Speciation of adsorbed arsenic (V) on red mud

using a sequential extraction procedure

D. A. RUBINOS1,*, M. ARIAS

2, F. DIÂAZ-FIERROS

1AND M. T. BARRAL

1

1Departamento de EdafoloxõÂa e QuõÂmica AgrõÂcola, Facultade de Farmacia, Universidade de Santiago de

Compostela, 15782 Santiago de Compostela, Spain2AÂ rea de EdafoloxõÂa e QuõÂmica AgrõÂcola, Facultade de Ciencias de Ourense, Universidade de Vigo, 32004 Ourense,

Spain

ABSTRACT

The distribution of sorbed arsenic(V) among different geochemical fractions for arsenic(V)-loaded red

mud, an oxide-rich residue from bauxite refining that has been proposed as an adsorbent for arsenic,

was studied as a function of sorbed arsenic(V) concentration using a sequential extraction procedure.

The release of previously sorbed arsenic(V) was also studied as a function of pH and arsenic(V)

concentration. Most sorbed arsenic(V) (0.39ÿ7.86 mmol kgÿ1) was associated with amorphous and

crystalline Al and Fe oxides (24.1ÿ43.8% and 24.7ÿ59.0% of total sorbed arsenic, respectively).

Exchangeable arsenic was the smallest fraction (0.4ÿ5.2% of total sorbed arsenic). The distribution of

sorbed arsenic(V) was related to the arsenic surface coverage. For arsenic surface coverages >~30% the

percentage of arsenic(V) associated with the amorphous Al oxide fraction increased and that associated

with the crystalline oxide fraction decreased. The arsenic(V) exchangeable fraction increased from 1.4

to 756 mmol kgÿ1

as surface coverage increased from 388 to 7855 mmol kgÿ1. The release of sorbed

arsenic(V) from red mud was greater at alkaline pH values (maximum release of ~33% of previously

sorbed arsenic at pH = 12), but for high arsenic(V) initial concentration (0.2 mM arsenic) considerable

amounts of arsenic (6.5% of previously sorbed arsenic) were released at pH 4, in accordance with the

dissolution of amorphous Al oxides in the red mud. The results obtained suggest a greater mobility of

sorbed arsenic(V) as its surface concentration approaches saturation.

KEYWORDS: red mud, arsenic, sorption, speciation, sequential extraction.

Introduction

THE extremely toxic properties of inorganic

arsenic compounds are well-known. Arsenic is

classi®ed by the International Agency for

Research on Cancer (IARC) as a human

carcinogen (IARC, 1980). The occurrence of

arsenic in the environment can have natural

(weathering of rocks and volcanism) or anthro-

pogenic (petroleum re®ning, thermal power

plants, pesticides, ceramics, non ferrous smelting,

gold-mine tailings, ¯y ash leachates and fertilizer

production) origins (Prasad, 1994). The introduc-

tion of arsenic compounds to the food chain,

mainly through ingestion of contaminated

drinking water, is especially dangerous.

Drinking water is contaminated by arsenic in

many areas of the world; for example, excessive

concentrations of arsenic in groundwater have

affected large populations in West Bengal and

Bangladesh (Anawar et al., 2004; Nickson et al.,

1998). Recent studies on chronic exposure to

arsenic have shown that the consumption of water

containing concentrations of arsenic <50 mg lÿ1

can cause cancer over long exposure periods

(WHO, 2001). As a consequence, the World

Health Organisation has recently reduced their

recommended provisional guideline value for

arsenic in drinking water from 50 to 10 mg/l

(WHO, 1993).

In response, the search for and development of

new technologies for the removal of arsenic from

* E-mail: edrubi@usc.es

DOI: 10.1180/0026461056950273

Mineralogical Magazine, October 2005, Vol. 69(5), pp. 591±600

# 2005 The Mineralogical Society

aqueous systems have increased considerably.

Sorption methods are promising because they are

simple to perform, can be used in small-scale

treatment plants or household systems and are

easy to operate (GencË-Fuhrman et al., 2004a).

