5.0 270.8 ± 1.7 388.1 ± 8.0 71.2 ± 2.3 Gypsum 5.6 ± 0.2 7.0 ± 0.2 392.9 ± 3.0 287.9 ± 0.8 334.9 ± 9.0 1170.7 ± 8.1 Zeolite 5.2 ± 0.06 6.5 ± 0.46 401.7 ± 2.14 279.3 ± 1.4 421.7 ± 11.1 77.8 ± 1.4
3.1.3 Column test
Leaching of As increased during the column test to a maximum of 277 mg/kg at L/S 10 in unamended thermally treated soil (REF, figure 6), while Cu (0.1 mg/kg), Cr (below detection limit) and Zn (0.25 – 0.8 mg/kg) remained at low levels in all columns. The thermally treated soil mixture with gypsum (5 wt%) and additionally amended with SA (1 wt %) lowered As leaching to 84 mg/kg. pH and redox-potential increased in all columns from 6.6 to 7.3 and from 200 to 370 mV respectively (table 5), while conductivity had a decreasing trend
throughout the test. The proportion of sulphides calculated by a difference between SO4 – S and total S increased with increasing amount of percolated water. Leaching of Fe was below detection limit (< 0.013 mg/kg) in soil mixtures with gypsum and/or SA throughout the test, while maximum Fe-concentration in unamended soil was 0.8 mg/kg. There were high levels of sulphates and calcium in the leachates (figure 7). Proportion of the leaching of SO4
decreased from L/S 2 to L/S 10 suggesting an increased reduction of sulphate to sulphide (figure 8).
0 50 100 150 200 250 300
GFFE GFF REF
mg/kg
L/S 2 L/S 4
L/S 10
Figure 6: Column test: Cumulative leaching of arsenic in L/S 2, 4 and 10. Thermally treated fraction < 0.125 mm, (±SD, n=3). GFFE: Gypsum (5 wt %) and untreated SA (1 wt %).
GFF: Gypsum (5 wt %). REF: Unamended soil.
1 10 100 1000 10000
As Ca Sulphur
mg/kg
GFFE GFF REF
Figure 7: Column test: Concentrations of As, Ca and S in leachates in accumulative L/S 10.
Thermally treated fraction < 0.125 mm, (±SD, n=3).
1 10 100 1000 10000
GFFE GFF REF
mg/kg
L/S 2 L/S 4 L/S 10
Figure 8: Column test: Difference SO4 - S – Total S in accumulative L/S 2, 4 and 10.
Thermally treated fraction < 0.125 mm, (±SD, n=3). GFFE: Gypsum (5 wt %) and untreated SA (1 wt %). GFF: Gypsum (5 wt %). REF: Unamended soil.
Table 6: pH, Redox and EC of leachates from the column test (±SD, n=3)
pH Redox (mV) EC(µS/cm)
GFFE
L/S 2 6.86 ± 0.23 213.3 ± 19.9 1 653.3 ±213.8
L/S 4 7.33 ± 0.09 204.4 ± 7.4 1 673.6 ± 41.6
L/S 10 7.51 ± 0.14 369.7± 6.1 1 011.3 ± 21.0
GFF
L/S 2 6.89 ± 0.36 196.5 ± 12.6 1 816.6 ± 45.6
L/S 4 6.75 ± 0.49 343.2 ± 192.0 1 423.5 ± 310.4
L/S 10 7.28 ± 0.25 283.3 ± 133.9 976.8 ± 11.6
REF
L/S 2 6.67 ± 0.06 184.7 ± 15.8 274.8 ± 36.1
L/S 4 6.88 ± 0.12 292.6 ± 151.2 96.9 ± 5.8
L/S 10 7.22 ± 0.08 372.7 ± 10.0 35.8 ± 6.8
3.1.4 Leaching in anoxic conditions
Eh – values fluctuated between (-23) and 245 mV during the test period. Leaching of As reached 203 mg/kg in unamended thermally treated soil while leaching of Cr, Cu and Fe was below detection limits. Zn leaching remained at low levels (0.2 - 0.4 mg/kg) in all mixtures and unamended soil. Most extended immobilizing effect of As (40 mg/kg) was reached in GFFE where Ca, S and Fe were present.
1 10 100 1000 10000
As Ca Mn Sulphur
mg/kg
GFFE GFF REF
Figure 9: Leaching in anoxic conditions (accumulated L/S 10). Thermally treated fraction
<0.125 mm, GFFE: Gypsum (5 wt %) and untreated SA (1 wt %). GFF: Gypsum (5 wt %).
REF: Unamended soil (±SD, n=3).
