Removal of arsenic from groundwater using adsorption to ferrihydrite-coated laterite in Burkina Faso

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INOM

EXAMENSARBETE TEKNIK,

GRUNDNIVÅ, 15 HP

STOCKHOLM SVERIGE 2017,

Removal of arsenic from

groundwater using adsorption to ferrihydrite-coated laterite in

Burkina Faso

AGNES HAGSTROEM

KTH

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This study has been carried out within the framework of the Minor Field Studies Scholarship Programme, MFS, which is funded by the Swedish International Development Cooperation Agency, Sida.

The MFS Scholarship Programme offers Swedish university students an opportunity to carry out two months’ field work, usually the student’s final degree project, in a country in Africa, Asia or Latin America. The results of the work are presented in an MFS report which is also the student’s Bachelor or Master of Science Thesis. Minor Field Studies are primarily conducted within subject areas of importance from a development perspective and in a country where Swedish international cooperation is ongoing.

The main purpose of the MFS Programme is to enhance Swedish university students’ knowledge and understanding of these countries and their problems and opportunities. MFS should provide the student with initial experience of conditions in such a country. The overall goals are to widen the Swedish human resources cadre for engagement in international development cooperation as well as to promote scientific exchange between unversities, research institutes and similar authorities as well as NGOs in developing countries and in Sweden.

The International Relations Office at KTH the Royal Institute of Technology, Stockholm, Sweden, administers the MFS Programme within engineering and applied natural sciences.

Erika Svensson Programme Officer

MFS Programme, KTH International Relations Office

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Title:

“Removal of arsenic from groundwater using adsorption to ferrihydrite-coated laterite in Burkina Faso”

Author:

Agnes Hagstroem

Supervisors:

Prof. Dr. Samuel Paré, Environmental Chemistry, Department of Chemistry, University of Ouaga 1, Pr Joseph KI-ZERBO, Ouagadougou, Burkina Faso

Prof. Dr. Jon Petter Gustafsson, Department of Sustainable Development, Environmental Science and Engineering, Royal Institute of Technology, Stockholm, Sweden

Prof. Dr. Ingmar Persson, Department of Chemistry, Swedish University of Agricultural Sciences, Uppsala, Sweden

PhD-Student Yacouba Sanou, Department of Chemistry, University of Ouagadougou, Burkina Faso

Granting institution:

International Relations Office, Royal Institute of Technology

Course title:

Bachelor Thesis in Civil Engineering, 15 HEC

Course code:

AL130X

Key words:

Burkina Faso, arsenic, laterite, ferrihydrite, adsorption, column experiment, drinking water, ground water

Front page picture:

Water collection from a well in Ziniaré, 25 km outside of Ouagadougou. Photo: Samuel Pare.

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Table of contents

ABSTRACT ... 4

ACKNOWLEDGEMENTS ... 5

1 AIM ... 6

2 INTRODUCTION ... 7

2.1 BURKINA FASO ... 7

2.2 PROBLEM BACKGROUND ... 8

2.3GEOLOGY OF THE REGION ... 9

2.4REMOVAL TECHNIQUES FOR ARSENIC-ENRICHED WATER ... 10

2.5 EARLIER STUDIES ... 11

3 THEORY ... 12

3.1 ARSENIC ... 12

3.2 LATERITE ... 13

3.3 FERRIHYDRITE ... 15

3.4 FACTORS AFFECTING ADSORPTION ... 16

3.4.1 Flow rate ... 16

3.4.2 Empty bed contact time (EBCT) ... 16

3.4.3 pH ... 17

3.4.4 Initial arsenic concentration ... 17

3.4.2 Competition from other anions ... 17

4 MATERIAL AND METHODS ... 19

4.2 COLUMN SET-UP ... 19

4.2.1 Material ... 19

4.2.2 Description ... 19

4.1 PREPARATION OF ADSORBENT ... 20

4.1.1 Laterite ... 20

4.1.2 Ferrihydrite-coated laterite ... 21

4.3 COLLECTION AND PREPARATION OF WATER ... 22

4.4RUNNING OF THE EXPERIMENTS ... 22

4.4.1 Experiment 1: Laterite ... 24

4.4.5 Experiment 5: Ferrihydrite-coated laterite (oven) ... 24

4.4.6 Experiment 6: Ferrihydrite-coated laterite (oven) ... 24

4.4.7 Experiment 7: Ferrihydrite-coated laterite (centrifugation) ... 24

4.5 ARSENIC ANALYSIS ... 25

5 RESULTS ... 26

5.1 INFLUENT WATER ... 26

5.2 EXPERIMENTS ... 27

6 DISCUSSION ... 29

6.1VALIDITY OF INFLUENT WATER ... 29

6.2 EVALUATION OF ADSORBENTS ... 29

6.2.1 Laterite from Burkina Faso ... 29

6.2.2 Ferrihydrite-coated laterite (oven) ... 30

6.2.3 Ferrihydrite-coated laterite (centrifugation) ... 30

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6.3 SOURCES OF ERRORS ... 31

6.4 POTENTIAL FOR UP-SCALING ... 32

7 CONCLUSION ... 34

8 REFERENCES ... 35

8.1PUBLISHED WORKS ... 35

8.2 UNPUBLISHED WORKS ... 36

8.3 OTHER ... 36

8 APPENDIX ... 38

8.1 APPENDIX 1: CALCULATIONS ... 38

8.2A^PPENDIX2:FERRIHYDRITE ADSORPTION CAPACITY ... 40

8.3A^PPENDIX3:CALCULATIONS OF COSTS FOR FERRIHYDRITE-COATED SAND ... 41

8.4 APPENDIX 4: FERRIHYDRITE SYNTHESIS ... 42

8.5 APPENDIX 5: RAW DATA FROM EXPERIMENTS ... 43

8.6 APPENDIX 6: PHOTOS FOR BETTER UNDERSTANDING ... 47

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Abstract

In several parts of Burkina Faso, there is an urgent need of making arsenic contaminated water potable. The shallow, dug out wells traditionally used in the rural areas provides small, inconsistent yields and are vulnerable to microbes. Consequentially, many of them have now been replaced with drilled tube-wells, intercepting water from fractures in the bedrock and providing reliable water sources. However, as arsenic is naturally present in the bedrock in many parts of the country, previous studies has shown that water from a lot of these wells is not safe to drink. In some cases, the wells had to be closed due to high arsenic concentrations.

The medical effect of the arsenic exposure has also been evaluated among the inhabitants of affected villages, indicating that the problem is critical.