Several materials have already been proposed for

the removal of metals and metalloids from water,

including natural materials and specially designed

technical particles (Daus et al., 2004). Amongst

proposed sorbents for the removal of arsenic from

aqueous solutions are activated carbon (Daus et

al., 2004), activated bauxite (Gupta and Chen,

1978), activated alumina (Lin and Wu, 2001),

amorphous Al hydroxide (Anderson et al., 1976),

amorphous iron hydroxide (Pierce and Moore,

1980), hematite (Prasad, 1994), granular Fe

hydroxide (GIH) (Driehaus et al., 1998), zeolites

(Shevade and Ford, 2004) and clays (Frost and

Grif®n, 1977).

Red mud is an oxide-rich residue generated by

alumina production from bauxite using the Bayer

process. About two tons of red mud is generated

per ton of metallic aluminium. World production

of red mud is roughly 60 million tons per year

(Glenister, 1987). Consequently, efforts have

been made to use the red mud bene®cially, for

example for leveÂe construction material, road

embankments, land®ll cover, synthetic soils,

fertilizer ®llers, remediation of coastal erosion,

manufacture of bricks, production of ceramic

glazes, as a ®ller for polymer reinforcement and,

of course, as a cheap adsorbent for removal of

potentially toxic metals, phosphorous and dyes

(Apak et al., 1998; Arias et al., 1999; Gupta and

Sharma, 2002; LoÂpez et al., 1998), and inorganic

arsenic from water and wastewater (AltundogÆan

et al., 2000; Rubinos et al., 1998a), in view of its

high Fe, Al and Ti oxides concentrations.

Recently, several studies have tested a derivative

product of red mud, BauxsolTM

(trademark name

of seawater-neutralized red mud), as an uncon-

ventional sorbent for arsenic removal from

aqueous solutions (GencË et al., 2003; GencË-

Fuhrman et al., 2004a,b). The most recent

studies on the utilization of red mud for the

removal of arsenic have been directed towards the

search for treatments to improve removal

ef®ciency. However, to our knowledge, the

distribution of adsorbed arsenic on red mud and

its implications for arsenic removal have not been

previously investigated, bearing in mind the

heterogeneous composition of red mud. The

objectives of this study are: (1) to determine the

distribution of sorbed arsenic(V) among different

geochemical fractions in red mud; and (2) to

understand the desorption behaviour of arsenic(V)

in relation to the distribution of arsenic(V) in the

different phases of the sorbent.

Experimental methods

Red mud

Red mud used in the present study was obtained

from ALCOA Europe Aluminium factory, Lugo,

Spain. The red mud was air-dried, crushed and

sieved (0.250 mm sieve). The chemical composi-

tion, determined after acid digestion, shows that

main constituents of the red mud employed in this

study (%w/w) are: 37.22Ô0.33% Fe2O3,

20.10Ô0.59% TiO2, 12.40Ô1.07% Al2O3,

6 .30Ô0 .20% CaO, 4 .64Ô0 .41% Na2O,

3.81Ô0.16% SiO2, 0.51Ô0.02% P2O5, 0.36Ô0.02%

ZrO2, 0.30Ô0.01% Cr2O3, 0.14Ô0.00% MgO, loss

on ignition 11.34Ô0.02%. Red mud composition

depends on bauxite origin, although typically it is

rich in Fe oxides (25ÿ40%) and Al oxides

(15ÿ20%) (Lombi et al., 2002). The main

mineral components (as determined by X-ray

diffraction) are hematite, rutile, magnetite, boeh-

mite, ilmenite and zeolite-type minerals. The

zeolite content, determined by a cation exchange

capacity method (Rubinos et al., 1998b) is ~9%.

The surface area of the red mud, determined by the

N2 adsorption-BET method, is 23.7 m2gÿ1

and its

pH (as a 1:5 mixture in water) is 9.7Ô0.1. Particle-

size distribution analysis, performed by wet sieving

(for the fraction >50 mm) and the pipette method

(for the fraction <50 mm) showed a predominance

of silt and clay fractions (80%). The cation

exchange capacity (CEC) of the red mud, as

determined by the ammonium acetate (pH 7)

method, is 10.83Ô0.02 cmolc kgÿ1. It is important

to note that ~50% of the total CEC of red mud is

zeolite CEC (Rubinos et al., 1998b). Selective

dissolution analyses performed showed that the red

mud studied contains 196Ô7 mg gÿ1

of crystalline

and amorphous Fe, extracted by dithionite-citrate-

bicarbonate (DCB) (Mehra and Jackson, 1960).