Table 7: pH, redox and EC of leachate from leaching test in anoxic conditions
Mixtures Eh (mV) pH EC (µS/cm)
GFFE 340 ± 1.8 7.40 ± 0.11 2390 ± 0
GFF 336.2 ± 2.0 7.47 ± 0.01 2407 ± 23.1
REF 340.1 ± 0.6 6.88 ± 0.02 225.2 ± 8.7
3.2 Soil W
3.2.1 Soil properties
Table 8: Main properties of bulk soil W (±SD, n=3) Units
Dry weight (%) 89.7 ± 4.0
pH 6.3 ± 0.1
EC µs/cm 199.8 ± 2.9
Redox potential mV 349.8 ± 80.7
Loss on ignition % 3.0 ± 0.2
Water holding capacity % 26.7 ± 8.5 Element concentration (mg/kg dw)
As 1 836.7 ± 77.7
Cr 832.3 ± 85.8
Cu 117.3± 17.6
PAH-16 3233.3 ± 723.4
PAH light molecular weight 166.7 ± 40.4 PAH medium molecular weight 2500.0 ± 529.1 PAH high molecular weight 563.3 ± 115.5
Figure 10: Particle size distribution, soil W dried in 35 0C. Fraction used in soil washing tests denoted
3.2.2 Peat properties
Table 9: Main properties of peat (±SD, n=3) Units
Dry weight % 95.73
pH 4.5 ± 0.3
Loss on ignition % 29.7 ± 12.7
DOC* mg/l 10 720
* Extracted from peat according to the method described by Anderson and Schoenau (1993).
Table 10: Temperature, pH, EC, Eh and trace element concentration in extraction solution before and after extraction step 1 (±SD, n=3).
45 º C 45 º C, pH –adjusted to 11 45 º C, DOC (1.7 %) in pH 11 Before
extraction
After extraction Before extraction
After extraction
Before extraction After extraction
Temperature (ºC) 45.2 20 44.4 20 44.9 20
pH 6.163 4.13 ± 1.81 10.924 8.89 ± 0.15 10.937 9.99 ± 0.03
EC (µS/cm) 7.0 835.0 ± 965.9 1385 467.8 ± 27.0 9350 7417 ± 157
Eh (mV) Unstable 360.5 ± 10.7 Unstable 324.9 ± 8.9 Unstable 276.6 ± 2.3 Trace elements
(mg/kg)
As 100.6 ± 7.5 358.9 ± 2.3 - 363.8 ± 13.3
Cu 1.4 ± 0.7 3.2 ± 0.0 10.0 ± 4.0 32.0 ± 1.1
Cr 1.1 ± 0.1 7.8 ± 0.4 2.0 ± 0.1 17.0 ± 0.6
Fe 8.3 ± 5.5 105.6 ± 6.1 150.0 ± 7.2 372.1 ± 6.8
Ca 135.1 ± 14.1 24.2 ± 3.1 58.4 ± 23.3 276.3 ± 5.89
Unstable: Analysis equipment adapted for room temperature solutions.
* = Contained filtered organic material (0.45 µm) which was dissolved in a mixture of HNO3 and H2O2.
Table 11: Temperature, pH, EC, Eh and trace element concentration in extraction solution before and after extraction step 2 (±SD, n=3).
50 º C 50 º C, pH –adjusted to 7 50 º C, DOC (1.7 %) in pH 7 Before
extraction
After extraction Before extraction
After extraction Before extraction
After extraction
Temperature (ºC) 52.0 20 51.8 20 49.8 20
pH 7.05 6.4 ± 0.1 6.7 7.4 ± 0.5 6.8 7.3 ± 0.1
EC (µS/cm) 5.2 36.6 ± 26.2 6.0 100.2 ± 20.8 45400 37397 ± 266
Eh (mV) Unstable 325.9 ± 1.6 Unstable 297.5 ± 9.0 Unstable 289.5 ± 1.1 Trace element
(mg/kg)
As 51.6 ± 0.6 89.78 ± 1.8 - 60.8 ± 2.1
Cu 0.33 ± 0.01 12.0 ± 0.8 10.0 ± 4.0 12.0 ± 0.8
Cr 1.3 ± 0.1 2.3 ± 0.3 2.0 ± 0.1 3.8 ± 0.1
Fe 14.4 ± 1.3 24.1 ± 5.3 150.0 ± 7.2 162.6 ± 5.3
Ca 28.3 ± 1.4 2.3 ± 0.3 58.4 ± 23.3 156.2 ± 22.4
Unstable: Analysis equipment adapted for solutions in room temperature.
* = Contained filtered organic material (0.45 µm) which was dissolved with a mixture of HNO3 and H2O2.
3.2.3 Chemical extraction
Figure 11: Soil concentrations of PAH-16, trace elements and As in 2-step chemical extraction solution with an initial pH of 11 lowered to 7 in the second step.
The addition of HA significantly enhanced the extraction of trace metals and PAH compared with the other heated solutions, but solely to a minor extent. Content of As, Cr, Cu, Zn and PAH-16 in soil was reduced by 56.3, 53.4, 58.7, 54.9 and 66.3% respectively, but the remaining concentrations of all the constituents in soil were still high after the extraction (figure 11). Limit values for less sensitive land –use were exceeded for As and Cr by a 40- and 3-fold, respectively. Limit values for low, medium and high molecular weights PAH were exceeded by a 3-, 25- and 45 –fold, respectively (table 12). Most abundant PAH in soil samples are medium molecular weight PAH. Content of low-, medium-, and high molecular weight PAH was reduced by 64.9, 78.2, and 66.3 % in soil extracted with HA. There was no significant difference between extraction solutions concerning low and medium molecular weight PAH. HA significantly increased extraction of high molecular weight PAH (figure 12).
