International Summer Water Resources Research School. Competitive adsorption of As(III) and As(V) on goethite By Erik Lidén

Transcription

1 International Summer Water Resources Research School Dept. of Water Resources Engineering, Lund University Competitive adsorption of and As(V) on goethite By 2011

2 Abstract Arsenic (As) is a semi-metal element which naturally occurs in soil, water, air, plants and animals. However, for most organic life forms arsenic is toxic in elevated quantities. Due to natural causes and human exploitation, arsenic contamination is one of the most widespread environmental problems in the world, especially when it comes to pollution of groundwater. Fortunately, there are a number of ways to reduce the and remove the arsenic compounds from the water; one of them is adsorption with iron oxides. This is one of the most common methods used today when dealing with low s of arsenic. In this experiment, the main goal was to investigate if there was a competitive adsorption of arsenite and arsenate on the iron oxide goethite. This was investigated for two different ph; 4,6 and 9,2 which is the natural range of ph. Batch experiments were carried out with solutions containing dosages of goethite powder. To these batches, arsenite and arsenate were added. The of arsenate was first kept constant at 10 mg/l and the of arsenite was changed (0, 10, 20 and 40 mg/l in four different batches). Then, in a new set of batches, the of arsenite was kept constant at 10 mg/l and the of arsenate was changed (0, 10, 20 and 40 mg/l in four different batches). The solutions were then allowed to reach equilibrium before the final ph was determined and the liquid and the solid were separated through vaccum filtration. Finally, a hydride generation atomic fluorescence spectroscopy was used to determine the different s of arsenite and arsenate left in the samples. The results showed no clear evidence of a competitive adsorption between arsenate and arsenite and the ph did not seem to have any obvious impact on the adsorption capacity of neither the arsenite nor the arsenate. However, at ph 9.2 there were some indications of competitive adsorption occurring and the amount of arsenite and arsenate adsorbed correlated fairly well with what has been found in previous studies. The uncertain results are primarily thought to be the cause of the HG-AFS not measuring the s in a proper way which lead to a lack of data. All in all, no clear conclusions can be drawn from this experiment as the results are too uncertain and in the future more experiments over a longer time period will be required. Keywords: competitive adsorption, adsorption, arsenic, arsenate, arsenite, goethite, HG-AFS 2011

3 Table of Contents Abstract Introduction Background theory Adsorption Adsorption of arsenite and arsenate Hydride generation atomic fluorescence spectroscopy (HG-AFS) Experiment Materials and reagents Equipment Method Sample preparation ph adjustment Addition of arsenite, arsenate and nitrogen gas ph determination and vaccum filtration Preparation of AFS solutions Results and discussion Final ph of the samples after adsorption Adsorption capacity of arsenite and arsenate at ph Adsorption capacity of arsenite and arsenate at ph Amount of arsenite and arsenate adsorbed Conclusion Sources of error Acknowledgements References Appendix

4 1 Introduction Arsenic is a semi-metal element which naturally occurs in soil, water, air, plants and animals (EPA, 2011). In the earth s crust it ranks as the 20 th most occurring trace element. Mainly, arsenic exists in the three different forms As(-III), and As(V). The last two species, arsenite and arsenate, are the most common in natural waters and are soluble over a wide range of ph and Eh conditions. In oxidizing conditions arsenate is the predominant species whereas in reducing environments arsenite the most common and stable form, which is also generally the most toxic species. Arsenic also exists in organic forms, however these are considered to be less toxic (Carranzam et al., 2005). At present, arsenic is used in a large number of areas such as glass- drugs- and pesticides manufacturing. Even though high levels of arsenic occurs naturally at several locations around the world, this additional exploitation is one of the reasons why arsenic is now a widespread contaminant in our environment (Carranzam et al., 2005). Another human activity connected to arsenic contamination is the geothermal power generation. This has increased over the last decades due to the increasing demand of non-fossil energy sources. However, the hydrothermal fluids that are extracted from the bedrock contain arsenic both as arsenite and arsenate and these tainted fluids then consequently pose a risk to contaminate the nearby groundwater and surface water. (Kersten & Vlasova, 2009) Groundwater with high contents of arsenic has been and still is one of the most serious environmental problems all over the world. The recommended limit of arsenic (both organic and inorganic forms) in standard drinking water is 0,01 mg/l according to U.S. EPA (EPA, 2011). Several hundreds of millions of people are exposed to this threat and for example in China the population in some provinces was found to drink water with an arsenic content exceeding 0,5 mg/l, fifty times higher than the recommended limit (Li et al., 2010). The distribution of arsenic in groundwater in China can be seen in figure 1 below. Figure 1. Distribution of arsenic in groundwater in China (Li et al., 2010). 1