Amorphous Fe and Al contents in red mud,

determined by extraction with 0.2 M ammonium

oxalate at pH 3 (Schwertmann, 1964), are

2.4Ô0.1 mg gÿ1

and 25Ô1.4 mg gÿ1, respectively.

Arsenic analysis

All arsenic analyses were performed by hydride

generation atomic absorption spectrophotometry

(HGAAS) using a Perkin-Elmer MHS-10 hydride

592

D. A. RUBINOS ET AL.

unit on an atomic absorption spectrophotometer

(Perkin-Elmer M2100). The system was cali-

brated using standard solutions of arsenic

prepared by dilution of a commercial standard

arsenic solution (1000 mg lÿ1

of arsenic). To

generate the AsH3, samples and standards were

reacted with a 3% NaBH4/1% NaOH solution (as

reducing agent) and 1.5% HCl solution. The

calculated detection limit for these conditions was

2 mg lÿ1

of arsenic. All solutions were prepared

with deionized MilliQPLUS

water.

Distribution of sorbed arsenic(V) on red mud

To study the distribution of sorbed arsenic(V) on

red mud, ®rst a series of samples of air-dried red

mud (1 g) were equilibrated in polypropylene

centrifuge tubes with 20 ml of solution containing

arsenic(V) (as Na2HAsO4´7H2O) and 0.1 M NaCl

(as background electrolyte). Initial arsenic(V)

concentrations employed ranged from 0.02 to

1 mM. The suspensions were shaken at room

temperature (20Ô1ëC) in a rotary shaker. The

arsenic(V) sorption stage was performed at natural

pH value of red mud (~9.2) without adjustment of

suspension pH during the equilibration step. After

24 h of shaking, suspensions were centrifuged

(14000 g, 10 min) and ®ltered through

WhatmanTM

40 ®lter papers (particle retention

rating at 98% ef®ciency = 8 mm). The concentra-

tion of arsenic in the ®ltered solution was analysed

by HGAAS and the concentration of sorbed

arsenic(V) on the red mud calculated as the

difference between the initial and ®nal concentra-

tion of arsenic in solution. Losses of arsenic by

adsorption to the centrifuge tubes or ®lters were

tested by preparing identical arsenic(V) solutions

and subjecting them to the same treatment as the

samples in the absence of red mud. The

concentration determined in these solutions was

assumed to be the arsenic(V) initial concentration.

Three arsenic(V)-loaded red mud samples were

prepared for each arsenic(V) concentration.

Immediately after the loading step, the red mud

samples containing arsenic were chemically

fractionated using the ®ve step sequential

extraction procedure for arsenic proposed by

Lombi et al. (1999) (Table 1). The extractant:red

mud ratio was 25:1. Sequential extractions

determine only operationally de®ned speciation

(Nirel and Morel, 1990), and depend on factors

including the concentration of the reagents,

duration of the extractions and the selectivity of

the reagents used to attack a given phase. Results

cannot simply be extrapolated to ®eld conditions

(Cappuyns et al., 2002). Despite their limitations,

sequential extraction procedures are a useful

approach to estimate partitioning of elements

between different geochemical fractions.

After the period of extraction, the ®ltered

extracts obtained from each step of the sequential

extraction scheme were analysed for arsenic by

HGAAS. The amounts of arsenic extracted in

each step were corrected (estimating the volume

of entrained solution after decanting the super-

natant by the difference in tube weight compared

to initial tube weight, accounting for the mass of

red mud) for arsenic contribution from previous

extraction step (Jackson and Miller, 2000).

Experiments were run in triplicate.

Arsenic(V) leaching at different pH

With the purpose of relating the distribution of

arsenic(V) between the different phases of the red

mud and the mobility of sorbed arsenic(V) on red

mud, we studied the remobilization of previously

sorbed arsenic(V) on red mud as a function of

solution pH. For this, arsenic(V)-loaded red mud

samples, prepared by reaction of 0.5 g of red mud

with 25 ml of 0.1 M NaCl solutions containing

0.02 mM or 0.2 mM As(V), were resuspended in

TABLE 1. Sequential extraction procedure for arsenic fractionation (Lombi et al., 1999).