pH was lowered during extraction in the first step in all solutions, while increased in the second step, probably due to liquid residues of NaOH. EC decreased in HA and pH adjusted – solutions after extraction, while the opposite prevailed in unamended solutions. EC increased from step 1 to step 2 for HA-solution, but decreased in pH-adjusted and solely heated
solutions. Eh decreased in HA – solution from step 1 to step 2, but decreased in pH-adjusted and unamended solutions (table 10 and 11). Extraction of all analyzed elements in solution decreased in the second step. In washing solutions, content of Fe, As, Cu and Cr increased in pH-adjusted and HA –solutions, while the content of Ca increased. HA increased desorption of all trace elements in both steps. Leaching of Ca was most prominent and increased by 10- fold in the first extraction step and by 25-fold in the second step.
167 40 39 36
900 897
563
247 240 190
2500
877
0 500 1000 1500 2000 2500 3000 3500
Untreated Hot water Hot water pH 11 Hot water HA + pH 11 mg/kg
Low molecular weight Medium molecular weight High molecular weight
Figure 12: Soil concentrations of PAH prior to and after the chemical extraction (±SD, n=3).
Table 12: Generic guideline values for contaminated land (NTV, 1999).
Element Unit Limit values
Less sensitive land-use (MKM)
Sensitive land –use (KM)
As mg (kg dw)-1 25 10
Cr “ ” 150 80
Cu “ ” 200 80
PAH low molecular weight
“ ” 15 3
PAH medium molecular weight
“ ” 20 3
PAH high molecular weight
“ ” 10 1
4. Discussion
4. 1 The mobility of Cu, Cr and As in thermally treated soil with and without amendments Leaching in oxidized conditions
As in Nordmark (2008) thermal treatment increased pH and leaching of As and Cr while leaching of Cu decreased. All amendments reduced leaching of As after thermal treatment.
Stabilization of As in gypsum and coal fly ash mixtures after thermal treatment is probably due to the formation of Ca-arsenates, which occurs in oxidizing conditions (Bothe and Brown;
1999; Porter et al., 2004). Sterling and Helbe (2003) showed that calcium species could bind arsenate in temperatures 600 – 800ºC. Jia and Demopoulos (2005) suggested that addition of gypsum could enhance adsorption of As onto ferrihydrite by a ligand exchange reaction between sulfate anions, ferrihydrite and arsenic. The participation of Ca-ions was found to enhance stabilization of As by co-precipitation with iron (III) in pH 3 – 8. Co-precipitation of
As was shown to be of most importance in comparison with adsorption processes in all evaluated conditions (pH 4 – 8, Fe/As ratios 2 – 8) (Jia and Demopoulos, 2005). Leaching of chromium was reduced in all mixtures except fly ash which raised pH. A raised pH favours Cr oxidation and the mobility of Cr (Fendorf, 1995). The mobility of Cr was reduced in mixtures with iron and sulphur present. Ferrous iron and iron sulphides have been shown to be effective for reducing Cr (VI) to Cr (III) (Patterson and Fendorf, 1997).
Leaching in anoxic conditions and column test
Leaching of As was most prominent in column tests and at anoxic conditions, where leaching of Cu and Cr were below 0.1 mg/l. Major difference between leaching of As in batch test, column test or leaching at anoxic conditions is that gypsum reduced leaching in the column test and leaching at anoxic conditions, while it had a minor effect in batch leaching test in aerobic conditions. Arsenic could form sulphides in anoxic conditions where reduction of SO4 occurs and gets incorporated in secondary sulfide minerals like As-bearing pyrite where Fe is present or orpiment, As2S3. In Maurice et al. (2007) and Kumpiene et al. (2008b), solubility of As increased in anoxic conditions, but decreased due to addition of Fe-materials. In anoxic conditions Cu is less soluble (Maurice et al., 2007) and Cr (VI) tends to be reduced in natural conditions or precipitate with iron oxides and thereby be stable for a long time (Fendorf, 1995). The stability of As in Fe-amended soil seems to be affected by increased L/S. Leaching of As at L/S 10 was more than 9 times higher than at L/S 4. A progressive leaching of As in anoxic conditions in Fe0- amended soil while L/S increased from 2 to 10 has been shown by Montesinos (2005).
Gypsum contains Ca and S which could bind As in oxidized or reduced conditions. Leaching of As decreased in the column test where anoxic conditions prevail, probably due to the formation of FeAsS or AsS, which occurs in moderately to strongly reduced conditions. The mixture GFFE contains iron and gypsum and could therefore bind As in differed formations where Fe and S are present. In reduced and oxidized conditions, the solubility of As increases with increasing pH. Eh – values varied from 200 to 370 mV in the column test and Porter et al. (2004) highlighted difficulties to predict the solubility of As; when pє-values are fixed at 5 (app: 300 mV) and pH increases from 6.2 to 7.2, the solubility of As increases by a factor of 29. Leaching in the column test and at anoxic conditions was supposed to be in reduced conditions but stable reduced conditions were not obtained.