5 Fortunately, there are a number of ways to reduce the and remove the arsenic compounds from the water. This includes adsorption, ion exchange, membrane separation (reverse osmosis), bio-reduction and electrolysis. Adsorption with iron oxides is one of the most common methods used today when dealing with low s of arsenic due to its effectiveness and low cost compared to the other methods, and this is the mechanism studied in this experiment. The most frequently occurring iron oxide mineral in nature next to hematite is goethite, which has been proven to be effective for arsenic adsorption. Goethite is able remove both arsenite and arsenate species as well as other harmful compounds from an aqueous solution simultaneously (Delides et al., 2004). In this experiment, the competitive adsorption of arsenite and arsenate on goethite is investigated by analyzing the adsorption capacity and the relative amount of arsenite and arsenate adsorbed on the goethite. This is done for two different ph values, 4.6 and 9.2, which is the natural range of ph in the environment, to see if the adsorption capacity changes with ph. After the adsorption, the final ph will be measured as well to ensure weather a chemical or physical adsorption is occurring, and what type of reactions that are in progress. The latter however, will be left for future studies. 2 Background theory 2.1 Adsorption The attachment of particles to a surface is called adsorption, where the substance that is being adsorbed is the adsorbate and the underlying material is the adsorbent. The process where a particle is detached from a surface is called desorption. The rate at which a surface is covered by adsorbate depends on the ability of the substrate to disperse the energy of the particle and the proportion of collisions with the surface that successfully lead to adsorption. This is called the sticking probability which is expressed in equation 1 below (Atkins & de Paula, 2006). Sticking probability = rate of adsorption of particles by the surface / rate of collision of particles with the surface (1) Since a surface only has a limited number of places where a particle can be adsorbed on to, called fractional coverage, this will cause competition among different species. This relation can be seen in equation 2 (Atkins & de Paula, 2006). Fractional coverage = number of adsorption sites occupied / number of adsorption sites available (2) An example of two competing species is arsenite and arsenate. Different conditions in ph and Eh will change the rules of contest, which is one of the reasons why this experiment will test the adsorption ability at both ph 4,6 and 9,2. One of the main purposes of this experiment is to find the adsorption capacity (often denoted milligram adsorbate per gram adsorbent), which is a way to describe the effectiveness of the adsorbate. This is displayed in equation 3 below Adsorption capacity = ( (original amount of adsorbate amount of adsorbate left) * volume ) / amount of adsorbent (3) 2

