IRON AND ALUMINIUM OXIDES POROUS MATERIALS FROM LATERITE: EFFICIENT ARSENIC ADSORBENTS

IRON AND ALUMINIUM OXIDES POROUS MATERIALS FROM LATERITE: EFFICIENT ARSENIC ADSORBENTS Y. Glocheux 1, S.J. Allen 1 and G.M. Walker 1 1. School of Chemistry and Chemical Engineering, Queen s University Belfast, United Kingdom; email: yglocheux01@qub.ac.uk ABSTRACT There is a need for low cost systems able to treat arsenic contaminated ground water. This work proposes different strategies to develop a low cost adsorbent using laterite; a natural ore easily available. Adsorption, kinetics and ph studies were carried out to assess the efficiency of the materials synthesised and were compared to Bayoxide; a commercial adsorbent. Results show that newly developed materials present very high removal capacity for As(V). The use of surfactant does increase the surface area of produced materials but do not influence the maximal arsenic adsorption. Adsorption capacities of these materials were evaluated at 4.5 mg As(III).g -1 and 20.2 mg As(V).g -1 respectively at 100 ppb ; a typical value for arsenic contaminated groundwater. Their very good arsenic removal capacities and their low cost make these materials excellent candidates for being used in continuous bed pack systems. Keywords: Laterite; Arsenic; Adsorption; Modelling 1 INTRODUCTION Arsenic groundwater contamination is a severe health issue and a global challenge requiring the efficient design of various treatment systems to attain high drinking water quality standards. Adsorption process is the most commonly used technology to treat arsenic contaminated groundwater as its running cost is usually very low and it is able to produced water below the WHO recommendation standard of 10 ppb. Iron oxide, aluminium oxides and titanium oxides have shown very high adsorption capacities toward both As(III) and As(V) species [1]. Several commercial adsorbents based on these elements already exist and are widely used. This work focused on the use of laterite as a low cost source of Fe, Al and Ti in order to produce highly porous material. Laterite is a natural iron, aluminium and titanium ore present in different parts of the world. It is also present in countries facing severe groundwater contamination like India for example. Some studies already showed the effectiveness of laterite in removing arsenic by adsorption. A recent study presented a modified laterite possessing very high removal capacity [2]. This study presents the use of a local Laterite mined in Northern Ireland to produce adsorbents composed of Fe, Al and Ti. There are several ways to produce ordered or dis-ordered porous metal oxides: EISA, hydrothermal processes, sol-gel methods or hydrolysis [3]. Here it was decided to use a controlled precipitation method of the metals into hydroxide by the slow addition of ammonium hydroxide. The effect of surfactant was also tested in order to increase the surface area of the materials produced. 2 MATERIALS AND METHODS 2.1 Materials A commercial adsorbent for arsenic removal was used in this study to compare results. Bayoxide E33 produced by Lanxess was supplied by Severn Trent (UK). Bayoxide is a granular iron oxide made of akaganeite [4], 500-710 µm particle size was used. All chemicals used were of reagent grade at least and all adsorption studies were carried out using deionised water. A laterite ore was used as a source of iron. The ore, extracted from a quarry in Northern Ireland was supplied by John Kinney from KESS Consulting. The laterite is divided into several layers at that location; for comparison purpose two layers were selected, an upper layer and a lower layer, the upper being richer in aluminium [5]. The iron, aluminium and titanium were extracted from the laterite by an HCl leaching at 60 C for 6 hours. 2.2 Synthesis Microporous adsorbents were produced using FeCl 3.6H 2 O or the laterite leaching as an iron source, P-123 as a surfactant and NH 3 OH as the precipitating agent. The solvent used was water. The effect of Fe/P123 ratio was investigated. In a typical synthesis 1 g of P123 was dissolved in 100 ml of deionised water in presence of 4.066 g of FeCl 3.6H 2 O. Then concentrated NH 3 OH was added drop wise, rising the ph from 1.5 to 9.00. The precipitate was allowed to age at 60 C in solution during 18 h and was then recovered by centrifuge,

