Arsenic removal from contaminated water by natural iron ores

Arsenic removal from contaminated water by natural iron ores

Minerals Engineering 17 (2004) 517–524 This article is also available online at: www.elsevier.com/locate/mineng Arsenic removal from contaminated wat...

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Minerals Engineering 17 (2004) 517–524 This article is also available online at: www.elsevier.com/locate/mineng

Arsenic removal from contaminated water by natural iron ores W. Zhang *, P. Singh, E. Paling, S. Delides Chemistry Program, Division of Science and Engineering, School of MPS, Murdoch University, South Street, Murdoch W.A. 6150, Australia Received 14 October 2003; accepted 30 November 2003

Abstract Natural iron ores were tested as adsorbents for the removal of arsenic from contaminated water. Investigated parameters included pH, adsorbent dose, contact time, arsenic concentration and presence of interfering species. Iron ore containing mostly hematite was found to be very effective for arsenic adsorption. As(V) was lowered from 1 mg/L to below 0.01 mg/L (US standard limit for drinking water) in the optimum pH range 4.5–6.5 by using a 5 g/L adsorbent dose. The experimental data fitted the firstorder rate expression and Langmuir isotherm model. The adsorption capacity was estimated to be 0.4 mg As(V)/g adsorbent. The presence of silicate and phosphate had significant negative effects on arsenic adsorption, while sulphate and chloride slightly enhanced. The negative effect of silicate could be minimised by operating at a pH around 5. The interference of phosphate would necessitate the use of a relatively high dose of the adsorbent to achieve arsenic levels conforming to drinking water standards. The mechanisms of interference of silicate and phosphate on As(V) adsorption are also discussed. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Iron ores; Waste processing; Environmental

1. Introduction Adsorption/co-precipitation with iron oxyhydroxides is the most commonly adopted method for arsenic removal from contaminated water because the method is both effective and cheap compared with other methods such as ion exchange, membrane separation (reverse osmosis), bio-reduction and electrolysis. Generally two ways of using iron oxyhydroxides for removal of arsenic have been widely investigated. The first is in situ coprecipitation, in which amorphous ferrihydrite is formed (Edwards, 1994; Stenkamp and Benjamin, 1994; Scott et al., 1995; Hering et al., 1996). The ferrihydrite is more porous and has larger surface area for adsorption than pre-formed material. However, filtration has proven to be difficult. The alternative method is the preformation and agglomeration of the iron oxides and or the coating the iron oxides on a substrate such as sand or a polymeric material (polystyrene) (Subramanian et al., 1996;

* Corresponding author. Tel.: +61-8-9360-6871; fax: +61-8-93101711. E-mail address: [email protected] (W. Zhang).

0892-6875/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2003.11.020

Vagliasindi et al., 1996; Vagliasindi and Benjamin, 1997; Khaodhiar et al., 2000; Yuan et al., 2002). The preformed iron oxides can be packed into a column or form a filtration bed to allow contaminated water to pass through. It has been reported that the residual arsenic in the underflow of a column of iron oxide coated sand can be lowered to below 10 ppb (Vagliasindi and Benjamin, 1997; Katsoyiannis and Zouboulis, 2002). The advantages of using the solid adsorbent are the elimination of filtration and the ability to reuse the adsorbent after regeneration with NaOH solution (Vagliasindi and Benjamin, 1997; Yuan and Luo, 2001; Yuan et al., 2002). Crystalline iron oxide minerals, which are either made through synthesises or selected from naturallyoccurring ores, have also been investigated for use as adsorbents for arsenic. These mineral particles are sufficiently hard and hence easy to separate from processed liquors. Hematite (Fe2 O3 ) is an effective adsorbent for arsenic removal from polluted river and ground water (Ashitani et al., 2000). The capacity of arsenic adsorption by Fe2 O3 materials can be improved if the surface area and porosity is increased by synthesising a composite SiO2 –Fe2 O3 material (Peleanu et al., 2000).