6 Regarding adsorption, an ion can bind to a surface in two different ways; with or without intervening water molecules. Ions that bind directly to the surface with covalent bindings i.e. has a high affinity for surface sites forms a so called inner-spehere surface complexes. The other case is when there is water molecules involved and is then called outer-sphere surface complexes (Atkins & de Paula, 2006). 2.2 Adsorption of arsenite and arsenate The adsorption of arsenite and arsenate onto the iron oxide goethite (α-feooh) will result in a few different types of molecular combinations. It is difficult to tell what kind combinations that will occur as this depends on many factors, including ph. A study made by Jain et al. (1999) suggest that some of the possible molecules formed with arsenite includes (FeO) 2 HAsO 3 and (FeO) 2 AsO 3 -. For arsenate there are also a few different molecules that will occur in this kind of solution, including; FeOAsO 3, Fe 2 O 2 AsO 2 and Fe 2 O 2 AsOOH. From the original and final ph values one can calculate the amount of hydroxide ions that are released from the goethite. This can in theory then be used determine the chemical reactions that are occurring and what type of molecules that are formed during the chemical adsorption. I n several previous studies, including one conducted by Duro et al. (2007) it has been observed that the adsorption capacity of both arsenite and arsenate is static at acidic to neutral ph with a decrease of arsenate adsorption capacity at alkaline ph. One of the objectives in this study is to determine weather this correlation appears or not. Further it has been showed by Kersten & Vlasova (2009) that for arsenite, the maximum amounts adsorbed on goethite are equal to 0.45, 1.75 and 2.25 µmol/m 2 in a solution of 10, 50 and 100 µm, respectively. This is lower than the theoretical maximum arsenite adsorption capacity calculated from the site density of goethite (5.8 µmol/m 2 ) which indicates that there is a formation of monolayer coverage by bidentate surface complexes. Regarding ionic strength, it has been established in recent studies conducted by Goldberg (2002) that both arsenite and arsenate adsoprtion on goethite has little ionic strength dependency as a function of ph, which suggest an inner sphere surface complex formation, described in section 2.1. However, arsenite adsorption showed more dependency which indicates that it is more weakly bond to goethite than arsenate. Concerning the amount of arsenite and arsenate adsorbed, it has been observed by Charnock et al. (2002) that iron oxide minerals in general adsorb between 49% and 74% of arsenic from the solutions and goethite adsorb around 49% of arsenite and 65% of arsenate. 2.3 Hydride generation atomic fluorescence spectroscopy (HG-AFS) The fluorescence from a molecule is the light that is emitted when a molecule is excitated to a higher electronic state by absorption of a photon. A HG-AFS is a device that measures this fluorescence from a solution in order to identify the compounds, in this case the arsenite. Hydrochloride and potassium borohydroxide is used to reduce the arsenite to AsH 3, a volatile gas. By driving this gas and argon through a flame, the AFS can register the flouresence intensity of the arsenic in the sample. The ph of the analyzed sample needs to be below 2,5 in order for this process to work. (Fidler et al., 2005). The AFS can detect both arsenite and arsenate compounds, however not separately. Therefore, a condition where the AFS can only detect arsenite alone needs to be determined. This was done a few months prior to this project and as can be seen from figure 2 below, when using 1,4 mol/l citric acid 3

7 Flourescense Competitive adsorption of and As(V) on goethite and 0,56 mol/l sodium hydroxide together with the arsenate and arsenite, the fluorescence from the arsenite is detected while the arsenate is not y = 9,6149x + 154,6 R² = 0,9986 y = 0,7304x + 166,91 R² = 0,9645 Arsenite Arsenate Concentration (µg/l) Figure 2. Arsenite and arsenate detection using the AFS (Hao, 2011). This relation is used in this experiment when the arsenite is determined. After this, the arsenate can be determined by reducing it to arsenite, detect the which is then the total arsenic, and then subtract the previous detected arsenite from the total arsenic. To be able to determine the from the flouresence data, one standard curve for each species is created with fixed values of arsenite or arsenate. 3 Experiment 3.1 Materials and reagents In the table 1 below, a complete list of material and reagents used in this experiment are displayed. Reagent Formula Concentration/Purity Sodium nitrate NaNO 3 96% Hydrogen nitrate HNO 3 0,1 mol/l Hydrogen chloride HCl 0,1 mol/l Sodium hydroxide NaOH 0,1 mol/l Goethite α-feooh particles ca 60 nm in diameter Sodium arsenate Na 3 AsO 4 ( As(V) ) 1 g/l Sodium arsenite NaAsO 2 ( ) 1 g/l Thiourea H 2 NCSNH 2 95% L-ascorbic acid C 6 H 8 O 6 99,7% Potassium borohydroxide KBH 4 95% Potassium hydroxide KOH 82% Citric acid C 6 H 8 O 7 H 2 O 99,5% Nitrogen gas N 2 100% Table 1. Material and reagents. 4