heavily washed with distilled water and dried at 60 C overnight. In order to remove remaining surfactant, the iron oxides newly created were re-dispersed in pure methanol at 60 C and washed 3 three times with fresh methanol and finally with deionised water before being dried at 60 C. 2.3 Adsorbent Characterisation BET surface area, porosity and average pore size were measured using a Nova 4200e Surface area and pore size analyser form Quantachrome Instruments run with N 2. Samples were degassed overnight at 60 C under vacuum and analysed at 77 K with 45 adsorption points and 30 desorption points. Points of zero charge of the adsorbents were determined using the method proposed by Reymond and Kolenda [6]. The surface morphology of adsorbents was evaluated by a JEOL JSM-6500F field emission Scanning Electron Microscope. Probe current was set at 2 A, external voltage between 2-3 kv and emission current at 71 µa. XRF analysis was performed using the Axios Advanced equipment from PANalytical. 2.4 Adsorption Studies Adsorbents produced were tested in removing both As(III) and As(V). ph studies, kinetics and concentration studies were carried out for selected adsorbents. Initial arsenic concentrations ranging from 0.5 to 50 ppm were chosen to study the effect of concentration onto the equilibrated adsorption capacity of the materials. The adsorbent dose was maintained at 1 g.l -1 and initial ph of 7 was used with a ph buffer constituted of 100 mg.l -1 of NaHCO 3. The experiments were carried out in 50 ml glass flasks and let to equilibrate for 48 hours onto a horizontal shaker set at 100 RPM. Kinetics experiments were carried out in 1 L Becker, adsorbent dose of 1 g.l -1 was used, initial ph was set at 7 with 100 mg.l -1 of NaHCO 3 and stirrers were set at 300 RPM. Based on the isotherms, initial arsenic concentrations of 25 ppm for As(III) and 50 ppm for As(V) were used to highlight the effect of ph onto the maximal adsorption capacity of the materials. Experiments using an adsorbent dose of 1 g.l -1 were performed in 50 ml glass flasks and let to equilibrate for 48 hours onto a horizontal shaker set at 100 RPM. Initial ph ranging between 4 to 9 were elected and adjusted using NaOH or HCl solutions with 100 mg.l -1 of NaHCO 3 as a ph buffer. Equilibrated ph was recorded by the mean of a Thermo Scientific Orion 3 Star ph meter equipped with a Camlab ph probe calibrated with 3 standard solutions at ph 4.01; 7.00 and 10.00. 2.5 Analysis Arsenic, iron, aluminium and titanium measurements were carried out using an ICP-AES HR duo IRIS Intrepid model from Thermo Elemental. Prior to analysis, samples were filtered using qualitative filter paper with a cut off around 13 µm and were acidified at 2% HNO 3. 2.6 Modelling Mathematical modelling were applied to experimental results; isotherms were modelled using Langmuir and Freundlich adsorption models and the pseudo first and pseudo second order models were applied to kinetics experiments. These models are presented by equations (1) to (4). Langmuir model: Freundlich model: (2) Where is the amount of pollutant adsorbed at equilibrium for a given initial concentration in mg.g -1. is the maximum amount of pollutant adsorbed at equilibrium in mg.g -1. is the concentration in bulk solution at equilibrium in mg.l -1. is the Langmuir constant in L.mg -1. is the Freundlich coefficient in mg 1-n.g -1.L n. is the Freundlich equation constant and is adimensional. Pseudo first order model: Pseudo second order model: (1) [ ] (3) ( ) (4)

Where is the amount of pollutant adsorbed at time t in mg.g -1. is the amount of pollutant adsorbed at equilibrium for a given initial concentration in mg.g -1. is the pseudo-first order kinetic constant in min -1. is the pseudo-first order kinetic constant in g.mg -1.min -1. 3 RESULTS AND DISCUSSION 3.1 Characteristics of Laterite Samples and Leaching Laterite ore and the liquor produced after HCl leaching of the laterite were analysed. Table 1 shows XRF analysis results of the two laterites used in leaching experiments; results are expressed in %. It can be seen that the laterite samples are mainly composed of aluminium, iron, silica and titanium oxides. Table 1. XRF analysis of laterite samples SiO 2 TiO 2 Al 2 O 3 Fe 2 O 3 Mn 3 O 4 MgO CaO Na 2 O P 2 O 5 V 2 O 5 Cr 2 O 3 LOI Total Upper 9.96 4.22 56.55 26.2 0.2 1.19 0.8 0.16 0.25 0.12 0.08 22.94 99.97 Lower 27.54 4.05 33.29 31.97 0.23 0.92 0.6 0.12 0.23 0.13 0.11 13.52 99.49 Table 2 shows ICP-OES results of the leaching; results are expressed in g.l -1 or % extracted, based on XRF analysis. Others elements were detected, but there concentrations were several orders of magnitude lower than the three elements reported in Table 2. Table 2. Laterite leakage analysis Al in g.l -1 Fe in g.l -1 Ti in g.l -1 Al in % Fe in % Ti in % Upper 5.282 36.89 2.754 7.06 80.52 43.55 Lower 4.911 37.59 1.274 11.15 67.24 20.99 The HCl treatment selectively extracts iron; aluminium is also leached out but in a smaller proportion compared to its initial presence in the laterite ore. Titanium concentration is significantly higher in the upper layer leaching while the total concentration of metals is slightly higher for the same sample. As a result it was decided to use only the upper layer leaching as it contained the higher concentration of metals. 