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In this method, nano-sized iron oxide particles are embedded into a SiO2 based solution gel matrix. Natural hematite was found to be a better adsorbent for arsenic than felspar (Prasad, 1994; Singh et al., 1996), maximum removal occurring at pH 4.2. A decrease in particle size from 200 to 100 lm increased adsorption capacity from 2.72 to 3.44 lmol/g, but an increase in temperature from 20 to 40 °C decreased the capacity from 2.72 to 2.34 lmol/g. Electrostatic attraction and surface complexation were proposed to be the major arsenic removal mechanisms. Ionic contaminants, such as Cd2þ and chromate, caused interference (Prasad, 1994; Singh et al., 1996). Natural hematite was also used in a pilot test in Mexico and a residual arsenic concentration below 0.05 mg/L was achieved (Simeonova, 2000). Goethite (a-FeOOH), the second most common naturally occurring iron oxide mineral after hematite, has also been reported to be effective for arsenic adsorption (Garcia-Sanchez et al., 1999; Lenoble et al., 2002). Goethite was capable of removing the toxic cation Cd2þ as well as As(III) or As(V) oxyanion from dilute aqueous solutions simultaneously (Matis et al., 1997; Matis et al., 1999a; Matis et al., 1999b). Ion removal strongly depended only upon the solution pH. Maximum adsorption of As(V) occurred in the pH range 3–6. Dissolution of iron increased at pH below 3 and above 9. Addition of a KNO3 electrolyte significantly improved As(V) adsorption in alkaline solutions in the pH range, 7–12, which was attributed to the depression of negative solid surface charges in the alkaline region (Matis et al., 1999b). Extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) spectroscopy examination indicates that arsenic(III) and As(V) species are adsorbed on the iron oxyhydroxide substrate through formation of inner sphere complexes. (Waychunas et al., 1993; Zhu et al., 1997; Manning et al., 1998; Farquhar Morag et al., 2002). Investigation of bonding structures of arsenite (As(III)) and arsenate (As(V)) on goethite (a-FeOOH), by Transmission-Fourier Transform IR (T-FTIR) and Attenuated Total Reflectance–FTIR (ATR–FTIR) spectroscopy (Sun and Doner, 1996), showed three bands of O–H stretch, corresponding to singly, doubly, and triply coordinated OH hydroxyls on the goethite surface. The arsenate and arsenite oxyanions replaced two singly coordinated surface OH groups (A-type) to form binuclear bridging complexes Fe–O–AsO(OH)–O–Fe and Fe–O–As(OH)– O–Fe. Iron oxides are adsorbents for other anions also and form similar surface complexes to those for As(V) and As(III) species. Silicate has been reported to have a strong ligand binding to iron oxides and therefore to influence arsenic adsorption on ferrihydrite (Swedlund and Webster, 1999; Meng et al., 2000), and goethite

(Waltham and Eick, 2002). Waltham and Eick (2002) observed that silicic acid sorption was characterized by biphasic kinetics with rapid adsorption followed by a much slower adsorption reaction. The rate of arsenate adsorption on goethite decreased as pH and silicic acid concentration increased. This has been attributed to a decrease in the goethite surface potential upon specific adsorption of silicic acid and deprotonation of the arsenate, creating an unfavourable electrostatic field. Swedlund and Webster (1999) noted that the interaction between silicic acid (H4 SiO4 ) and ferrihydrite involved both adsorption and polymerisation, depending on the Si/Fe mole ratio, and concluded that H4 SiO4 adsorption inhibited arsenic adsorption to a greater degree than H4 SiO4 polymerisation. The presence of silicate is also known to inhibit the transformation of aged ferrihydrite into goethite (Anderson and Benjamin, 1985). The point of zero charge (PZC) of the solid decreased from 8 to 4 as the Si/Fe ratio in the solid increased from 0 to 3.5. Phosphate is another ion which strongly competes with arsenate for adsorption on iron oxides due to its similar dominant dissociation species (Pourbaix, 1974) and comparable intrinsic affinity for iron oxide surface (Pierce and Moore, 1982; Roy et al., 1986; Manning and Goldberg, 1996; Jain and Loeppert, 2000). Parfitt (1989) reported that the amount of phosphate adsorbed on natural ferrihydrite, goethite and hematite up to 10 days followed the order Hematite < goethite < ferrihydrite. Phosphate also competed with silicate. The competitive effect of phosphate on arsenate or arsenite adsorption is also influenced by the way phosphate is introduced. Addition of phosphate, after arsenate or arsenite had been adsorbed, has very little effect on arsenic adsorption. However, a significant negative effect on adsorption of arsenic at low concentration is evident after phosphate or sulphate had already been adsorbed (Pierce and Moore, 1982). Sulphate also competes with As(III) and, to lesser degree, with As(V) for adsorption on iron oxides at pH 4–7 (Wilkie and Hering, 1996). The effect of sulphate addition is similar to that of phosphate (Pierce and Moore, 1982). Naturally occurring iron oxides are more attractive for arsenic removal from contaminated water than the synthetic oxides because they are more cost effective. However, few studies have been carried out on the feasibility of their use as adsorbents for arsenic removal, particularly on the effects of interfering species such as silicate and phosphate and interactions between ores and contaminated water. This work therefore focussed on investigating how various experimental parameters influence arsenic adsorption. These parameters included: adsorbent dose, pH, contact time, adsorbent particle size and surface area and the presence of interfering silicate, phosphate, chloride and sulphate ions.