8 3.2 Equipment In the table 2 below, a complete list of equipments and instruments used in this experiment are displayed. Equipment Model Parameters ph-meter Titroline Temperature: 25 C Shaker DSHZ-300 rpm: 170 Vaccum filtrator Membrane size: 0,22 µm Pressure: 0,05 MPa Hydride generation atomic fluorescence spectroscopy SK-2002 AFS analyzer Negative high voltage of multiplier: -380 V Hollow cathode lamp current: 110 ma Integral time: 5 seconds Speed of peristaltic pump: 40 rpm Transmission gain: 1 Flow rate of air: 750 ml/min Flow rate of argon: 600 ml/min Partial pressure of argon: 0,25 MPa Table 2. Equipments and instruments. 3.3 Method Sample preparation The first step is to prepare the samples. This is done by adding 0,085g of the polyethylene adsorption substrate sodium nitrate to 95, 97, 98 or 99 ml of distilled water depending on the s of arsenite and arsenate that should be added later (for details, see table 3 in the Appendix). The addition of sodium nitrate will change the ionic strength of the sample in order to simulate a natural environment. Second, 0,05 g of goethite particles with a diameter of 60 nm are added to the solution, which is then put on a shaker for 3 hours to be allowed to reach equilibrium ph adjustment After 3 hours, the ph in the samples is adjusted to 4,6 or 9,2 using hydrogen nitrate, sodium hydroxide and a ph-meter with an uncertainty interval of ±0,05. The volume of hydrogen nitrate and sodium hydroxide added can be neglected. The reason of choosing ph 4.6 and 9.2 is due to that it is the natural range of ph in the environment and outside these values solubility of goethite changes and the ph will be more difficult to regulate Addition of arsenite, arsenate and nitrogen gas Arsenate and arsenite is then added to the solution in the form of sodium arsenate and sodium arsenite. The of arsenate will first be kept constant at 10 mg/l and the of arsenite will be changed. Then, in a new set of batches, the of arsenite will be kept constant at 10 mg/l and the of arsenate will be changed. 5

9 Since the original of arsenate and arsenite used in this experiment is 1 g/l and the desired s in the batches will vary from 0 to 10, 20 and 40 mg/l, the volume of added arsenate and arsenite spans over 0, 1, 2 and 4 ml in the different batches. First, the arsenate is added. Then, pure nitrogen gas is blown into the samples for around 2 min in order to remove the oxygen, which will otherwise oxidize the arsenate and convert it into arsenite. After this, the arsenite is added and the samples are put on a shaker for 20 hours to be able to reach equilibrium again. All batches are made in two sets to enable a better estimate of the final ph and arsenate/arsenite s, which will result in a total of 32 batches of 100 ml each. This is displayed in table 3 in the Appendix ph determination and vaccum filtration After 20 hours, the final ph is determined by using the ph-meter. The goethite particles of the samples are then filtrated out using a vaccum filtration system. The samples then contain the remaining arsenate and arsenite which did not get adsorbed by the goethite. The of these substances are then determined using the HG-AFS Preparation of AFS solutions The reduction agent used when determining the total arsenic is produced by mixing 5 g of thiourea and 5 g of L-ascorbic acid in 100 ml of water. 5 ml of this agent is then added to each sample. In another solution the potassium borohydride, used for reduction of arsenic into AsH 3, is made by mixing 1 g of potassium hydroxide and 10 g of potassium borohydroxide to 500 ml. This solution together with hydrogen chloride is then pumped into the HG-AFS as described in section 2.3. A third solution, made to keep the ph stable under 2,5 at all times and to ensure the conditions for arsenite determination, is made by mixing 58,84 g of citric acid and 4,5 g of sodium hydroxide to 200 ml. 5 ml of this agent is added to each sample. When all reagents are added, 5 ml of the filtered arsenite /arsenate sample is added. Finally, the sample is diluted to 50 ml and tested in the HG-AFS. 6

10 Final ph Final ph Competitive adsorption of and As(V) on goethite 4 Results and discussion 4.1 Final ph of the samples after adsorption Below are the results of the final ph values in the 32 samples. 6,2 6 5,8 5,6 5,4 5,2 5 4,8 4,6 ph Variyng (mg/l) Varied arsenate, fixed arsenite Varied arsenite, fixed arsenate Figure 3. Final ph in the samples with original ph values of 4.6. ph 9.2 9,2 8, Varied arsenate, fixed arsenite 8,2 7,7 Varied arsenite, fixed arsenate 7,2 Variyng (mg/l) Figure 4. Final ph in the samples with original ph values of 9.2. By observing the results in figure 3 and 4 it is clear that when the original ph is 4.6, the ph is increased after the adsorption of the arsenite and the arsenate for both cases of varied. 7