Step Treatment Fraction

1 (NH4)2SO4 0.05 M ÿ 1 h shaking Exchangeable

2 NH4H2PO4 0.05 M ÿ 1 h shaking Specifically sorbed

3 NH4F 0.05M pH 7.0 ÿ 1 h shaking Al and organic matter-associated

4 NH4-oxalate 0.2 M pH 3.25 ÿ 4 h shaking

in the dark

Bound to amorphous oxides

5 NH4-oxalate 0.2 M + ascorbic acid 0.1 M ÿ

pH 3.25. 30 min shaking in water bath at 96ëC

Bound to crystalline oxides

SPECIATION OF ADSORBED AS(V)

593

arsenic-free 0.1 M NaCl solutions, the pH of which

was adjusted to the same pH value as that in the

arsenic(V)-loading step. pH adjustment was by

addition of small quantities of 0.1 M HCl or 0.1 M

NaOH using a Metrohm 702SM automatic titrator.

The pH values studied were moderate acidic (pH

4), neutral pH (pH 7), equilibrium pH of red mud

in the release solution without addition of base or

acid (pH 8.3) and an extremely alkaline value (pH

12). The suspensions were shaken for 1 h in a

rotary shaker at room temperature, then centrifuged

and ®ltered (WhatmanTM

40 ®lters). The same

process was repeated for the centrifuged solid

samples with another 25 ml of arsenic-free

solution and the pH of the suspensions was

readjusted again to the initial corresponding pH

value. After the shaking period, the suspensions

were centrifuged and ®ltered and the ®ltrates

obtained from the two desorption steps were

mixed and analyzed for arsenic by HGAAS. Al,

Fe, Si and Ca were also analyzed in the extracts by

atomic absorption spectrophotometry (AAS) to test

the solubility of the main components of red mud

at the pH values studied. The experiments were

conducted in triplicate.

Statistical analysis

Analyses of variance (ANOVA) and least

signi®cance difference tests (LSD) were

performed on the data using version 12.0 of the

SPSS for WindowsTM

program.

Results and discussion

Distribution of sorbed arsenic(V) on red mud

The arsenic loading procedure resulted in samples

of red mud with concentrations of sorbed

arsenic(V) of 0.39, 1.91, 3.58, 6.68 and 7.86

mmol of As kgÿ1. These values represent 4.9,

24.1, 45.3, 84.5 and 99.4% of the maximum

arsenic(V) sorption capacity for red mud,

calculated by ®tting the experimental sorption

data to the Langmuir equation (data not shown).

The red mud is a heterogeneous solid with a

number of components to which the arsenic(V)

species in solution may show different af®nities.

Therefore, arsenic species may sorb preferentially

onto some components of red mud to which they

show greater af®nity. As the arsenic(V) concen-

tration in solution increases and red mud

approaches saturation, arsenic will also sorb on

the `lower af®nity' components of red mud. It is

possible that this aspect can be re¯ected in the

distribution of sorbed arsenic(V) species between

the different components of red mud as a function

of surface coverage. Moreover, the nature and

strength of the bonds established between the

arsenic(V) species and the different phases of the

sorbent cannot be the same. Also, the components

of the red mud present different solubility

behaviour. This aspect would directly affect the

remobilization of associated arsenic(V), which in

turn would affect the net ef®ciency of the removal

process.

The amounts of arsenic extracted in each step

of the sequential fractionation for the arsenic-

loaded red mud samples are shown in Table 2.

The arsenic recovered with the sequential

extraction ranged between 74 and 99% of the

sorbed arsenic(V). The distribution of arsenic

among the different (operationally de®ned) phases

of the sorbent, expressed as percentage of

recovered arsenic, are shown in Fig. 1.

Two main aspects must to be noted. First, most

sorbed arsenic on red mud was extracted in steps 4

TABLE 2. Arsenic extracted in each step of the sequential fractionation procedure for the As(V)-loaded red

mud samples (data in mmol of As kgÿ1

of red mud). The ratio between arsenic extracted (% of total

extracted) in steps 5 and 4 is also shown (standard deviation in parenthesis, N = 3).