4.2 The mobility of Cu, Cr and As with and without amendments with no prior thermal treatment
Leaching in oxidized conditions
The mobility of As in soil is primarily governed by sorption processes and binding to metal- oxides, especially Fe-oxides. These Fe-As complexes are stable in oxidized conditions at low to neutral pH (Porter et al., 2004). OSG and SA stabilized As in untreated soil whereas other amendments had no effect or increased the mobility of As. The addition of iron-containing materials (OSG and SA) could have increased the number of sorptive sites in soil and thereby reduced the mobility of As. An addition of fly ash which raised pH (8.3) increased the
mobility of As. Addition of materials which increase pH increase the mobility of As (Hartley et al., 2004). An addition of iron materials (OSG and SA) could mean that soil is reclassified to be accepted as non-hazardous waste at a landfill in regard to leaching of As (table 8).
Leaching of Cu was reduced by the addition of fly ash which raises pH and leaching of Cu is primarily governed by pH (Hartley et al. (2004). The mobility of Cr was reduced by an addition of OSG. Cr could co- precipitate with hydrous iron oxides in the present of divalent iron (Fendorf, 1995).
4.3 The mobility of Cu, Cr, As and PAH affected by an addition of HA
The addition of HA significantly enhanced the extraction of As, Cr, Cu, low- and medium molecular weight PAH compared to the other heated solutions, but only to a minor extent.
The addition of HA increased extraction of high molecular weight PAH most extensively.
Kim and Osako (2003) highlighted that pH has a stronger effect on the surrounding soil than on the PAH whenever organic content increases. Leaching of medium molecular weight PAH was unaffected by raised pH and L/S in a sandy soil with 1.7 % organic matter (OM) content (Kim and Osako, 2003). OM in extracted soil was 3.0 ± 0.17 % and low organic content could have meant that leaching of medium molecular weight PAH was less affected by raised pH.
The ratio of coal/hydrogen within the structure of a PAH increases with molecular weight.
High molecular weighted PAH have lower solubility and a higher boiling point probably due to higher inherent coal (C) content (Connell, 1997). High molecular weight PAH is therefore less affected in heated solutions, but could be more affected by their surroundings due to a larger C content. Kögel-Knabner et al. (2000) showed that the solubility of a high molecular weight PAH was enhanced at a higher rate than a medium molecular weight in the presence of dissolved organic matter (DOM). Desorption of PAH is moreover positively correlated with content of high molecular weight DOM (> 14000 Dalton) (Kögel-Knabner et al., 2000).
Content of dissolved calcium was reduced in pH-adjusted extraction solutions compared to distilled water. Analysis of HA – solution showed that Ca and trace metals has been adsorbed in washing solution. Dissolved calcium is highlighted by Rastas – Amofah et al. (2010) and Li et al. (2002) to possibly hamper the extraction of As and to bind humic acids or NaOH used in extraction solutions. Extraction solutions were pH-adjusted with 0.5 M NaOH. The solubility of PAH decreases with increased ionic strength or salt coefficient. Addition of NaOH changes ionic strength in solution and thereby could decrease leaching of PAH.
Negative charges of the HA is shielded by Na+ and the charge density of humic acids
decreases (Tanaka et al., 1997). Kögel-Knabner et al. (2000) and Yang et al. (2001) showed that the addition of divalent or monovalent metal ions decreased desorption of PAH,
regardless of ion type.
Soil W was not recently contaminated but age was unknown and several studies have concluded that aging of contamination decreases the extractability of PAH (Chaker Ncibi et al., 2007, Nam and Kim, 2002; Kögel-Knabner et al., 2000). Due to severe difficulties filtering, HA extraction solutions were left for 40 minutes for sedimentation after extraction prior to filtering which could have caused readsorption of PAH (Haritash and Kaushik, 2009).
5. Conclusions
Thermal treatment increased pH and the mobility of Cr and As while the mobility of Cu was reduced. All amendments (iron compounds, fly ash, gypsum and zeolite) decreased leaching of Cu, Cr and As in thermally treated soil; most prominent exception was the addition of fly ash which raised pH and increased the mobility of Cr. The mobility of As after thermal
treatment was reduced by all amendments but leaching still exceeded the leaching limit values for acceptance at landfills for hazardous waste and needs further treatment. Leaching of Cu was below the limit for inert waste in amended and unamended soil. Leaching of Cr exceeded the limit for inert waste in unamended soil but decreased below this limit with the addition of amendments (except fly ash). An addition of gypsum reduced the mobility of Cu and As most efficient. The mobility of Cr was mostly reduced by an addition of zeolite. However, an addition of zeolite reduced leaching of As and Cu solely to a minor extent. Arsenic was the only significantly leaching trace element in anoxic conditions exceeding limits for acceptance at landfills for hazardous waste. A mixture of steel abrasive and gypsum decreased leaching of arsenic at a higher extent than gypsum. The addition of a mixture of gypsum and steel abrasive decreased leaching of As in anoxic conditions but leaching still exceeded the leaching limit values for acceptance to landfills for hazardous waste and soil needs further treatment.