11 Arsenite adsorbed per unit goethite (mg/g) Competitive adsorption of and As(V) on goethite For ph 9.2 it is the other way around, the ph values are in all cases lowered. At original ph of 9.2 there is a trend of decreasing ph change with increasing for both arsenite and arsenate. For ph 4.6 there is instead a small increase in ph change with increasing of arsenite in particular. There is no clear trend regarding which of the two species that affects the ph the most. However, due to the relative high change in ph, the results might be an indication of that the adsorption type occurring here is chemical rather than physical. Due to this, the model of inner sphere surface complex is applicable where the chemical reactions work through ion exchange, as described in section 2.2. By using the change in ph to calculate the amount of hydrogen and hydroxide ions that are released from the goethite, one can determine the chemical reactions that are occurring and what type of molecules that are formed during the chemical adsorption. However, what type of molecules that are formed and additional investigation of the inner sphere ion exchange mechanisms falls out of range of this project and the data received will be investigated further in future experiments. 4.2 Adsorption capacity of arsenite and arsenate at ph 4.6 Below are the results from the arsenite and arsenate analysis conducted at ph Adsorption capacity of arsenite at ph Arsenate (mg/l) Figure 5. Adsorption capacity of arsenite with fixed of arsenite at 10 mg/l and ph at 4.6. There is no clear correlation between the increasing amount of arsenate and the amount of arsenite adsorbed. However, a trend with small increase with can be observed, indicating that an increase in arsenate actually promotes the adsorption capacity of arsenite. This is the opposite of what is to be expected as when the competitive adsorption mechanism is present, the adsorption capacity of arsenite should decrease with increasing s of arsenate. 8

12 Arsenite adsorbed per unit goethite (mg/g) Arsenate adsorbed per unit goethite (mg/g) Competitive adsorption of and As(V) on goethite Adsorption capacity of arsenate at ph Arsenite (mg/l) Figure 6. Adsorption capacity of arsenate with fixed of arsenate at 10 mg/l and ph at 4.6. The results in figure 6 shows that the adsorption capacity of arsenate does not change with arsenite, similar to the results in figure 5 above. From observing the results presented above, one could draw the conclusion that in an acid environment, competitive adsorption is not present or that the low ph value in some way prevents or at least decrease the effectiveness the mechanism of competitive adsorption. Another reason why no real correlation is seen might be that the maximum adsorption capacity of the goethite is higher than expected and therefore there is no need for competition between the two species. However, there are too few datasets available to come to any such conclusions. 4.3 Adsorption capacity of arsenite and arsenate at ph 9.2 Below are the results from the arsenite and arsenate analysis conducted at ph Adsorption capacity of arsenite at ph Arsenate (mg/l) Figure 7. Adsorption capacity of arsenite with fixed of arsenite at 10 mg/l and ph at

13 Arsenate adsorbed per unit goethite (mg/g) Competitive adsorption of and As(V) on goethite This is one of the few results that show the presence of a competitive adsorption between arsenite and arsenate. At higher s of arsenate, the adsorption of capacity of arsenite decreases accordingly. The negative value at 40 mg/l is due to measurement faults while using the HG-AFS and should be neglected Adsorption capacity of arsenate at ph Arsenite (mg/l) Figure 8. Adsorption capacity of arsenate with fixed of arsenate at 10 mg/l and ph at 9.2. Here, there are some indications that there is a competitive adsorption between the two species; however the data at 20 mg/l do not correlate with this. The fact that it is three times higher than the other values received can be interpreted as a measuring fault and if that is the case, the value could be neglected. Once again, the negative value at 40 mg/l can also be neglected. By observing the results from the measurements at ph 9.2, there are some indications that the competitive adsorption mechanism here is more distinct apparent. Nevertheless, one can not from these results alone deduce that competitive adsorption mechanism would be promoted in an alkaline environment rather than in an acidic environment. To do this, more experiments over a longer time period would be required. 10

14 Arsenate adsorbed (%) Arsenite adsorbed (%) Competitive adsorption of and As(V) on goethite 4.4 Amount of arsenite and arsenate adsorbed Below is the amount of arsenite and arsenate adsorbed in percentages Amount arsenite adsorbed Arsenate (mg/l) ph 4.6 ph 9.2 Figure 9. Amount of arsenite adsorbed (%) with fixed of arsenite at 10 mg/l Amount arsenate adsorbed Arsenite (mg/l) Ph 4.6 ph 9.2 Figure 10. Amount of arsenate adsorbed (%) with fixed of arsenate at 10 mg/l. In these two figures it can be seen that the amount of arsenite adsorbed varies between 40% and 95% with an average around 50% and the amount of arsenate between 40% and 60% with an average also around 50% (despite from two values at 40 mg/l and the value at 20 mg/l that are unrealistic and can be neglected). This correlates fairly well with what was found by Charnock et al. (2002) (see section 2.2). It does not seem as an increasing of arsenite or arsenate has any impact on the 11