W. Zhang et al. / Minerals Engineering 17 (2004) 517–524

2. Experimental The iron ore samples used in this study were taken from deposits in the Hamersley Ranges (provided by Hamersley Iron Pvt Limited Australia). The major iron minerals in the samples were hematite and goethite, with iron content being in the range of 59–64% (Table 1). The ore samples were ground and sieved to produce various particle size fractions. Surface area measurements were carried out by using a quantachrome Autosorb-1 cell according to ASTM C1069 –’86 followed by BET analysis (CSIRO Division of Minerals, Western Australia). Stock solutions containing 1000 mg/L As(V) were prepared by dissolving AR Na2 HAsO4 Æ 7H2 O in deionised water. Experimental solutions were prepared freshly from these stock solutions as required. Most experiments were, otherwise stated, performed by adding a dosage of 5 g/L adsorbent (with )75 + 63 lm particle size) to 100 ml of solution containing 1 mg/L As(V) in a 250 ml conical flask with magnetic stirring at a constant speed for 2 h. To determine how the presence of interfering species (silicate, phosphate, chloride and sulphate) along with As(V) in water affected the adsorption of these ions on the iron ore, experiments were conducted where various mixtures of As(V) and the interfering ions were kept in contact with 5 g/L iron ore at pH 7. In these mixtures, As(V) concentration was fixed at 1 mg/L but those of the interfering ions varied. The solution pH was monitored by an Activon pH probe and meter (Model 209), and adjusted by addition of dilute HCl or NaOH solutions. Samples were collected at regular intervals and filtered through 0.2 lm membrane filter followed by centrifugation at 3000 rpm for 30 min. The supernatant was sent for analysis of As, Fe, P, Si by a Varian inductively coupled plasma spectrometer (ICP), carried out by the Marine and Freshwater Research Laboratory, MurTable 1 Composition of iron ore samples from Western Australia (RM 11264 from Yandicoogina fines, RM 12318––Brockman type ore from Pit 4 at Brockman, RM 12258 from Tom Price) Elements (%)

RM 12264

RM 12318

RM 12258

Fe

59 (20% hematite, 80% goethite) 4.1 1.4 0.05 0.008 0.06 0.007 9.9

62.7 (mostly hematite)

64.1 (>90% hematite)

2.9 2.3 0.136 0.029 0.01 0.06 4.5

3.5 1.8 0.057 0.041 0.05 0.12 2.4

SiO2 Al P S Ca MgO Othersa a

Loss on ignition at 900 °C (Chemically bound water in CaCO3 Æ xH2 O).

519

doch University. The detection limits were 0.01, 0.001, 0.05, and 0.02 mg/L for As, Fe, Si and P, respectively.

3. Results and discussion 3.1. Comparison of As(V) adsorptions on various iron ores The ore (RM 12264) which had the highest surface area (30 m2 /g) did not produce the best adsorption results (Table 2). Instead, the sample RM 12318 (10 m2 /g) showed the best adsorption for As(V). The residual As(V) concentration was lowered to below 0.01 mg/L (detection limit) when 10 g/L of RM 12318 ore was used. The above results suggest that the surface area was not the dominating factor for As(V) adsorption in the investigated system. Unlike synthetic iron minerals, the naturally occurring iron ores contain a variety of contaminants. The observed results could well be related to the blockage of the ore surface by these contaminants, thereby preventing access of As(V) to the adsorption sites. Silicate and phosphate could also compete with As(V) adsorption. The natural iron ore RM 12318 was selected for further investigation as described in the following sections. 3.2. Effect of adsorbent particle size The effect of particle size on arsenic adsorption was investigated over the range +125 to )38 lm. Such data would be valuable for scale-up and for practical application in a fixed-bed-column adsorption system. Surprisingly, the experimental results did not show any significant variation of residual As(V) concentration over the whole range of the investigated particle size. To understand why this was so, the BET surface area of three representative size fractions: +125, )75 + 63, )38 lm was obtained. All three size fractions were found to possess similar surface area per gram of the solid (9.3– 10.2 m2 /g). This probably suggests that internal porosity rather than outer surface area is the major determinant of surface area.