In soil with no prior thermal treatment an addition of iron materials reduced leaching of Cr and As most efficiently while an addition of fly ash was most effective reducing leaching of Cu. An addition of fly ash increased leaching of As and Cr. Leaching of Cr was below the limit for acceptance at a landfill for inert waste in unamended soil. Leaching of Cu exceeded limit values for acceptance at a landfill for inert waste in all mixtures (except fly ash) and unamended soil. The leaching of As was reduced in soil amended with iron materials below limit values for the acceptance at a landfill for non-hazardous waste.
In comparison to a pH-adjusted solution the peat extract only slightly enhanced the extraction of trace metals and low and medium molecular weight PAH. Enhancement was most
significant for the extraction of high molecular PAH. Levels of Cu decreased to 50 mg/kg after chemical extraction which is below the limit for sensitive land –use (KM). Other trace elements and PAH was in need for further treatment before soil usage.
Appendix A: Soil washing design
Soil washing is efficient for soil fractions 0.2 – 2 mm while smaller particle sizes could be difficult to cleanse (Kuhlmann and Greenfield, 1999). The efficiency of humic acids as an extraction agent for PAH and arsenic is primarily dependent on:
1. Chemical structure and composition of humic acids 2. Co – existing cations
3. pH
4. Age of contamination 5. Temperature
6. Contact time and rate of humic acid addition
Arsenic is a trace element in soil which is most mobile in its reduced state and appears as an anion in solution. The mobility of As in soil is primarily governed by pH and the redox – potential (Eh). Extraction of As often means that pH is adjusted to extremely acidic or basic values (< 3 or >10) but has also been enhanced by the addition of dissolved organic matter (DOM) decreasing redox-potential (Mulligan and Wang, 2009). PAH adsorbs primarily to organic matter in soil and the solubility is primarily governed by their molecular weight, temperature and pH. Remediation of PAH: s includes, thermal treatment, solvent extraction and short- or long-term degradation. Degradation could be enhanced by adding oxidizing materials (e.g.: H2O2 or ozone), otherwise land farming or phytoremediation are most common for the remediation of PAH. Techniques used for remediation of PAH have been summarized by Gan et al. (2009).
A humic acid (HA) includes a variety of functional groups. A majority of these functional groups are more soluble and bind more strongly cations primarily Fe3+, Al3+, Pb2+, Ca2+, Mn2+
or Mg2+ in high pH. Due to their negative charge and affinity to metal (hydro) oxides they tend to compete with As (III) and As (V) for sorptive sites (Wang and Sullivan, 2005). The chemical structure and composition of HA could vary significantly even in the same soil (Kang and Xing, 2005; Ping et al., 2006). Interactions of PAH-HA and As-HA are highly dependent on the concentration of divalent cations, the properties of iron oxides and humic acid functional groups (Wang and Sullivan, 2005).
Variations in pH could change the behavior of material co-existing with PAH in soil reducing the binding of humic acids to minerals and thereby mobilize contaminants (Lesage et al., 1995). Extraction of PAH are enhanced in pH above 10 when HA is present (Kim and Osako, 2003; Liang et al., 2007), but numerous studies have concluded that an addition of humic substances at neutral pH increases solubility and leachability of PAH (Kim and Osako., 2003;
Lassen and Carlssen, 1997; Tanaka et al., 1997). Lassen and Carlssen (1997) increased the solubility of PAH to a limited extent, while Tanaka et al. (1997) increased the solubility by a 100-fold in neutral pH and in the present of HA. Acidic conditions could increase adsorption of HA to mineral grains. Ping et al. (2006) evaluated leaching of PAH in the acidic range (pH 3 – 6) where adsorption increased at lower pH.
Several authors have concluded that aging of contamination decreases the extractability of PAH (Chaker Ncibi et al, 2007, Nam and Kim, 2002), while recently contaminated soil is more easily leached than aged. Temperature has been identified to be a significant factor
governing leaching of PAH and As (Viamajala et al., 2007, Kim and Osako, 2003; IVL 2003, Ping et al., 2006; Rastas – Amofah et al., 2010), where leaching increases as temperatures raises from 20 to 60ºC.
Increasing the concentrations of HA caused increased leaching of phenanthrene and pyrene (Kim and Osako 2003). The addition of 1% HA enhanced the solubility of PAH while temperature was raised from 15 to 25 °C. Addition rates of HA for removal of PAH in earlier research has been 0.2 – 1.6% of HA in soil at L/S 10 (Ping et al., 2006; Lin et al., 2009;
Conte et al, 2004 and Berselli et al., 2004) where removal efficiencies ranged between 50 and 85% at contact times 24 – 168 h in room temperature. Most studies reached solution
equilibrium in 12 – 48 h (Tanaka et al, 1997; Hwang and Cutright, 2002; Wang and Mulligan 2009; Kim and Osako, 2003), but there is a large variety of recommended contact times and Kim and Osako (2003) reported contact times between 48 – 240 h to be sufficient for PAH removal. Contact time exceeding 200 h has been recommended in soils with high clay content (Hwang and Cutright, 2002). Haritash and Kaushik (2009) recommended constant steering to prevent the readsorption of PAH.