Previous studies in this project were focused on using the commercial material Granular Ferric Hydroxide (GFH) as a possible method for arsenic removal in the villages. As the price of GFH however is too high, other methods has been researched instead. In this study, natural laterite rock has been crushed and used as adsorbent in column experiments, but indicating a low arsenic capacity. The same lateritic sand has then been coated in ferrihydrite both by drying the materials together in the oven and by a centrifugation method previously evaluated in Uppsala, Sweden. Although results after the ferrihydrite-coating improved, the adsorption capacity was still significantly lower than that of GFH. The results from the ferrihydrite-coated sand also differed largely from those previously found in Uppsala, and though this could be due to the fact that this study was made with natural water instead, it is thought that the inevitable alterations in the coating method might have affected the result of the coating to a great extent. The conclusion of this study is therefore that ferrihydrite-coated sand still is a possible low-cost adsorbent material for arsenic contaminated water in developing countries, though factors such as sand material, grain size and coating method should be evaluated further.

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Acknowledgements

I would like to thank both my great supervisors in Sweden, Ingmar Persson and Jon Petter Gustafsson, for introducing me to the project and thereby giving me the opportunity to do something I can see myself working with in the future for my bachelor thesis. I have broadened my knowledge a lot in the chemical field, both theoretically and experimentally, but even more so have I got to know a new country, a new continent in a way, and gotten a taste of the conditions and joy of working in projects abroad, in a development country.

I would also like to thank my burkinabé supervisor Samuel Pare for whole-heartedly supporting my work, both through great theoretical guidance and through help in finding materials and methods that made this study possible. Special thanks also to Yacouba Sanou for a lot of practical help, for letting me use your office as working space and for welcoming me to Burkina Faso and Ouagadougou during my first week. Additional thanks to Labaratoire National d’Analyse des Eaux for carrying out the water analysis in this study and thanks to SIDA for the financial support that made my work in Burkina Faso possible.

My visit in this amazing country would not at all have been the same without my wonderful friends Toussiane Sankara and Rasmir Talato Lallogo with families, who made my time in Burkina Faso unforgettable. The many family visits, dinners and adventures in Ouagadougou, both by day and night, gave me an opportunity to experience the city in a way I could never have done without you. The visit to the town Yako in my last weekend also left me with numerous beautiful memories of the country and its fantastic people. And the opportunity to explore the countryside was as joyful to me as it was valuable to the project. The visit gave me a greater understanding of life and conditions in the villages where this project is aimed to be implemented and additionally, provided me with samples of another type of laterite that potentially could be researched as adsorbent in the future.

Also special thanks to Mats and Göran at the Swedish embassy for welcoming me with warmth to Burkina Faso and for letting me take part in the weekly floor ball games, which quickly became an appreciated break from the sometimes stressful workload. The opportunity to be part of the floor ball project in the schools and villages in and around Ouagadougou felt both

fun and important to me, and I wish you all the best with the project in the future.

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1 Aim

This study aims to contribute to the development of a low-cost yet efficient method for arsenic removal in drinking water in the rural villages of Burkina Faso. By filtration-adsorption process through laterite materials and/or ferrihydrite-coated laterite, the project aims to find a purification method suited for developing countries. The set up should be made with the final result in mind, meaning it should be easy to upscale for use in villages with up to 200 people, reasonably priced and with a need of minimum technical knowledge for maintenance. The research on ferrihydrite-coated sand is partially done as a complimentary study to Rapid adsorption of arsenic from aqueous solution by ferrihydrite-coated sand and granular ferric hydroxide by Mähler and Persson (2013).

To concretise the aim of this study, the work was based on the following questions:

Is laterite from Burkina Faso efficient as an adsorbent for arsenic in contaminated drinking water?

Can the crushed laterite be coated in ferrihydrite through drying ferrihydrite suspension together with the sand particles at high temperature? Or can the ferrihydrite be attached to the laterite by centrifuging the materials together?

Is the adsorption capacity of the laterite material increased by adding ferrihydrite?

Studied only to a certain extent: What parameters affect the adsorption to the laterite and the ferrihydrite-coated laterite?

If the project is successful: What would be needed to upscale the filtration method to enable treatment of tube-well water in villages with several hundred residents?

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2 Introduction

2.1 Burkina Faso

Burkina Faso is a relatively small landlocked country in West Africa. The country is located partly in the dry Sahel region which reaches across the continent south of Sahara, and the climate is hot and dry. Water is a scarce resource and can be a problem in the lengthy dry season, especially in the northern parts of the country where conditions turn almost desert- like during the hottest months (Landguiden, 2016).

Burkina Faso is also one of the poorest nations in the world. According to the UN organ UNDP’s index for human development (HDI) Burkina Faso was ranked as 183 out of 188 countries (UNDP, 2015). Poverty is widely spread and over half of the population (55,3 %) were living under extreme poverty conditions in 2009, although the amount has decreased rapidly from 1998 where the number was over 80 % (UN Stats, 2016).

Figure 1. Satellite map of Burkina Faso (Google Maps, Collected: February 2017).

Being a former French colony, the official language in Burkina Faso is French, though the population consists of over 60 different ethnic groups, many with their own language. In 1960, Burkina Faso became an independent state and since then the political situation has been unstable and, in times, resulting in coup d’états and dictatorships. The last leader, Blaise Compaoré, had led the country for 27 years, officially winning the elections by far but accused for electoral fraud, when the peoples’ voices finally got through and he gave up the role in 2014. After this followed a year of political instability until the current government, led by Roch Marc Christian Kaborés could seize power in a democratic election in 2015 (Landguiden, 2016).

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While the country is struggling in many ways, the burkinabés are well known for being open and friendly people and the country is considered one of the safest in the region. Culture is a central part of Burkina Faso, and the country is famous for its density of music, dance and colourful artwork and clothing. Africa’s largest film festival is also hosted yearly in the capital Ouagadougou (Landguiden, 2016).

2.2 Problem background

In Burkina Faso, especially in its northern part, the traditional source of drinking water consists of dug wells, often less than 25 meters deep. The groundwater yields from these wells are typically small and the wells are vulnerable to microbes and to drying out in the dry season.

Consequently, new borehole-drilling programs have been undertaken by administrative authorities. Many of the villages now have tube-wells (boreholes) down to 50-120 metres and most of them intercept groundwater from fractures in the basement. However, as arsenic is naturally present in the bedrock, water from many of the new wells contain high arsenic concentrations and some wells has had to close due to the high concentrations (Smedley et al., 2007).

Figure 2. Estimated number of persons at risk of being exposed to groundwater arsenic concentrations 10 μg/L per km² in Burkina Faso (Bretzler et al., 2017).