15 adsorbed percentage of the other species, as has also been noted in the results in section 4.2 and 4.3. No clear connection between the ph value and the amount of arsenite and arsenate adsorbed could be found, in contrast to previous studies. 5 Conclusion The results in this study do not give any clear verification that there is a competitive adsorption between arsenite and arsenate, although a few of them gives indications of it. The ph do not seem to have any obvious impact on the direct adsorption capacity of either the arsenite or the arsenate as all values ranges between 0 and 20 mg/g with an average around 12 in both cases, which contradicts previous findings. However, at ph 9.2 there were some indications of competitive adsorption occurring and the amount of arsenite and arsenate adsorbed correlated fairly well with what had been found in previous studies. The varying of arsenite or arsenate did not have any impact on the adsorbed percentage of the other species. The uncertain results are primarily thought to be the cause of the HG-AFS not measuring the s in a proper way, which lead to a lack of data. No clear conclusions can be drawn from this experiment as the results are too uncertain and do not correlate with any parameter. In order to receive more reliable results in future experiments, carefully checked conditions of the HG- AFS, more samples and measurements over a longer time period with additional ph values, a more oxygen free environment and more even goethite particle size distribution are some of the actions that are suggested. This is more thoroughly discussed in section 6 below. 6 Sources of error There might be several reasons for the flawed results. For one, there are some major differences in between some of the duplicated samples made (see table 5 through 8 in the Appendix); this might be due to the fact that the surface area of the goethite particles varies between the samples. Smaller particles give a larger total surface area which in turn promotes a higher adsorption capacity. Another source of error might be that the conditions determined to enable the AFS to only detect arsenite described in section 2.3 were no longer valid while conducting this experiment. If both arsenate and arsenite was detected, the values received during the fluorescence measurement were incorrect. This gives a large impact on the end results and can explain the negative values and why no real evidence for a competitive adsorption between the two species was found. An additional reason for the flawed values is that when measuring the total arsenic in the HG-AFS, the arsenate was not fully reduced to arsenite when adding the reduction agent as described in section In order to not oxidize the arsenate into arsenite, the environment needed to be kept oxygen free during the adsorption stage by adding nitrogen gas. This condition might not have been fulfilled and in that case the measurement was later defective. The condition of the instruments used in this experiment, especially the HG-AFS, might be another source of error. 12

16 7 Acknowledgements The author would like to thank Dr. Tong Ouyang, Lin Hao, Lin Haiying and Jingjing Ma at Xiamen University. This project has been financially supported by Tyréns, ITT Water & Wastewater, Sweco and Lions. 8 References Atkins, P. & de Paula, J., Physical Chemistry. United States: W.H. Freeman and company. Carranzam, E., Duker, A.A. & Hale, M., Arsenic geochemistry and health. Environment International, (31), pp Charnock, J.M., Farquhar, M.L., Livens, F.R. & Vaughan, D.J., Mechanisms of Arsenic Uptake from Aqueous Solution by Interaction with Goethite, Lepidocrocite, Mackinawite, and Pyrite: An X- ray Absorption Spectroscopy Study. Environmental science and technology, pp Delides, S., Paling, E., Singh, P. & Zhang, W., Arsenic removal from contaminated water by natural iron ores. Minerals Engineering, (17), pp Duro, L. et al., Arsenic sorption onto natural hematite, magnetite, and goethite. Journal of Hazardous Materials, (141), pp EPA, Basic information about the arsenic rule. [Online] Available at: [Accessed 7 April 2011]. Fidler, V., Hof, M. & Hutterer, R., Fluorescence Spectroscopy in Biology Advanced Methods and their Applications to Membranes, Proteins, DNA, and Cells. Germany: Springer. Goldberg, S., Competitive adsorption of arsenite and arsenate on oxides and clay minerals. Soil Science Society of America Journal, (66), pp Hao, L., Calibration of HG-AFS. Xiamen: Xiamen University. Jain, A., Raven, K.P. & Loeppert, R.H., Arsenite and arsenate adsorption on ferrihydrite: Surface charge reduction and net OH release stoichiometry. Environmental science and technology, (33), pp Kersten, M. & Vlasova, N., Arsenite adsorption on goethite at elevated temperatures. Applied Geochemistry, (24), pp Li, Z., van Halemb, D. & Verberk, J.Q.J.C., Review of High Arsenic Groundwater in China. IEEE. 13