Table 2 Comparison of As(V) adsorption at pH 6.5 on various natural iron ores Iron ore sample

RM 12264 RM 12258 RM 12318

Surface area (m2 /g)

Residual As(V) (mg/L) 1 g/L adsorbent

10 g/L adsorbent

30 5.3 10.2

0.70 0.83 0.52

0.05 0.03 <0.01

520

W. Zhang et al. / Minerals Engineering 17 (2004) 517–524

5

The residual As(V) concentration decreased exponentially with an increase in the amount of adsorbent (Fig. 1). An adsorbent dose of 5 g/L effectively decreased residual As(V) concentration to below 0.01 mg/L (detection limit).

4 ln(1/(Qe-Q))

3.3. Effect of adsorbent dose

3 2 1 0

3.4. Effect of contact time

-1

The results of effect of contact time on As(V) adsorption by the iron ore is shown in Fig. 2. The data were analysed according to the Lagergren (Altundogan et al., 2000) rate equation (1). Lnð1=ðQe  QÞÞ ¼ ðKad tÞ  LnQe

ð1Þ

where Qe and Q are the amounts of arsenic adsorbed at equilibrium and at time t; Kad is the adsorption rate constant. A linear relationship of Lnð1=ðQe  QÞÞ with contact time was obtained with correlation coefficient being 0.985 (Fig. 3). This indicates that As(V) adsorption

Residual As(V), mg/L

1

0.1

0.01 Below detection limit 0.001

0

2.5

5 Adsorbent, g/L

7.5

10

Fig. 1. Residual As(V) as a function of adsorbent (iron ore powder) dose at pH 6.5.

-2 0

40 60 Contact time, min

80

Fig. 3. Lagergren plot for As(V) adsorption at pH 6.5.

follows the first-order kinetics under the investigated conditions. From this graph, the rate constant Kad is calculated to be 0.077 min1 . 3.5. Effect of pH The residual As(V) concentration goes through a minimum in the pH range 4.5–6.5 where it is below the detection limit (0.01 mg/L) (Fig. 4). The pH (4.5–6.5) at which maximum adsorption of As(V) occurs differs significantly from the PZC for pure iron oxide minerals (pH 7.4–8.7) (Parks and De Bruyn, 1962; Lumsdon and Evans, 1994; Cornell and Schwertmann, 1996; Manning and Goldberg, 1996). Thus, some mechanism in addition to the simple electrostatic attraction model is operative. It is interesting to note that the pH range (4.5–6.5) where the adsorption of As(V) is maximum coincides with the pH range where the amount of iron resulting from the dissolution of the iron ore (adsorbent) is minimum. For As(V) adsorption, the iron ore samples (mainly natural hematite and goethite) are very similar to ferrihydrite (Pierce and Moore, 1980, 1982) which

1

1

0.8

Residual As(V), mg/L

Residual As(V), mg/L

20

0.6 0.4 0.2

0.1

0.01 Below detection limit

0 0

30

60 90 120 Contact time, min

150

180

0.001 0

2

6

4

8

10

pH Fig. 2. Effect of contact time on As(V) adsorption by iron ore at pH 6.5.

Fig. 4. Effect of pH on As(V) adsorption of As(V) on iron ore.

W. Zhang et al. / Minerals Engineering 17 (2004) 517–524

Mathematically the Langmuir isotherm model is defined by the following equation:

10 9

Initial pH

521

5 g/L iron ore

Final pH

Ce =Qe ¼ ð1=bQ°Þ þ Ce =Q°

ð4Þ

where, Ce (mmol/L) is the equilibrium concentration in the solution, Qe (mmol/g) is the amount adsorbed on the adsorbent at equilibrium, Q° and b are the Langmuir constants related to adsorption capacity and binding energy of adsorption respectively. The experimental results fit the Langmuir model, indicating a monolayer adsorption isotherm (Fig. 7). From the slope of best fit, the adsorption capacity of the iron ore (RM 12318) is determined to be 5.38 lmol (0.4 mg) As(V)/g adsorbent. Thus, use of 2.5 g/L of the adsorbent is expected to lower 1 mg/L initial As(V) to below 0.01 mg/L at equilibrium.