In 2012, a study in the Yatenga Province showed that more than one half of the tube-wells (52 %) had an arsenic concentration exceeding the World Health Organization (WHO) guideline limit on drinking water of 10 μg/L. A little bit more than 6 % of the wells contained more than 100 μg/L (Somé et al., 2012). Previously, concentrations of as high as 1630 μg/L has been observed in the area (Smedley et al., 2007). Arsenic concentration in the urine samples of the residents correlated with the arsenic concentration in the water, yet only 3 % of the population showed an arsenic concentration above the WHO guideline of 40 μg/L. The

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adverse health effects of arsenic in drinking water depend strongly on the dose and the duration of exposure. Many diseases are linked with long-term use of arsenic polluted water, e.g. skin and internal (lung, bladder, kidney) cancers. Dermatological effects, such as melanosis (hyperpigmentation) and keratosis (rough, dry, papular skin lesions), are typical signs of chronic exposure to arsenic (Somé et al., 2012). In the studied villages in Yatenga Province, melanosis and keratosis were respectively found among one third (29 %) and one half (46 %) of the population. Among those suffering from the skin lesions nine out of ten (90

%) were over 18 years old, which indicates the effects are due to long term use of the contaminated water (Somé et al., 2012).

While the Yatenga Province for long has been known to have issues with arsenic polluted water, a recent study by Bretzler et al. (2017) shows that the problem is widely spread throughout Burkina Faso (Fig. 2). The situation is also thought to exist in neighbouring countries such as Mali and Niger due to similar geological conditions (Bretzler et al, 2017).

While arsenic polluted water might be bad to drink, the access to any kind of drinkable water is a vast issue in Burkina Faso, leaving people with little choice. According to the UN, one quarter of the population (24 %) in the rural areas are still lacking access to improved water sources. The number shows people lacking access to water sources such as pipes, pump wells, protected springs or collected rainwater providing at least 20 litres of water per person and day and located within one kilometre from the habitation (UN Stats, 2016).

2.3 Geology of the region

Burkina Faso is largely covered by a few metres deep laterite crust, underlain by a few tens of metres of clayey saprolite¹. Below lays the so-called fissured layer (densely fissured and fractured weathered bedrock), which can extend to a depth of 80-100 m below ground level (Bretzler et al., 2017). Ground water levels are typically 10 – 20 m beneath ground surface.

Yet, water yields from the upper part of the aquifer are usually small, and the fissured layer represents the main aquifer for groundwater storage (Smedley et al., 2007).

The basement rock in Burkina Faso consists mainly of the Birimian Formation with belts of metasedimentary, (meta-)volcanic and plutonic rocks, as well as large intrusive bodies of Eburnean granitoids (granite, tonalite, granodiorite). The Birimian Formation has undergone considerable mineralisation together with regional metamorphism and deformation, which has led to the formation of high-grade ore deposits, mainly gold. Arsenic is primarily found in these gold-rich formations, as sulphide minerals such as pyrite (FeS2) and arsenopyrite (FeAsS) also occur in high frequencies. The oxidation of such sulphide minerals when in contact with oxygen-containing groundwater and subsequent release of arsenic to solution can lead to considerably elevated groundwater arsenic concentrations (Bretzler et al., 2017).

1 saprolite = chemically weathered rock

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Figure 3. Simplified geological map of Burkina Faso, showing the main rock types, major faults and the location of mineral deposits (metal ores) (Bretzler et al., 2017).

2.4 Removal techniques for arsenic-enriched water

As arsenic polluted drinking water is a critical, worldwide problem, many different removal techniques has been studied and developed during the last century. Lots of the methods in practice today uses several steps of treatment, combining different removal techniques.

Among the most commonly used are; oxidation, conventional coagulation, ion-exchange, adsorption, i.e. to activated carbons, activated alumina and granular ferric hydroxide (GFH), and different membranes, including reverse osmosis (Crittenden et al., 2012; Ungureanu et al., 2015).

However, many of the established methods today are not available for small scale use, as the set up requires advanced techniques and equipment. Furthermore, even less treatment strategies would be applicable in a developing country like Burkina Faso due to the high expense of materials and chemicals. Consequentially, arsenic removal with low-cost, low-tech methods are researched in numerous countries, and several techniques has indicated promising results. Adsorption, advantageous in terms of simplicity of operation, cost- effectiveness, regeneration capability and minimal sludge production, still seems to be one of the strongest candidates to a method for developing countries. Many different materials, both natural and chemically or physically enhanced, has been tested as adsorbents (Ungureanu et al., 2015). Iron-rich laterite and lateritic soils has showed good results, but with factors such as iron content and particle size having a large impact on the adsorption capacity (Maiti et al., 2013; Maji et al., 2008). Other interesting techniques are filtration through and adsorption to different types of biomass, i.e. rice husk and different types of algae (Ungureanu et al., 2015).

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However, for many of the treatment strategies, and perhaps even more so for the simpler, inexpensive alternatives, the water content and characteristics such as pH and presence of competitive ions are often considered to strongly affect the arsenic removal capacity (Ungureanu et al., 2015). This, of course, makes the research harder as one technique can work well on one type of natural water, while not working at all on another.

2.5 Earlier studies

In 2013, two previous Minor Field Studies (MFS) were carried out in this project. In both cases the adsorbents were studied by column experiments in the laboratory and using natural water collected from the affected tube-wells in northern Burkina Faso, as to as closely as possible mimic the real conditions. Lundin and Öckerman (2013) presented a complimentary study to Competition for Adsorption Sites on Iron Oxyhydroxide Based Column Adsorbents for the Removal of Arsenic Oxyacid Species (Paper IV in Mähler, 2013), evaluating the properties of the commercial material Granular Ferric Hydroxide (GFH) as adsorbent for arsenic in natural water. Frid and Haglind (2013) continued the study on GFH at a larger scale and also researched the possibility of using natural lateritic soil from Burkina Faso for the same purpose. While the experiments with GFH in both studies showed good results (~106-463 µg/cm³), it was concluded that the laterite tested had a low arsenic removal capacity (~7 µg/cm³), approximately 66 times lower than the GFH (Frid and Haglind, 2013).

As lateritic soil from Burkina Faso showed a low adsorption capacity, a recent study in this project has examined the properties of GFH and laterite gathered from Lâm Dông Province, Vietnam, respectively (Sanou et al., 2016). This study indicated the laterite to have a good arsenic removal capacity, although the work was done with very high arsenic concentrations (mg/L) and using enriched deionized water rather than natural water, why the results might not be comparable with earlier results in Burkina Faso.

Recently, another Minor Field Study in this project were carried out by Ambjörnsson et al.

(2016) in Ouagadougou, in collaboration with another team in Uppsala. This time, the iron- oxyhydroxide material ferrihydrite was studied for arsenic removal. Both column and batch experiments were made, and while the adsorption capacity was very high in the batch experiment in Uppsala, it was concluded by both groups that the set up for the column needed to be improved.