As(V)RM1

Step 1 Step 2 Step 3 Step 4 Step 5 AsASC/AsOX (%)2

388 1.4 (1.3) 46.7 (5.4) 17.2 (4.2) 92.7 (1.3) 227.3 (3.2) 2.45 (0.00)

1909 17.3 (4.5) 352.3 (50.4) 58.8 (4.3) 474.7 (25.8) 499.9 (19.2) 1.06 (0.10)

3580 55.1 (9.9) 556.8 (19.0) 119.4 (18.6) 1189.9 (129.3) 807.8 (55.7) 0.71 (0.17)

6680 168.2 (23.8) 1227.9 (11.4) 117.8 (30.9) 2102.5 (178.9) 1181.8 (102.7) 0.57 (0.10)

7855 755.5 (56.9) 1908.0 (28.9) 316.8 (139.1) 2739.7 (120.8) 1779.0 (145.1) 0.65 (0.02)

1As(V)RM is sorbed arsenic(V) on red mud

2AsASC and AsOX are the arsenic percentages extracted in steps 5 and 4, respectively.

594

D. A. RUBINOS ET AL.

(by ammonium oxalate) and 5 (by ammonium

oxalate + ascorbic acid), although signi®cant

amounts were also extracted in step 2 (displaced

by ammonium phosphate). Second, the distribu-

tion of sorbed arsenic(V) was related to the

arsenic(V) concentration in red mud. Ammonium

sulfate (step 1) extracted the least arsenic. This

fact is important because this extractant is

expected to remove oxyanions from exchangeable

sites (Lombi et al., 1999); therefore this fraction

represents weakly bound arsenic, considered to be

the most bioavailable and most easily leached to

water (Lombi et al., 2000). Another important fact

is that as the concentration of sorbed arsenic on

red mud increased from 0.39 to 7.86 mmol kgÿ1,

the percentage of exchangeable arsenic also

increased from 0.4(0.3) to 5.2(0.4)% of total

sorbed arsenic (standard deviation in parenthesis);

this means that as the red mud approaches

saturation with arsenic(V) a greater fraction of

sorbed arsenic is susceptible to easy remobiliza-

tion. In any case, the amount of sorbed arsenic

corresponding to this fraction is small compared to

the amounts of arsenic extracted in steps 4 and 5.

Ammonium phosphate (step 2) extracted

considerably more arsenic than ammonium

sulphate. This is not surprising because the

capacity of phosphate to displace arsenic is well

documented (Jackson and Miller, 2000). The

percentage of sorbed arsenic displaced from

sorption sites on red mud by phosphate was

in¯uenced by arsenic(V) concentration. As the

sorbed arsenic concentration increased from 0.39

to 1.91 mmol kgÿ1, a sharp increase in the arsenic

extracted in this fraction was observed. For

arsenic concentrations above 1.91 mmol kgÿ1

the differences were not signi®cant. This must

be taken into account if red mud is used to remove

arsenic(V) from solutions with high concentra-

tions of phosphate, especially for the case of

heavily arsenic-contaminated systems.

Step 3 (0.05 M NH4F) extracted less arsenic

than step 2 and, for the two greatest arsenic(V)

concentrations, the amounts of arsenic extracted

were comparable to, or slightly less than, the

exchangeable arsenic fraction. The percentage of

sorbed arsenic associated with this fraction was

not dependent on the arsenic surface concentra-

tion and only represented ~3ÿ4% of total

extracted arsenic. The NH4F-extractable arsenic

has been associated with poorly ordered alumi-

nosilicates (Lombi et al., 2000) and organic

matter, although this is not applicable to the red

mud, since it does not contain organic matter. The

amounts of dissolved Al and Si were small

(82Ô11 and 124Ô14 mg kgÿ1, respectively),

compared with the amounts of Al and Si dissolved

in step 4.

Most arsenic was extracted with NH4-oxalate

(between 24.1(0.6) and 43.8(3.0)%) and NH4-

oxalate + ascorbic acid (between 24.7(2.6) and

59.0(1.4)%). The distribution of sorbed arsenic in

these two fractions was related to the arsenic

concentration in red mud (Fig. 1). As arsenic

surface coverage increased from 4.9 to 45.3%, the

percentage of arsenic extracted in step 4 also

increased from 24.1(0.6) to 42.9(4.7)% (standard

deviation in parenthesis). For arsenic surface

coverages >45.3% the differences in the percen-

tage of arsenic associated with this fraction were

not signi®cant. Ammonium oxalate dissolves

am o r p h o u s F e a n d A l c om p o u n d s

(Schwertmann, 1964); therefore the extracted

arsenic in this fraction represents arsenic asso-

FIG. 1. Distribution (% of total arsenic extracted) of sorbed arsenic(V) on red mud as a function of arsenic surface

concentration. Error bars represent the standard deviation of three replicates. Different letters for each step of the

extraction scheme represent statistically signi®cant differences (LSD test) at P <0.05 between the percentages of

arsenic(V) extracted for each sorbed arsenic(V) concentration on red mud.