Appendix B:
Review: Soil remediation studies at the Division of Waste Science and Technology at the Luleå University of Technology
1 Introduction
There are about 80 000 potentially contaminated sites in Sweden which could be in need for remediation. Many of these are contaminated with constituents with differed behavior in various environments. Conditions in nature are much variable and a contaminant most likely will not behave similar at two different areas. Knowledge about contaminated areas and their properties is therefore essential. The Division of Waste Science and Technology at the Luleå University of Technology has been developing soil remediation techniques since 2001.
The aim for this study is to summarize the obtained results of the researchers at the Division of Waste Science and Technology at the Luleå University of Technology (LTU) and possible gain for the soil remediation industry, as well as to give suggestions for further research by reviewing similar studies.
2 Methods
A literature study of performed and documented soil remediation studies at the Division of Waste, Science and Technology at Luleå University of Technology in 2003- 2010. Similar studies in a minor selection of data have been reviewed for comparison. Studies from the Division of Waste, Science and Technology at Luleå University of Technology are numbered 1 – 15 and compared studies 20 – 56.
3 Results
14 soil remediation studies have been presented. Soil remediation technologies used:
• Chemical stabilisation
• Thermal treatment
• Soil washing (Chemical extraction)
3.1 Remediation of arsenic-contaminated soil: Chemical stabilisation
Arsenic (As) is a metalloid which is present in nature primarily as As (III) or As (V) where As (III) is more toxic. In solution arsenic is present as an oxyanion and therefore binds more strongly to positively charged surfaces at acidic conditions. Arsenic could precipitate as sulphides in anoxic reduced conditions, while higher pH means an excessive amount of hydroxide ions where As could precipitate as hydroxides. Mobility of As in soil is most governed by metal oxides, primarily Fe-oxides. Adsorption capacities of Fe-oxides are dependent on redox-potential and pH.
Stabilisation of trace metals and arsenic refer in summarized studies to chemical stabilisation induced by immobilizing soil additives (amendments). Stabilization techniques are developed to convert contaminants into less soluble or less toxic forms. Soil stabilization immobilizes contaminants by adding materials that enhance adsorption, complex binding or precipitation.
Typical immobilizing agents in field studies are: zerovalent iron materials (ZVI), lime, phosphates, organic matter induced additives (peat, manure) and industrial co-products based synthetics. In situ stabilization often involves an introduction of a treatment material or a chemical into the contaminated soil. Soluble and insoluble amendments are often mixed in soil by saturation or mechanical mixing.
3.1.1 Environments for evaluation
Metals and metalloids (As) are indestructible. Remediation goal for metal-contaminated soil could therefore be to immobilize contaminants. There are several factors that affects the stability of As and metals in treated soil. Efficiency of amendments is evaluated by batch leaching tests (short term) and column tests or lysimeter tests (long term). For the assessment of the stability of amended soils factorial tests and fractionation are used. In order to evaluate stabilisation effects of Fe-treated soil, a sequential chemical extraction test according to Tessier (1979) was performed before and after Fe-treatment to determine As bound fractions (I – V) (2). In nature conditions such as pH, redox, aeration and soil properties like organic matter content often interacts. Factorial tests are made to resemble these conditions by
evaluating interaction between factors likely to occur in nature. Factors are also evaluated separately with no interaction.
3.1.2 Amendments
Amendments in studies concerning chemical stabilisation has been chosen based on their supposed ability to reduce levels of As – contamination. Most thoroughly used amendment is different kinds of iron materials. Zerovalent iron (ZVI) material oxidizes to Fe-oxides which could bind As. ZVI is supposed to have a minor effect on soil pH (2). Adsorption of As to Fe- oxides is more effective for As (V) than As (III) and occurs when hydroxyl groups are replaced with As-ions (17). In some studies (5 and 6) mixtures of amendments has been used in order to stabilize As. Amendments used in summarized studies are given in Table 1.
Table 1: Amendments used in studies of chemical stabilisation
Amendments Mixing ratios (wt %) References
Steel abrasives (SA) 0.1 – 1 1, 2, 3 , 4
Oxygen scarfing granulate (OSG)
1 – 25 4, 9, 10, 11
Blast furnace slag (BFS) 5 6
Gypsum, sulfurous material 5 5, 12
Coal fly ash (CFA) 3 – 5 6, 12
Organic material (sludge, peat) 3 – 5 6, 12
Fe0 -materials 0.1 – 1 (Mol rate Fe/As = 2 – 2.5) 2, 3, 4, 5, 6, 8 , 10, 11, 12
3.1.3 Amendments used in compared studies
Materials that have been used as amendments in reviewed studies for the remediation of As are compost (5 – 20 wt %), green waste compost (GWC) (30 – 50 wt %), iron materials (0.1 - 1 wt % or Fe/As = 2 -100), peat (5 wt %), beringite (Al-silicate 5 wt %) and iron-sulfur materials (1 wt %)(20, 21, 22, 25, 28, 34, 35, 38, 39, 40 and 41).