In 2013, Mähler and Persson carried out a study on ferrihydrite-coated quartz sand as a potential treatment method for arsenic-enriched water in developing countries. The study was done with deionized water, evaluating the effect of parameters such as empty bed contact time (EBCT), flow rate, initial pH and arsenic oxidation state. Additionally, Mähler (2013) studied the impact of competing ions. The two studies revealed ferrihydrite-coated sand to have a good adsorption capacity (~79-349 µg/cm³), and to be a possible candidate for a low-cost full scale system in the rural areas of developing countries such as Burkina Faso.

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3 Theory

3.1 Arsenic

The toxicity of arsenic on humans depends on its chemical forms: both inorganic and organic trivalent arsenicals are more potent toxicants than pentavalent forms (Hughes, 2002).

Inorganic arsenic exists in natural water in the two predominant oxidation states As(III) (arsenite) and As(V) (arsenate), where arsenite is more mobile and 60 times more toxic than arsenate. In addition, arsenite is more difficult to remove from water. Arsenate has high adsorption ability, while arsenite is less likely to adsorb to surfaces and in many cases has to be oxidized to arsenate before final treatment. Oxidation of arsenite to arsenate in water with pH under 10 does not happen readily without the presence of additional oxidizers (Maji et al., 2007).

The occurrence and distribution of arsenite and arsenate is largely influenced by pH and the redox conditions of the environment. Typically, arsenite is more likely found in anaerobic groundwater while arsenate is more common in aerobic surface water (Saha et al., 2005). In the Yatenga Province however, the groundwater from the wells is mainly oxic and of near- neutral pH and arsenic is mainly present as arsenate (Smedley et al., 2007). In slightly oxidizing environments and near-neutral pH, arsenite mainly occur as H3AsO3 and arsenate as H2AsO4– and HAsO42–, with the equilibrium between the two species pKa = pH 7,0 (Fig. 4; Fig. 5) (Crittenden et al., 2012).

Figure 4. Predominance diagram for As(III) and As(V) as function of pH (Crittenden et al., 2012).

There are several techniques for arsenic removal from water, including conventional coagulation, sorption with ferric and aluminium oxides, ion exchange and membranes. In this study the treatment strategy active in the method is adsorption, both to the GFH-material and to the laterite.

Adsorption occur as either physical adsorption or chemisorption, where physical adsorption is most common for water treatment. Physical adsorption is caused by electrostatic interactions creating weak binding mechanisms such as Van der Waals forces, making the process a reversible one. Chemisorption is caused by chemical reactions and the exchange electrons between adsorbent and adsorbate, creating chemical bonds. It is usually not

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reversible, and if desorption occurs, it entails a chemical change in the adsorbate (Crittenden et al., 2012).

Figure 5. The EH –pH diagram for arsenic at 25 ^◦C and 1 atm with total arsenic 10⁻⁵ mol/L and total sulfur 10⁻³ mol/L. Solid species are enclosed in parentheses in cross-hatched area, which indicates solubility less than 10^−5,3 mol/L (Crittenden et al., 2012).

3.2 Laterite

Laterite is the name of deeply weathered red, brown or yellow rock and soil, rich in iron and aluminium and with a low silicon content (Nationalencyklopedin, 2017). However, several attempts have been done to redefine or entirely abandon the term, as it has been used for all types of reddish Al/Fe-rich soils. Though, the term laterite is so widely used nowadays that an abandonment seems unrealistic. The persisting problem is therefore based on the fact that when studied in detail, in spite of apparent similarities, each lateritic profile is unique because of its development depending on several factors, such as climate, morphology, bedrock composition and hydrogeology (Giorgis et al., 2013). Furthermore, or perhaps consequentially, the lateritic formation process has been discussed by many up to this day.

The general conclusion seems to be that laterite is formed over long periods of time in tropical and subtropical climates with a heavy rainfall due to chemical weathering and leaching of the parent rock, resulting in an enrichment of more insoluble ions, predominantly iron and aluminium (Giorgis et al., 2013). The location of the parent rock is among others one of the topics still discussed today. The classic laterite concept, largely based on Schellmann’s work (Schellmann, 1994), states that the source of the laterite is the underlying bedrock and thus that lateritic formation is a result of chemical leaking and enrichment in a vertical direction.

Bourman and Ollier (2002) criticised this statement, proposing instead that most laterites are formed from chemical and physical lateral movement, thus leaking and enrichment from aside.

Research on laterite earlier carried out in this project were made on lateritic soil collected some ten kilometres outside of Ouagadougou, near the village Pabre. The soil was dug out of

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the top layer of the ground and sieved to a particle size between 0.125-2 mm and washed with tap water before being put into the column. However, the results from the small scale column experiment showed the lateritic soil to have a low arsenic removal capacity and therefore not being an efficient adsorbent (Frid and Haglind, 2013). Since then, no further research on laterite from Burkina Faso has been done in this project.

Instead, Sanou et al. (2016) recently carried out a study laterite in Vietnam which indicated that laterite, in the type of hard rock from northern Vietnam has a higher arsenic removal capacity. The laterite was reddish brown in colour and collected from Lâm Dông Province at 300 km from Ho Chi Minh City. Prior to the column experiment, the adsorbent was prepared by; crushing into average pieces, washing with tap and distilled water, drying in a stove at 105

°C for 24 hours, cooling in desiccator and crushing in smaller particles for usage. The properties of the laterite after preparation are shown in table 1.

Table 1. Properties of the laterite from Vietnam used by Sanou et al. (2016).

Properties Quantitative value Elemental composition

(%, wt/wt)

pHZPC 7.6 – 7.8 Si 34.27

BET Surface (m²/g) 240 – 300 Al 19.86

Micropore volume (cm³/g) 0.01 Fe 7.79

Pore radius (nm) 1.16 Mg 5.06

Grain size (µm) 450 – 2000 Ca 4.59

Bulk density (g/mL) 1.91 Ti 1.68

Porosity (%) – Na 1.31

Moisture content (%) 0.43 C 4.4

Residual porosity (%) 97 – 99 O 19.87

In 2013, a study on laterite and the formation of the different rocks and soil layers in Burkina Faso was carried out by Giorgis et al. (2013). Samples were gathered from the area of Balkouin, a village located approximately 10 kilometres outside central Ouagadougou, and properties of geochemistry, minerology and genesis were determined. In this study, it is clear the top most layer, in the form of rock, contains a significantly higher concentration of Fe2O3 (54.90 %) than all the other samples. The amount of Al2O3 is also high (16.25 %), although not the maximum, and the quantity of SiO2 (19.05 %) is considerably lower than all other samples.

The composition of this rock has lead us to think it will be good for arsenic removal, why the laterite rocks of presumably the same type and from the same location, has been used in the following experiments (Fig. 6).