7 6 5 4 3 EX9 EX10 EX11 EX12 EX13 EX14 EX15 EX16

Experiment number Fig. 5. pH ‘‘buffering’’ effect of iron ore.

suggests that the surface properties of the natural iron oxides are similar to those of synthetic ferrihydrite.

3.8. Effect of silicate The presence of soluble silicate (Si) in water is found to strongly influence adsorption of As(V) on iron ore (Fig. 8). As can be seen from Fig. 8, an increase in Si from 1.4

The addition of iron ore to water at different initial pH’s is found to have a ‘‘buffering’’ effect (Fig. 5). The resultant pH is around 7. This ‘‘buffering’’ effect can be explained by the amphoteric nature of the iron oxide as given in Eqs. (2) and (3) (Bdonates surface group) and in Fig. 6 (Ahmed, 1966; Cornell and Schwertmann, 1996) BFeOH2þ ¼ BFeOH þ Hþ

ð2Þ

BFeOH ¼ BFeO þ Hþ

ð3Þ

In a low pH medium, the equilibrium equations (2) and (3) shift towards left, resulting in an increase in the bulk solution pH. In a high pH medium, the acid dissociation dominates which causes a decrease in the bulk solution pH. 3.7. Effect of initial As(V) concentration The effect of initial As(V) concentration on As(V) adsorption was investigated over the range 0–30 mg/L As(V) using 1 and 5 g/L doses of the adsorbent. Fig. 7 shows Langmuir isotherm plots for both the doses.

120 100

Ce/Qe, g/L

3.6. Effect of iron ore on solution pH

Langmuir isotherm

80 60 40

5 g/L iron ore

20

1 g/L iron ore

0

0

0.1

0.2

0.3 0.4 Ce, mmol/L

0.5

0.6

0.7

Fig. 7. Langmuir plot for As(V) adsorption on iron ore (12 h contact time).

Residual As(V) and Fe(III), mg/L

pH

8

1

[As(V)] 0.1 [Fe(III)] 0.01

0.001 0

5

10

15

20

Si (as silicate), mg/L Fig. 6. Iron oxide/aqueous solution equilibria.

Fig. 8. Effect of presence of silicate on As(V) adsorption at pH 7.

W. Zhang et al. / Minerals Engineering 17 (2004) 517–524

to 5 mg/L resulted in an increase of residual As(V) concentration from 0.03 to about 0.35 mg/L which amounts to a 30% decrease in As(V) adsorption. Above 5 mg/L Si, the effect levelled off. An important point to note is that the effect of silicate on soluble iron (from the iron ore) follows the same pattern as As(V) in solution. Thus, As(V) and Fe(III) both increase initially but then level off at the Si concentrations above 5 mg/L. One possible explanation for this observation could be that some soluble complex species consisting of Fe(III), As(V) and silicate is formed through mutual interactions. Control of solution pH is found to be a key parameter for minimising the silicate effect on As(V) adsorption (Fig. 9).The adsorption of As(V) is favoured at low pH. For example, the residual As(V) is effectively lowered from about 0.9 mg/L at pH 9.8, to 0.065 mg/L at pH 5.

Adsorbed P(V) and As(V), µmol/g

522

Residual As(V), mg/L

1.00

16 mg/L Si 5 g/L iron ore

4

6

6 Total adsorbed(As + P) Adsorbed P(V) Adsorbed As(V)

4 2 0

0

2

6 4 Initial P(V), mg/L

8

10

8

10

12

Adsorbed P, µmol/g

10

The results of the effect of P(V) as phosphate on As(V) adsorption on the iron ore are plotted in Fig. 10. It is evident from these results that when P(V) is present along with As(V), the two ions compete for adsorption sites on the iron ore. The relative adsorption of the two species as a function of P(V) concentration in the P(V) and As(V) solution mixture (As(V) ¼ 1 mg/L) can be seen from Fig. 10. While the adsorbed P(V) increases, the adsorbed As(V) decreases as the initial P(V) concentration in the solution mixture is increased. The adsorbed P(V) and As(V) both reach saturation levels when the P(V) concentration in the solution mixture exceeds approximately 3 mg/L. The adsorption capacity of the ore for combined [P + As] is 8.4 lmol/g which is much higher than the value, 5.4 lmol/g, when As(V) is adsorbed in the absence of P(V). The plot of [P(V)]ad vs [As(V)] is a straight line (Fig. 11) with a slope approximately )3.8 and best fit coefficient R ¼ 0:986. The magnitude of this slope provides an insight into the relative affinity of P(V) as compared to

0.01

8

Fig. 10. Adsorbed As(V) and P(V) as a function of initial P(V) concentration at pH 7.