3.2 Efficiency of amendments
In batch leaching tests most of stabilization trials including Fe0 - materials reduced leaching of arsenic (1, 2, 3, 4, 5, 6, 8, 9, 10, and 11) with an exception of an addition of 0, 1 % Fe0 in Kumpiene et al., (10). Mixtures of Fe0 and other materials (gypsum, fly ash, peat or sludge) in
soil were evaluated in Schulenburg, (12), all mixtures but fly ash stabilized arsenic. Moreover (4) investigated stabilization differences and concluded that fresh fine grained iron was more effective than old outside stored coarse grained material (4). In Olsson, Persson, (5) column tests with various additions of Fe, S and Ca- materials were used for stabilization of soils with high levels of clay, sand and OM. FeSO4 was the only amendment stabilizing As in sandy soil, Fe0 had no significant effect on As but addition of CaS or sulfurous water decreased leaching of As (5). FeSO4 and sulfurous materials reduced leaching of As in soils with high levels of clay or OM but decreased pH and thereby increased leaching of other trace metals (5). To access the potential use of treated CZA-contaminated soil in a vegetation layer at a landfill a gravity column test was used in Schulenburg, (12). Mixtures of Fe0 and gypsum, coal fly ash, peat or sludge were used as potential amendments in soil. As-content in pore water from these soil mixtures decreased in column depths up to 95 cm but increased in deeper levels (195 cm). All mixtures except coal fly ash showed promising results for usage (12).
Water saturation levels of 30 – 100 % have been used for the evaluation of the efficiency of Fe-treated soil during anaerobic and aerobic conditions. Treatment of As-contaminated soil in lysimeters resulted in reduced mobility of As in oxidized conditions while the mobility of As increased with time, water saturation level (1, 11, 10 and 9) and decreased with depth (10).
Although, in Maurice et al., (1) leaching of As was on the contrary less in saturated conditions with amendments than in oxidizing conditions, these results could be dependent on aerobic filtering or other disturbing laboratory conditions. The opposite was obtained in Kumpiene et al., (10). Differences in stabilizing effects due to Fe-amendments have been dedicated to inadequate mixing of amendments into soil, watering of soil, quality of amendments or disturbance caused by sampling and analyses (1, 2, 4, 8, 9 and 11). To resemble low L/S with no additional water added to Fe-amended soil, As- concentration in pore water decreased almost to a similar extent compared to laboratory experiments (4).
The reduction of As has occurred in saturated and unsaturated conditions (10). While water saturation levels increased from 50 to 100 %, solubility of As increased and conversion rate of As (V) to As (III) significantly increased and causes As (III) to be the dominating specie even in untreated soil (1, 10). Reduction rates of As increased with higher amounts of Fe- amendments (10). In pore water samples the solubility of As and Fe correlated with water saturation level. Solubility of As increased as saturation levels increased but decreased with amount of Fe-amendments regardless of saturation level (10). Although, an addition of OSG (17 wt %) decreased the solubility of arsenic in saturated conditions while the solubility of Fe and Mn increased (1). In simulated natural conditions (600 mm precipitation/year and field moisture: 30 – 50 % of WHC) there were no detection of As (III) in lysimeter pore water, levels of DOC decreased, pH increased and redox-values were unaffected over time (9).
3.2.1 Efficiency of amendments in compared studies
In field trials, the mobility of As was effectively reduced by an addition of iron compounds (20, 21, 22, 34 and 37). Mixtures of iron compounds, beringite and sulfurous materials were more effective than materials added separately (34 and 35). A column test with acidified water for percolation was used in Hartley and Lepp (35). Other materials such as peat (5 - 10 wt %), compost (5 – 20 wt %) and green waste compost (30- 50 wt %) increased the mobility of As (20, 21, 25, 28). 10 wt% peat was added in columns with contaminated sand the mobility of As increased with depth of 6 cm and then became constant (25). Fe-materials with higher specific surfaces were more effective reducing leaching of As. Efficiency rose with molar ratio of Fe/As (41). The dissolution of Fe –oxides decreased over time in a 34 –day period in saturated conditions. Dissolution of Fe was not immediately followed by a release of As. Readsorption of arsenic occurred and dissolution rates decreased over time. Conversion rate of As (V) to As (III) was initially rapid but decreased over time (37). Release of As in reduced conditions in the presence of Fe are dependent on temperature. Release of As and Fe is 10 times smaller in 5 ºC compared to 23 ºC. Extraction experiments suggest a co- precipitation or readsorption of As (III) to sorbents that are less sensitive for leaching (39).
For assessment of the behavior of As in flooded soil, an As-contaminated soil was amended with iron materials and stored at 100 % saturation level for 6 weeks. Redox-values and rate of As V/ As III was unaffected in saturated samples but when organic matter was added redox- values decreased, levels of As (III) increased but decreased in the presence of goethite (0.1 wt %). In saturated samples the mobility of As (III) increased in the presence of high levels of dissolved organic matter (DOM) and in pH 7 rather than 4 (41).