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Figure 6. Photo of the top most layer of the laterite profile in Balkouin, kindly borrowed by Giorgis et al. (2013).

3.3 Ferrihydrite

Ferrihydrite is a poorly crystallized iron oxy-hydroxide with the approximate formula of Fe5HO8 × 4H2O, and thus a less well defined structure than many other iron hydroxides such as goethite, akaganeite and lepidocrocite (Mähler and Persson, 2013). In nature ferrihydrite occurs both naturally and through antropogen sources such as mining, as one of the most common iron oxy-hydroxides in soils and sediments. Because it typically forms nanoparticles (2-6 nm), ferrihydrite has a very high surface area and thus a high surface reactivity.

Theoretical numbers of a surface area up to ~1250 m²/g has been reported (Bompoti et al., 2016). The same report also later concludes the point of zero charge of ferrihydrite to lie between 8.0-8.7, depending on the size of the ferrihydrite crystals which in turn differ with the preparation method. Fresh ferrihydrite suspension has smaller crystals and consequentially a lower pHZPC, while aged suspensions have a higher pHZPC. In Mähler and Persson’s report (2013) the pHZPC of ferrihydrite is said to be pH ~8.1.

The large surface area and the relatively high pHZPC of ferrihydrite makes it a suitable adsorbent for arsenic. However, the small particle size poses practical difficulties when used as filter materials due to the risk of seepage to the effluent water. In 2016 a study in this project was carried out on ferrihydrite alone as an adsorbent. Students from Uppsala University studied ferrihydrite solution alone as adsorbent of arsenic through batch and column experiments in both Uppsala and Ouagadougou respectively. In Uppsala it was concluded in the batch experiment that 10 g of ferrihydrite could adsorb 0.7 g arsenic with a contact time of 0.5 hour. The column experiment in Uppsala indicated that by using a very small flow velocity, drop by drop flow, glass wool would work as a filter and the effluent water would be clear, free from ferrihydrite. However, after calculations it was found that the amount of ferrihydrite needed to adsorb the amount of minimum 0,02 g/L arsenic (which is the lowest amount that can be measured by the atom absorb spectrometer) is too big to be used in the set up in the previous experiment. Therefore, no further column experiments were

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done. In Ouagadougou the same experiments were carried out, and the column were also tested for arsenic removal. The results were however not satisfactory, both the batch and column experiment showed the ferrihydrite solution had little or no adsorption capacity for arsenic. The unanimous conclusion from both groups was that ferrihydrite works well as an adsorbent but that the set up needs to be developed.

Mähler and Persson (2013) assumed that ferrihydrite, with its nanoparticles, would have to be attached to surfaces of larger mechanically stable particles or incorporated in granular materials, in order to practically work as a filtration method. They carried out both batch and column experiments on ferrihydrite-coated quartz sand for arsenic removal from water, and found that the material has an adsorption capacity between 79-349 µg/cm³(depending on arsenic specie, flow velocity etc).

Our hypothesis, for this experiment, is that laterite with an adsorption ability in itself could be coated in ferrihydrite and reach a high arsenic removal capacity, to what would still be a low cost and considered a low-tech method.

3.4 Factors affecting adsorption

The chemical water properties are expected to affect the adsorption capacity of adsorbed arsenic onto the adsorbent, as showed in the preceding study of natural laterite and the commercially available material GFH (previously studied in this project), carried out by Sanou et al. (2016). However, that study was made with distilled water and it is reasonable to assume the amount of adsorbed arsenate would be lower in natural water due to for example the competition with other anions. Results in the following sections, if not stated otherwise, is given by Sanou et al. (2016).

3.4.1 Flow rate

Adsorption to both GFH and the laterite diminished with an increased flow rate, with the effect most evident on the laterite. The occurrence can be explained by insufficient residence time of the solute to the column, and the reason to why GFH works better as adsorbent even in higher flows is explained as a result of a larger surface area and lower porosity of the GFH.

3.4.2 Empty bed contact time (EBCT)

A slight upsurge in the arsenate removal was observed between contact time of 5 – 45 min for the GFH and 5 – 30 min for the laterite. Thereafter, no or nearly no change in adsorption was found. Thus, according to this data, material surface saturation was reached after 45 min of contact time for the GFH and 30 min for the laterite. The difference is thought to be due to a higher porosity of laterite. Still, it is evident that adsorption occur rapidly, but as Mähler (2013) states, a somewhat longer EBCT is desirable for a more efficient material utilization, as both diffusion into porosities and desorption of competing species are kinetic hindrances.

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Figure 7. Effect of flow rate (left) and EBCT (right) on arsenic removal using 10 g of adsorbent and C0 = 20 mg/L in a previous study in the project (Sanou et al., 2016).

3.4.3 pH

As described earlier, the surface charge of the adsorbent as well as the specie in which the adsorbate occur depends on the pH in the solution. GFH has a point zero charge at pHZCP 7.6 – 7.8 but adsorption was not affected much by variations in pH in previous column experiments in the project (Sanou et al., 2016). In research studies on GFH done by Saha et al.

(2005) adsorption of arsenate decreased when pH increased. Still, as arsenate switches form from H2AsO4– to HAsO42– inner-sphere complexes form with the new adsorption sites available, and the effect of the increased pH is dampened. For the laterite from Vietnam used by Sanou et al. (2016), pHZCP is 7.97 which also was reflected on the results as the amount of arsenate adsorbed to the laterite decreased with increased pH.

3.4.4 Initial arsenic concentration

Arsenate removal by the laterite was highly affected by the initial concentration of adsorbate.

For GFH the effect was considerably smaller. The phenomenon is thought to be a result of the occupation of free adsorption sites, inaccessible at low concentrations of adsorbate.

Figure 8. Effect of initial pH (left) and initial concentration (right) on arsenic removal using 10 g of adsorbent for 15 min with C0 = 20 mg/L in a previous study in the project (Sanou et al., 2016).

3.4.2 Competition from other anions

The change in adsorption over different pH values show that hydroxide ions (OH^-), directly or indirectly, can compete with anionic arsenic species for available adsorption sites (Mähler, 2013). Phosphate is stated to compete strongly with arsenic as adsorbate, likewise observed by Saha et al. (2005) earlier. The correspondingly high competition between phosphoric and

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arsenic anions is thought to depend on the similar chemistry of the two substances, i.e. the pKa-values of arsenic acid and phosphorous acid.

Table 2. The species in which arsenic and phosphor is present depend on the pH of the solution.