3.9. Effect of phosphate

0.10

10

8

y = -3.7552x + 9.9724 2

R = 0.986

6 4 2 0 0

1 2 Adsorbed As(V), µmol/g

3

Fig. 11. P/As mol ratio as a function of initial P concentration at pH 7.

As(V) for adsorption on iron ore. This implies that when P(V) is present, approximately four times higher dose of the adsorbent would be needed to achieve residual As(V) comparable to what could be obtained when P(V) is absent. Fig. 12 compares the effect of iron ore dose on As(V) adsorption from initial 1 mg/L As(V) solution with and without concurrent presence of 2 mg/L P(V). As seen, the residual As(V) concentrations in both cases decrease exponentially as the iron ore dose is increased. However, the capacity for As(V) adsorption in the presence of initial 2 mg/L P(V) is dramatically lower. The results in Fig. 12 suggest that while adsorbent dose of 5 g/L would be enough to lower the residual As(V) concentration from initial 1 mg/L As(V) to below the standard drinking water levels (<0.01 mg/L), a six time higher dose (30 g/L) would be needed to do the same if the solution was contaminated with 2 mg/L P(V). 3.10. Effect of chloride/sulphate

pH

Fig. 9. Effect of pH on residual As(V) in the presence of added 16 mg/L soluble silicon in solution.

The presence of chloride is found to improve the As(V) adsorption on the iron ore (Fig. 13). It appears

W. Zhang et al. / Minerals Engineering 17 (2004) 517–524

1 Residual As(V), mg/L

With initial 2 mg/L P(V) Without added P(V) 0.1

0.01 0

10

20 30 Iron ore dose, g/L

40

Fig. 12. Comparison of effect of iron ore dose on As(V) adsorption with and without initially added 2 mg/L phosphate at pH 7.

effect on As(V) adsorption. This effect can be minimised by operating at a lower pH 5. The presence of phosphate also significantly lowers the ability of iron ores to remove arsenic by adsorption. Initial addition of 6.5 mg/L phosphate can lower the As(V) adsorption on 5 g/L iron ore dose by 30–50% at pH 7. This is related to the competition of phosphate with arsenate for adsorption sites on iron ore. Up to six times more iron ore dose is needed when 2 mg/L P(V) is co-present with initial 1 mg/ L As(V) to bring down the residual As(V) below the drinking water standards. The presence of chloride and sulphate both have a slightly positive effect on As(V) removal by adsorption on iron ore. Under optimum conditions, iron ore can lower the residual As(V) in water to below 0.01 mg/L (detection limit).

Acknowledgements

0.1

Residual As(V), mg/L

523

This research is funded by AusAID under the AusAID Australian Arsenic Mitigation Program (CONTRACT: 9779). Partial funding of the project by Murdoch University is also acknowledged.

Chloride medium

0.01 Sulphate medium

References

0.001 0

10

20

30

40

50

Chloride and sulphate, mg/L Fig. 13. Residual As(V) as a function of chloride and sulphate concentrations at pH 7.

that the presence of chloride alters the surface characteristics of the iron ore making it more conducive to As(V) adsorption. Chloride and sodium ions are reported to form ion pair, outer sphere surface complexes (Smith and Jenne, 1991). These outer sphere surface complexes help adsorption of As(V). The presence of sulphate is found to be even more favourable than chloride for As(V) adsorption. The residual As(V) is effectively lowered to below the detection limit (0.01 mg/L) at sulphate concentrations above 10 mg/L (Fig. 13).

4. Conclusions The iron ores from the Hamersley Ranges of Western Australia can effectively remove arsenic (initial concentration 0–1.0 mg/L) from water. The ore sample RM 12318 containing mostly hematite is particularly effective. Maximum arsenic removal occurs in the pH range 4.5–6.5. The presence of silicate have strong negative

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