3.2 Adsorbents Characteristics The BET surface analysis results of the different samples prepared are presented in Table 3. Table 3. Adsorbents characteristics BET Surfactant Fe source Fe/P-123 m 2.g -1 ml.g -1 nm µm Particle size PZC Bayoxide No Akaganeite n.a 124.15 0.7658 12.34 500-710 8.3 Fe No FeCl 3.6H 2 O n.a 138.96 0.2212 10.12 180-710 8.1 Fe + P-123 Yes FeCl 3.6H 2 O 100 208.63 0.2022 1.94 < 500 8.0 Laterite No HCl leaching n.a 75.63 0.0591 1.56 180-710 7.7 Laterite + P-123 Yes HCl leaching 100 111.42 0.0766 1.38 < 500 7.6 The materials produced using FeCl 3.6H 2 O as the source of iron possesses similar surface area than Bayoxide. The presence of surfactant increases the surface area of the materials produced while decreasing the average pore size. At a molecular ratio of 100 Fe:P-123 it was found an optimum in term of surface area. (a) (b) (c) Figure 1. SEM pictures of (a) Bayoxide, (b) Fe adsorbent and (c) Fe + P-123 adsorbent

Increasing Fe:P-123 ratio results in low surface area material having a high porosity. Decreasing this ratio results in material with smaller surface area and bigger average pore size. It is worth noticing that materials produced with the laterite leaching possesses very small porosity values compared to the others materials. Their surface areas are nevertheless quite similar. The Bayoxide E33 is an iron oxide adsorbent made of akaganeite needles measuring around 200 nm long as shown on the SEM picture in Figure 1 (a). In the other hand the crystals produced by NH 3 OH precipitation are spherical and smaller as seen in Figure 1 (b) and (c). 3.3 Isotherms Analysis Figure 2 presents the isotherms for the different adsorbents used in the study. The adsorbents produced by FeCl 3.6H 2 O have lower adsorption capacity than Bayoxide for As(III) and similar adsorption capacity for As(V) removal. On the other hand the material produced using the laterite leakage present similar removal capacity for As(III) and higher adsorption capacity for As(V) than Bayoxide. There are no real difference between the adsorbent produced with or without surfactant in term As(III) and As(V) adsorption capacity. As(III) As(V) Figure 2. As(III) and As(V) concentration study. Visible lines are Freundlich or Langmuir model. Based only on the material characteristics presented in Table 3 it could not be predicted the very high As(V) removal capacity of the Laterite adsorbent. Chemical surface analysis of these materials is needed to better understand the adsorption mechanism involved. The presence of aluminium and titanium but also the possible high concentration of chloride ion might be responsible for this high removal efficiency. Table 4. Isotherms modelling data q m b L r 2 K f n r 2 As mg.g -1 L.mg -1 As mg 1-n.g -1.L n Laterite III 31.67 0.35 0.9896 Fe III 3.73 0.65 0.9960 Laterite + P-123 III 33.50 0.60 0.9720 Fe + P-123 III 5.05 0.55 0.9819 Fe + P-123 V 12.85 0.28 0.9986 Bayoxide III 11.67 0.32 0.8477 Laterite V 43.54 8.63 0.9177 Fe V 2.36 0.56 0.9663 Laterite + P-123 V 38.38 12.28 0.9901 Bayoxide V 7.03 0.28 0.9788 The very high adsorption capacity of the material produced by laterite leakage is promising to treat groundwater contaminated with low arsenic concentration. Table 5. Arsenic adsorption capacity comparison As removal capacity at 100 ppb Reference Adsorbent As(III) As(V) Model [7] Ordered mesoporous alumina 5.0 19.8 Column study [2] Modified laterite 3.13 14.54 Langmuir This study Bayoxide 5.63 3.7 Langmuir This study Precipitate from laterite 4.5 20.2 Langmuir

The removal capacity of the materials produced by laterite leakage can be compared to the best materials present in the literature as shown in Table 5. 3.4 Kinetic Results The kinetics studies were carried out over 48 hours; Figure 3 presents the first 24 hours results. Adsorption is very fast for both As(III) and As(V). It can be noticed that As(V) removal is faster and also total; whereas some As(III) still remain after 24 hours as trace. As(III) As(V) Figure 3. As(III) and As(V) kinetics study. Visible lines are 1 st or 2 nd pseudo order kinetic model. Based on the adsorbent characteristics presented in Table 3, it could be expected that Bayoxide would possess the better kinetics properties having the higher porosity and the bigger average pore size. Figure 3 and Table 5 show that in contrary the adsorbents produced from the laterite leakage have better kinetics parameters for both As(III) and As(V). The difference observed for As(V) removal could be explained by the much higher adsorption capacity of the produced