The following pKa-values are given by Saha et al. (2005).

pKa1 pKa2 pKa3

Arsenic acid H3AsO4 2.2 7.0 11.6 Phosphorous acid H3PO4 2.1 7.2 12.2

Additionally mentioned as possible adsorption competitives are hydrogen carbonate (HCO3-) and fluoride, also studied by Mähler (2013). Although hydrogen carbonate does not at all have an adsorption capacity close to that of arsenite or arsenate, when compared to the high concentrations sometimes found in natural water, it is evident the substance can be a fierce competitor for adsorption sites. For fluoride, the ions are small anions isoelectric with the hydroxide anion, and thus expected to form strong inner-sphere surface complexes affecting the adsorption of arsenate (Mähler, 2013).

Figure 9. Arsenic(III) and arsenic(V) breakthrough curves for different pH and competitors, as stated in the graphs. Where not stated otherwise, pH is close to 6.5. The graphs are borrowed from Mähler (2013) and present column experiment on ferrihydrite-coated quartz sand.

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4 Material and methods

4.2 Column set-up 4.2.1 Material

Chromatic column Electrical pump Tubes

Chemistry stand Electrical tape Glass wool

1.5 L and 0.5 L Plastic bottles 20 L containers (influent water) Long, thin tool to for fitting of filter

Figure 10. Column set up for the experiments.

4.2.2 Description

The column was set up in an office at the university, rather than in the laboratory, and experiments were therefore carried out in room temperature (30-35 °C). After the filter of glass wool had been put in place with the aid of a long, thin piece of iron bought in the market, the column was filled with distilled water. The adsorbent was then added one spoonful at a time and the column was drained halfway between every addition to create an even density and mimicking the method used by Lundin and Öckerman (2013) with the GFH experiments.

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Though, as the particle size of the sand varied quite a lot, the effect was rather that of sorting the layers into different grain sizes. However, it was still considered the best option as the attempt of stuffing the column without water lead to visible, large gaps and an uneven density.

The electrical pump brought water from the 20 L container of the arsenic-enriched influent water, to the top of the column where the tube end was fastened with electrical tape. The flow rate was regulated by the turning mechanism at the bottom of the column, and stroke length and rate of the electrical pump was adjusted as to balance the inflow with the flow out of the column.

Samples of the flow was collected at even time intervals into a 250 mL container, while the time to fill the sample was measured and the flow rate then calculated. The regulation of the flow with the mechanism at the bottom of the column was quite unprecise, and after every other break the flow was kept uneven for some minutes during regulation and even then, the resulting flow often differed a bit to what it was before. Though, when a good flow rate had been found, the flow was kept even.

When a pH meter was available (after the first experiments), pH and temperature of the influent water was measured at the start of every new run of the experiment and for all samples of effluent water right after collection.

4.1 Preparation of adsorbent 4.1.1 Laterite

The laterite was collected at the village of Balkouin, approximately 10 kilometres from central Ouagadougou. It was then crushed with a hammer into small particles and sieved by hand both dry and under running tap water as for removing the finest particles. The crushed laterite was then thoroughly washed in tap water until the red colour had gone and finally washed in distilled water before left to air dry in room temperature (30-35 °C) for three days.

Three different samples were prepared from different sites and ground levels in the sampling area of Balkouin. The samples were collected within a radius of 100 metres from each other.

Sample 1: This was collected from a pile of rocks on the ground. The gentlemen who showed us around in the area had collected the rocks from down one of the wholes in the ground where he dug out bricks for construction of houses. Supposedly, this is therefore not from the very top of the ground profile, although it might have been there once and then gotten covered by sand. During and after preparation it was observed this sample contains more quartz and glimmer (white pieces) than the other samples. The red laterite gluing the smaller stones together was easily crushed and a lot of particles finer than 125 µm was left in the sieve.

Sample 2: This was collected on top of the ground profile, removed from the hard rock using a hack. During and after preparation this was thought to be the best sample. Nearly all of the

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crushed rock was dark red in colour and it was harder to smash into really small pieces, resulting in more particles of a larger size and less fine material left in the sieve.

Sample 3: This was collected in a similar place as sample 2, some 50 metres away. During and after preparation, this was considered to be somewhere in between sample 1 and 2 in characteristics. The crushed sample contained some white particles, although most of it was of the dark red colour. It was also more easily smashed than sample 2, creating a lot of finer powder left in the sieve.

4.1.2 Ferrihydrite-coated laterite

The ferrihydrite used in this experiment was prepared using 40 g of Fe(NO3)3×9H2O dissolved in 500 ml distilled water. Approximatly 300-330 mL 1 M NaOH was then added to the solution at a slow rate and while using a magnetic stirrer, until pH of 7.4 ± 0.1 was reached.

Preparation by drying in the oven

In the first method the laterite sand was coated in ferrihydrite by drying the materials together in the oven for one day. Inspiration to this method came from the work by Jonsson et al. (n.d.) where olivine sand was coated in iron-oxide by drying the materials together in the oven.

However, as the oven was only available for one day, the coating was done in one step instead of repeated three times as in the method used by Jonsson et al.

After ferrihydrite synthesis, the solution was left to sediment during the night and excess water was removed the next morning. Thereafter, the solution was filtrated using an electric air pump and the resulting paste was added to 120 ml (93 g) laterite from sample 2 (grain size:

400 – 700 µm) and left to dry in the oven at 110 °C for 24 hours. The coated sand was then washed thoroughly with tap water before being put in the column.

It was hard to tell how much ferrihydrite actually attached to the laterite, due to the similar reddish brown colour of the materials. The dried ferrihydrite that did not attach to the sand could be seen as a black flaky material. Although some of this was washed away while cleaning, a bit also remained in the column, creating thin black layers as the fine particles sedimented slowly while packing the column.

Preparation by centrifugation

In the second method the laterite was coated in ferrihydrite by centrifugation, according to the method used by Mähler and Persson (2013). However, as a shaker was not available, the samples were shaken by hand to a reasonable extent instead.

Two runs of preparation were done, the first one with laterite from sample 3 (grain size: 400 – 1000 µm) and the second one with laterite from sample 2 (grain size: 400 – 700 µm). For both attempts the same method was used, as closely as possible following the protocol used by Mähler and Persson (2013). Ferrihydrite suspension was placed in 50 mL tubes and

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centrifuged for 5 min at 1700g. Excess water was replaced with more suspension and centrifuged again. Thereafter followed four cycles of centrifugation and replacement of supernatant with distilled water in order to remove dissolved salts. In the last replacement water was added up to 35 mL and the suspension was then shaken by hand for 10 min (original method; 2 h in a shaker). Afterwards, 15 mL laterite was added to each tube and the samples were shaken by hand every now and then for 24 h (original method; 48 h in a shaker). The mixture was then centrifuged for 60 min at 2300g, after which the brown supernatant was removed and material was left to air dry for 2 days. The sand was finally rinsed thoroughly with tap water before being put in the column.