17 9 Appendix Batch number Final ph of batch Amount of water added (ml) Amount of goethite added (g) Amount of As(V) added (ml) Amount of added (ml) 1 4,6 99 0, ,6 99 0, ,6 98 0, ,6 98 0, ,6 97 0, ,6 97 0, ,6 95 0, ,6 95 0, ,6 99 0, ,6 99 0, ,6 98 0, ,6 98 0, ,6 97 0, ,6 97 0, ,6 95 0, ,6 95 0, ,2 99 0, ,2 99 0, ,2 98 0, ,2 98 0, ,2 97 0, ,2 97 0, ,2 95 0, ,2 95 0, ,2 99 0, ,2 99 0, ,2 98 0, ,2 98 0, ,2 97 0, ,2 97 0, ,2 95 0, ,2 95 0,

18 Table 3. The batches and their properties. Origin ph Type 0 mg/l 0 mg/l 10 mg/l 10 mg/l 20 mg/l 20 mg/l 40 mg/l 40 mg/l 4,6 Varied 5,3 6,2 5,91 6,03 5,99 6,08 5,92 5,95 arsenate, fixed arsenite 4,6 Varied 5,98 5,13 6,11 5,82 6,02 5,92 6,06 5,89 arsenite, fixed arsenate 9,2 Varied 7,59 7,67 7,62 7,80 7,78 7,96 7,80 8,05 arsenate, fixed arsenite 9,2 Varied arsenite, fixed arsenate 7,52 7,42 7,56 7,71 7,67 7,99 8,06 8,33 Table 4. Final ph of the 32 samples. Amount arsenate (mg/l) Fluorescence intensity Average (ppb) (ppm) Average (ppm) absorbed (mg/g) Amount adsorbed (%) ,666 21,964 4,392 4,58 10,832 54, ,666 23,874 4,774 5,82 8,357 41, ,666 28,468 5,693 3,44 13,123 65, ,741 5,948 0,59 18,823 94, ,560 6, ,333 0,821 0, ,293 0, ,666 3,586 0,717 Table 5. Results of AFS measurements with fixed arsenite at 10 mg/l and ph at 4.6. Amount arsenite (mg/l) Fluorescence intensity Average Total arsenic (ppb) Total arsenic (ppm) As(V) (ppm) As(V) absorbed (mg/g) Amount As(V) adsorbed (%) ,333 1,415 1,414 1,414 12,508 62, ,333 6,343 6,343 6,078 13,634 68, ,333 8,403 8,403 2,332 8,835 44, ,333 9,286 9,285 4,034 12,890 64, ,333 17,672 17,672 4, ,206 17,206 6, ,667 14,313 28,625 3, ,822 27,644 3,213 Table 6. Results of AFS measurements with fixed arsenate at 10 mg/l and ph at 4.6. Amount arsenate (mg/l) Fluorescence intensity Average (ppb) (ppm) Average (ppm) absorbed (mg/g) Amount adsorbed (%) ,667 28,618 0,286 0,235 19,529 97, ,398 0,183 5,429 9,141 45,705 15

19 ,667 27,392 5,478 6,001 7,996 39, ,667 26,901 5,38 30,785-41, , ,417 6, ,667 29,6 5, ,153 31, ,417 30,417 Table 7. Results of AFS measurements with fixed arsenite at 10 mg/l and ph at 9.2. Amount arsenite (mg/l) Fluorescence intensity Average Total arsenic (ppb) Total arsenic (ppm) As(V) (ppm) As(V) absorbed (mg/g) Amount As(V) adsorbed (%) ,667 3,911 3,911 3,582 12,382 61, ,667 4,163 4,163 4,0349 7,791 38, ,817 9,817 6,693 30, ,993 10,993 5,514-6,769-33, ,23 20,23-6, ,062 20,0629-3, ,667 19,867 39,734 15, ,643 39,286 11,566 Table 8. Results of AFS measurements with fixed arsenate at 10 mg/l and ph at