adsorbent which is an indication of a higher density of adsorption site for As(V) at their surface. Having a higher adsorption site density would lead to a quicker initial adsorption; this can also explain why the pseudo second order fit better the laterite adsorbent kinetics data. Nevertheless this phenomenon cannot explain the better kinetics for As(III) removal; the presence of titanium and aluminium in the produced adsorbents might be responsible for this quicker arsenite uptake. Table 6. Kinetics modelling parameters k 1 q e r 2 k 2 q e r 2 As min -1 µg.g -1 g.mg -1.min -1 µg.g -1 Bayoxide III 1.73E-02 1024.9 0.999 2.03E-05 1143.1 0.999 Laterite III 2.09E-01 888.9 0.997 4.41E-04 923.4 1.000 Laterite + P-123 III 2.67E-01 934.2 0.998 6.05E-04 963.6 1.000 Bayoxide V 9.21E-03 1065.9 0.999 1.03E-05 1178.8 0.993 Laterite V 7.62E-01 1021.4 1.000 7.46E-03 1024.8 1.000 Laterite + P-123 V 6.85E-01 1039.7 1.000 5.18E-03 1044.3 1.000 3.5 ph Study The ph studies presented in Figure 4 show normal behaviour of the materials produced toward ph changes based on their respective point of zero charge. Between ph 4 and 9 As(III) is mainly present as H 3 AsO 3 and is not a charged specie. ph in this range should not then affect the adsorption process of As(III) onto the adsorbents used. The adsorption process in this case is governed by physisorption phenomenon. - 2- While at the same ph range, As(V) is negatively charged as H 2 AsO 4 or HAsO 4 (pka = 6.94). The materials used possess points of zero charge ranging from 7.6 to 8.3. There are then positively charged at ph lower than their PZC attracting more efficiently anions. In this case adsorption is predominantly governed by chemisorption processes.

As(III) As(V) Figure 4. As(III) and As(V) removal ph study. As(III) 0 = 25.30 ppm and As(V) 0 = 52.91 ppm 4 CONCLUSION The adsorbents developed using laterite as a low cost source of iron, aluminium and titanium show very high removal capacities, especially toward As(V). While the one produced using only FeCl 3.6H 2 O present similar As(V) removal than Bayoxide but lower As(III) adsorption capacity. The use of surfactant does influence the physical properties of the materials synthesised but no major changes are observed for both As(III) and As(V) removal adsorption capacities. The physical properties of these adsorbents are not the main factors explaining the remarkable removal values obtained. The adsorbent produced using laterite leakage have very quick uptake for both As(III) and As(V) compared to Bayoxide while their porosity and average pore size are smaller. The ph study confirms that As(V) adsorption is mainly governed by chemisorption processes. Thus a more detailed study of the surface chemical groups should be carried out to explain the very good As(V) removal capacity noticed. The particle sizes of the adsorbents used in this study were different. The next step would be to study the granulation of the powder produced with laterite leakage as grains. Granules having a particle size of 500 µm to 1 mm are expected to be produced and would be more suitable for bed pack systems. If adsorption capacities and kinetics properties of these granules are similar to the powder presented here; these adsorbents will have ideal properties for arsenic removal from groundwater. ACKNOWLEDGEMENTS This work was performed as part of the EU Framework 7 project ATWARM (Marie Curie ITN, No. 238273). The authors would like to thanks QUESTOR centre staff for their technical support. REFERENCES [1] K. Hristovski, A. Baumgardner, and P. Westerhoff, Selecting metal oxide nanomaterials for arsenic removal in fixed bed columns: From nanopowders to aggregated nanoparticle media, Journal of Hazardous Materials, Vol. 147, pp. 265 274, 2007. [2] A. Maiti, J. K. Basu, and S. De, Experimental and kinetic modeling of As(V) and As(III) adsorption on treated laterite using synthetic and contaminated groundwater: Effects of phosphate, silicate and carbonate ions, Chemical Engineering Journal, 2010. [3] U. Schwertmann and R. M. Cornell, Iron Oxides in the Laboratory: Preparation and Characterization. John Wiley & Sons, 2000. [4] A. Schlegel, Contact and adsorbent granules, US Patent 2001. [5] I.. Hill, R.. Worden, and I.. Meighan, Geochemical evolution of a palaeolaterite: the Interbasaltic Formation, Northern Ireland, Chemical Geology, Vol. 166, pp. 65 84, 2000. [6] J.. Reymond and F. Kolenda, Estimation of the point of zero charge of simple and mixed oxides by mass titration, Powder Technology, Vol. 103, pp. 30 36, 1999. [7] W. Li, C.-Y. Cao, L.-Y. Wu, M.-F. Ge, and W.-G. Song, Superb fluoride and arsenic removal performance of highly ordered mesoporous aluminas, Journal of Hazardous Materials, Vol. 198, pp. 143 150, 2011.