3.2.2 Similarities and differences
Mixtures including Fe and S were effective for the stabilisation of As (5 and 35) but mixtures of Al-like materials and Fe was not evaluated in studies from LTU. Mixtures of gypsum, coal fly ash, peat or sludge and Fe were effective for the stabilisation of As (5, 6 and 12), while peat, compost and GWC increased the mobility of As when added separately (20, 21, 25 and 28). The addition of Fe –materials in all studies stabilized As in saturated conditions with exception for Schulenburg in (12), and leaching of As increased in saturated conditions compared to unsaturated with an exception in Maurice et al., (1). The efficiency of Fe- materials increased as more amendments were added and with higher specific surface and freshness of the Fe-material (4 and 41). Weber et al, (39) evaluated temperature as a factor affecting the mobility of As.
3.3 Stability and toxicity of amended soil
ZVI-materials have reduced mobility of As in soil in all summarized studies (except in 6) and have therefore been further evaluated in various levels of pH, organic matter (OM), microbiological activity (MA), redox-potential and L/S (3, 6, and 8). L/S and MA were the
most significant factors for As-leaching in Montesinos, (3) and Kumpiene et al.,(8).pH was not interpreted individually in Montesinos, (3) but was significant in interactions with MA, L/S and OM in Kumpiene et al.,(8). pH was the most significant factor where 3 factors (L/S, redox and pH) were evaluated (6). However, in Lundberg, (6) there were no statistical correlation between factors and low redox values were not obtained. Interactions between different factors affecting leaching of As has been evaluated (3, 6, 8). Leaching of As was smallest at pH 3 and 5, high MA, and low L/S (8). L/S increased As-leaching in pH 3, but L/S had no significant effect on As-leaching in pH 8 (3, 8). Significance of factors affecting Fe- leaching were evaluated in Montesinos, (3) and Lundberg, (6) and low pH and high MA increased leaching of Fe (3).
Treatment with iron containing materials reduced As bound to exchangeable (I) and As- fraction bound to Fe-oxides (III), but had no effect on As bound to organic matter (fraction IV) and significantly increased As bound to residual fraction (V) (2). Conversion to As- bound to residual fraction indicates the formation of geochemical stable species (2).
Solution toxicity correlates with higher concentration of As (III) rather than pH. Cell survival was higher in 50 % of water saturation level (WSL) of all Fe-amended samples compared to unamended soil (10). In Fe-treated soil cytotoxicity was introduced and increased with depth and short termed but decreased over time (9). Bioaccessibility, toxicity, enzyme activity and plant uptake showed a positive effect in Fe-amended soil (2). OSG-addition in Maurice et al., (1) decreased cytotoxicity remarkably. Cytotoxicity was further correlated (R2 = 0, 85) with concentration of As (III) (1).
3.3.1 Stability and toxicity of amended soil in compared studies
The addition of peat (5 – 10 wt %) increased leaching of As where As was redistributed to less stabile fractions (water soluble and exchangeable) (20). In resemblance to Kumpiene et al., (2) Fe-treatment reduced As bound to exchangeable fraction (I) in favor of the As-fraction bound to Fe-oxides (III) and the residual fraction (3).
Treatment with Fe-materials has reduced plant uptake of As (20, 34, 36, 37 and 40). ZVI was less effective for biomass production in comparison with goethite. In a long term study, plants were cultivated and harvested after 6 years. Mixtures of ZVI, beringite (Al-silicate), sulphate was more effective reducing leachable As and plant uptake of As than materials added separately (34). Mixtures of Fe and organic materials have increased the mobility of As, biomass and microbiological activity (40).
3.3.2 Similarities and differences
There were no compared studies evaluating how the interaction of factors (pH, L/S and MA) affects the mobility of As. Treatment with Fe-materials transferred As to more stable fractions and reduced plant uptake of As (2, 20, 34 , 36, 37, 37 and 40). Mixtures of Fe-materials,
beringite and sulphate were more effective reducing plant uptake of As than materials added separately (34).
4 Remediation of arsenic-contaminated soil: Soil washing
Soil washing is a method where chemical or physical processes separate, concentrate or convert contaminants to have less negative effects on the environment. This method is done on-site where soil has been excavated. Soil washing is done with solely water or water mixed with different amendments. Adsorption is the process that primarily governs the mobility of metals in soil. Contaminants must be available for treatment and metals must be in solution or made more soluble. Acids are used to desorb metals or other inorganics from soil particles while surfactants (surface-active-agents) are mainly added for less soluble organic contaminants. Reductive or oxidizing materials are used to enhance availability of metals;
reductive amendments aim to dissolve Fe-Mn-oxides and release contaminants bound to these oxides (23).
4.1.1 Environments for evaluation
Efficiency of soil washing solutions is evaluated in batch leaching tests.
4.1.2 Amendments
Table 2: Amendments used in study for soil washing (13)
Amendments Mixing ratios
NaOH 0.05 – 0.1 M
Oxalic/citric acid (OC) 0.2 M / 0.1 M (OC)
Na2S2O4 (Na dithionite)/Na citrate/Na oxalate (DCO)
0, 03 M/ 0.1 M / 0.05 M (DCO)
The addition of NaOH and other solutions (table 2) was done in concentrations (table 3) and temperatures (20, 35 and 50°C). Mixtures of OC and DCO were tested in pH 3, 5, 7 or 5, 6, 7 at temperatures of 20, 35 and 50°C.