4.3 Collection and preparation of water

Water was collected in two wells near central Ouagadougou on the 15^th of April and the 6^th of May. The first well was located in the village of Ziniaré, some 25 km from Ouagadougou, and was handled by hand. The flow from this well was low and the well was therefore thought to be quite shallow. The second well was located in the suburbs of north-east Ouagadougou, and was driven by solar power and thought to be deeper. For the first occasion, two 20 L containers were used for sampling at each well, while for the second time 40 L were collected only from the first well. After collection, the water was sent to an external laboratory for analysis of the water content.

The wells used for sampling do not contain arsenic naturally, though due to shortage of time and because of the safety issues in the northern part of country where naturally arsenic contaminated water previously was collected, this was considered a good option. The water was contaminated with arsenic after collection. According to calculations using Equation 1 (Appendix 8.1), between 2.5 mL and 3 mL of standard arsenic solution (H3AsO4) with a concentration of 1000 mg/L was added to a 20 L container to create a concentration between 125-150 µg As/L. The sample was then shaken well to make an even solution, and the initial concentration of the water was measured.

4.4 Running of the experiments

Several column experiments have been carried out during this study. First, the removal capacity of the laterite itself was tested in numerous experiments with different grain sizes, flow rates, adsorbent volume and natural water from both wells. Thereafter, the same laterite was attempted to be coated by ferrihydrite by drying ferrihydrite paste together with the laterite at high temperature in the oven for one day. Finally, the laterite sand was coated in ferrihydrite by centrifuging the laterite together with the ferrihydrite suspension, as closely as possible following the method used by Mähler and Persson (2013).

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Table 3. Summary of the experimental set ups.

Experiment 1 2 3

Material Laterite (s. 2) Laterite (s. 2) Laterite (s. 2) Grain size (µm) 125 – 2000 ~ 700 – 2000 ~ 400 – 700

Column height (mm) 75 75 75

Column diameter (mm) 15 15 15

Height of glass wool (mm) 10 10 10

Flow rate (L/h) 0.68 – 0.125 1.54 0.77

Water source Well 1 (15 April) Well 1 (15 April) Well 1 (15 April)

Material Laterite (s. 2) Laterite (s. 2) +

Ferrihydrite Laterite (s. 2) + Ferrihydrite

Coating method – Oven Oven

Grain size (µm) ~ 400 – 1000 ~ 400 – 1000 ~ 400 – 700

Flow rate (L/h) 0.54 0.74 0.65

Water source Well 2 (15 April) Well 2 (15 April) Well 1 (15 April)

Material Laterite (s. 3) +

Ferrihydrite Laterite (s. 3) +

Ferrihydrite Laterite (s. 2) + Ferrihydrite Mixing rate (ferrihydrite,

laterite) Centrifuging Centrifuging Centrifuging

Grain size (µm) ~ 400 – 1000 ~ 400 – 1000 ~ 400 – 700

Flow rate (L/h) 0.83 0.91 0.70

Water source Well 1 (6 May) Well 1 (6 May) Well 1 (6 May)

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4.4.1 Experiment 1: Laterite

The set up for this experiment was done to mimic the initial experiment done by Lundin and Öckerman (2013) on GFH. However, it was quickly understood that the small grain size set the limit for the flow rate. With the hose of the column completely open the flow rate was limited to a constant flow the first day. The second day clogging could be seen, resulting in an abandonment of the experiment after 3,5 litres had passed the column.

This experiment was set up with larger grain size, though due to lack of a sieve in the desired size, the difference was considerable. The flow rate was regulated to more than the double of that in experiment 1. Considering the breakthrough had been reached after only one litre of water the experiment was abandoned after this.

For this experiment, a smaller sieve had been found and the grain size was limited between two sieves bought in the market, resulting in a small and even but approximate size. The flow was unimpeded, but was kept low due to the low capacity observed in previous experiments.

The same laterite and grain size was used as in experiment one but with several changes in the set up due to lack of satisfactory results. Water from well 2 was used instead, the height of the column was doubled and the flow rate was regulated to be even lower.

4.4.5 Experiment 5: Ferrihydrite-coated laterite (oven)

At this experiment, laterite with the same grain size as in experiment 4 was coated in ferrihydrite through drying at high temperature in the oven. The flow rate was unhindered but still kept quite low.

4.4.6 Experiment 6: Ferrihydrite-coated laterite (oven)

This experiment was made to mimic experiment 5, but with water from well 1 instead, because of the significantly higher phosphate concentration in the second well. Due to electricity blackouts and contemporary preparation of material for the following experiments, this experiment was run for several days and with only a few samples taken every day. The water source was also changed after 6.4 litres had passed the column, to the second container from the same well, although breakthrough had happened before this.

4.4.7 Experiment 7: Ferrihydrite-coated laterite (centrifugation)

Here, the laterite (from sample 3) was coated by centrifugation according to the method used by Mähler and Persson (2013). For this first experiment the material had not yet dried completely, though due to shortage of time the experiment was started anyway.

Prior to this experiment, the same ferrihydrite-coated laterite as in experiment 7 was used, although it had now dried for 2 days in room temperature (30-35 °C).

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For this experiment, the second run of preparation of ferrihydrite-coated laterite was used.

This contained laterite from sample 2 instead (the same as in most of the other experiments), but was otherwise a repetition of experiment 8.

4.5 Arsenic analysis

The analysis’ of the water content were made at a laboratory outside of the university, Labaratoire National d’Analyse des Eaux. For analysis of arsenic concentrations, a Wagtech Arsenator was used, measuring the total amount of arsenic within a range of 0-100 μg/L. The equipment was evaluated by Safarzadeh-Amiri et al. (2012) and found to be good and reliable, providing results comparable to those provided by analytical laboratories. The detection limit was found to be 4 μg/L and the accuracy for 50 μg/L was about ± 10 μg/L.

In a typical analysis, sulfamic acid is added to the sample (50 mL) to make the arsenic disperse as free ions. Thereafter, a sodium borohydride tablet is added and, if arsenic is present in the water, arsine gas (H3As) of equivalent concentration would evolve. The gas is passed through a filter and the arsenic concentration is determined by analysing the colour of the filter after 20 min with a spectrophotometer. If the concentration is higher than 100 μg/L, the sample is diluted appropriately and the above procedures were repeated again (Safarzadeh-Amiri et al., 2011; Georges et al., 2012).

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5 Results

5.1 Influent water

Water was collected at two occasions from two different wells. Consequentially, analysis of the water content was carried out on in total three samples, two from well 1 (for each date of sampling) and one from well 2. Further analysis of only the arsenic concentration was done on one additional container from well 1. The samples were sent for analysis after arsenic standard solution had been added and the container had been shaken thoroughly.

Table 4. Results of analysis of the influent water used in this study, displayed together with the average values of earlier results of water analysis from the Yatenga province. Numbers are borrowed from Lundin and Öckerman (2013) (left) and Frid and Haglind (2013) (right).

The Initial Molar Ratios (IMR) of arsenic and important competitive ions are given as well.

Well 1 Well 1 Well 2 Wells in the Yatenga province

Date for sampling 15 April

(2017) 6 May

(2017) 15 April

(2017) June-August

(2013) October-

December (2013)

Arsenic [µg/L] 98/45 100 94 156 162

Phosphate [µg/L] 80 790 1140 585 350

IMR (PO43- : As) 0.8/1.8 7.9 11.4 2.95 2.16

Bicarbonate

[mg/L] 19.88 0 13.05 220 66

IMR (HCO3- : As) 203/442 0 139 1502 407

Fluoride [mg/L] - - - 465 -

Iron [mg/L] 0.30 - 0.05 - 0.01

Conductivity

[^µS/cm³] 498 - 130 - 345

Colour Yes Yes No - -

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5.2 Experiments

Table 5. Summary of set up and results from experiments.

Experiment # 1 2 3 4 5

Influent Min. T (°C) - - - 32.5 -

Max. T (°C) - - - 34.2 -

Av. T (°C) - - 33.5 33.1 32.5

Min. pH - - - 6.72 -

Max. pH - - - 7.05 -

Av. pH - - 8.15 6.92 7.01

Water source Well 1 Well 1 Well 1 Well 2 Well 2

[As] (µg/L) 98 98 98 94 94

Effluent Min. T (°C) - - 32.7 32.7 31.6

Max. T (°C) - - 33.8 34.4 32.9

Av. T (°C) - - 33.2 33.4 32.3

Min. pH - - 8.09 6.86 7.31

Max. pH - - 8.96 7.37 7.43

Av. pH - - 8.44 7.19 7.37

Column set up Material L L L L L + Fh

Coating method - - - - Oven

Ads. height (cm) 7.5 7.5 7.5 15 7.5

Ads. volume (cm³) 13.25 13.25 13.25 26.5 13.25

Grain size (µm) 125 -

2000 700 -

2000 ~ 400 -

700 ~ 400 -

1000 ~ 400 - 700

Av. Flow (L/h) 0.68 - 0 1.54 0.79 0.55 0.74

EBCT (min) 1.17 0.52 1.00 1.45 1.07

Breakthrough of 10-15 ^µg/L

Time passed

(hr:min) 3:24 < 1:10 1:50 << 4:28 < 4:20 Water added (L) 2.25 < 1.79 1.41 << 2.41 < 3.0

Bed volumes 170 < 135 106 << 91 < 226

µg As/cm³ 16.6 < 13.2 10.4 << 17.1 < 21.3

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Experiment # 6 7 8 9

Influent Min. T (°C) 30.9 - 32.3 31.9

Max. T (°C) 33.8 - 33.4 33.0

Av. T (°C) 32.3 32.5 32.9 32.5

Min. pH 6.83 - 6.90 7.05

Max. pH 7.89 - 7.05 7.31

Av. pH 7.53 7.01 6.98 7.18

Water source Well 1 Well 1 Well 1 Well 1

[As] (µg/L) 110 100 100 100

Effluent Min. T (°C) 31.0 31.4 32.3 30.9

Max. T (°C) 33.6 34.3 33.1 33.5

Av. T (°C) 32.2 32.4 32.7 32.1

Min. pH 7.19 7.25 6.95 7.16

Max. pH 8.03 7.70 7.24 7.56

Av. pH 7.55 7.37 7.13 7.38

Column set up Material L + Fh L + Fh L + Fh L + Fh

Coating method Oven Centrif. Centrif. Centrif.

Ads. height (cm) 7.5 7.5 7.5 7.5

Ads. volume (cm³) 13.25 13.25 13.25 26.5

Grain size (µm) ~ 400 - 700 ~ 400 -

1000 ~ 400 -

1000 ~ 400 - 700

Av. Flow (L/h) 0.65 0.83 0.91 0.70

EBCT (min) 1.22 0.96 0.87 1.14

Breakthrough of 10-15 ^µg/L

Time passed

(hr:min) 5:52 / 8:53 < 4:18 4:17 < 6:21 Water added (L) 3.76 / 5.80 < 3.57 3.89 < 4.50

Bed volumes 284 / 438 < 269 294 < 339

µg As/cm³ 31.2 / 48.1 < 26.9 29.4 < 34.0

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6 Discussion

6.1 Validity of influent water

Analysis of the influent water indicates that the water content of different wells from the same geographical area, possibly even from the same well at different times, varies a lot. As this also might be true for different wells in the Yatenga province, it is somewhat hard to say weather the natural water used in this study is a valid substitute or not. Another explanation to the dissimilarities in the results (for well 1) could be that the accuracy of the analysis is low, in which case not too much value should be put in the results and the comparison to the waters in previous studies.

If the results are assumed to be valid though, we can directly see that none of the waters in this study are very similar to the ones collected from the Yatenga province in 2013. While the phosphate concentration is quite a bit lower in well 1 (15 April) it is instead significantly higher in well 1 (6 May) and well 2 which should affect the results negatively. The concentration of hydrogen carbonates is considerably lower in all wells in this study, and was even measured to be inexistent in well 1 (6 May), and thus would affect the results positively compared to those where the water from Yatenga was used. Analysis of fluoride concentrations were unavailable in this study, due to lack of required chemicals.

6.2 Evaluation of adsorbents

To evaluate the different adsorbents used in this study, the set up of most experiments was kept similar as to be able to compare the adsorbents to one another. Nonetheless, some changes were made and the results of breakthroughs were approximate,

6.2.1 Laterite from Burkina Faso

Experiments carried out in this study indicates that the laterite sampled in Balkouin area, close to Ouagadougou, has a low arsenic removal capacity. Breakthrough (10 µg/L) was reached after only 1-2 L with an initial concentration of ~100 µg As/L. However, a smaller grain size seems to have a great impact of the adsorption capacity as the experiment with 125-2000 µm grains adsorbed approximatly the double amount of arsenic as the sample with ~400-1000 µm. It is therefore thought that the adsorption capacity could be improved by using a small and even grain size that still is large enough to avoid clogging. Another possible solution to the problem of clogging would be to use an upward flow instead. Further research would be needed in this area, desirably with sieves at specific sizes, ranging from 150-700 µm to find an optimal particle size distribution.

Self-generation was observed at several occasions and are thought to have a significant impact on the adsorption capacity at a full scale system. Samples were generally collected up to a few minutes before the break and then approximatly 50 minutes to one hour after the break