SPECIATION OF ADSORBED AS(V)

595

ciated with amorphous oxides of the red mud. The

amounts of Al and Si extracted in step 4 were

26Ô1.4 and 2.5Ô0.4 mg gÿ1, respectively. The

ability of amorphous oxides to bind arsenic is well

established (Anderson et al., 1976) and several

works have found a strong correlation between

amorphous oxide content and arsenic(V) sorption

parameters for soils and sediments (Livesey and

Huang, 1981).

The percentage of arsenic extracted with

oxalate + ascorbic acid decreased from ~60 to

~25% of the total extracted arsenic as the

concentration of sorbed arsenic increased from

0.39 to 7.86 mmol kgÿ1, and, speci®cally, we

observed a sharp decrease in the arsenic extracted

as the arsenic surface coverage increased from

0.39 to 1.91 mmol kgÿ1

(Fig. 1).

When comparing the percentages of arsenic

extracted in step 5 with those extracted in step 4

we observed a reduction from 2.5 to ~0.6 as

surface coverage increased (Table 2). This shows

a preferential association of arsenic(V) with

crystalline oxides of red mud at low arsenic(V)

concentrations. It has been shown that arsenic

binds preferentially to Fe oxides in soils, and to a

lesser degree to Al oxides (Wauchope, 1975). At

surface coverages >~30%, and the sites in the

crystalline oxide surfaces of red mud become

occupied, arsenic(V) tends to sorb preferentially

onto amorphous oxides of red mud, probably a

consequence of their greater surface area and

porous structure. This suggests greater arsenic(V)

sorption capacity for amorphous oxides than for

crystalline oxides of red mud. Higher arsenic(V)

sorption capacities have been reported for

amorphous Al hydroxides than crystalline Fe

oxides (GarcõÂa-SaÂnchez et al., 2002).

Some sorbed arsenic(V) on red mud was not

extracted with the sequential extraction scheme

employed. This can be explained by the fact that

red mud contains 20% TiO2 as rutile. The ability

of rutile to sorb arsenate to a signi®cant degree

has been described (Fordham and Norris, 1979).

TiO2 in red mud is extremely stable, remained

undissolved (85%) even in strong acid (6 M HCl)

solutions.

It was observed that ~80% of sorbed arsenic(V)

was associated with the two oxalate fractions for

small arsenic concentrations, and this percentage

decreased to ~60% for high arsenic surface

concentrations. Arsenic associated with oxalate-

extractable fractions represents less mobile forms

of arsenic (Lombi et al., 1999), therefore the

reduction in the percentage of sorbed arsenic

associated with these fractions observed at high

arsenic(V) concentrations must be appreciated.

Selective dissolution analyses of red mud

resulted in calculated ratios Alox/Altotal = 0.38

and Feox/Fetotal = 0.007, i.e. the amorphous Fe

oxides content in red mud represent <1% of total

Fe, whereas amorphous Al oxides represent ~38%

of total Al content. Since at surface coverages

>~30%, arsenic is extracted predominantly in step

4, and considering the low amorphous Fe oxide

content of the red mud, the relative contribution

of amorphous Al oxides to the overall sorption

process increases above this surface coverage.

The predominant association of arsenic(V) with

amorphous Al oxide fraction at high surface

concentrations is very important, because some

amorphous aluminum compounds in red mud are

aluminosilicates integrated in the so-called

desilication product (DSP). The DSP is formed

during the Bayer process by reprecipitation of

dissolved silicates as sodium-alumino-silicates.

The DSP is not a single compound, but rather a

series of zeolites. Glenister and Thornber (1985)

observed that at pH values near 4 the Al and Si of

red mud show a sharp increase in solubility,

suggesting that at this pH the zeolite materials of

red mud could be dissolved. Therefore, if part of

the amorphous Al compounds are dissolved at

these pH values, the associated arsenic can

potentially be released. The results obtained

from the arsenic remobilization experiments at

different pH values seem to con®rm this

hypothesis.

Arsenic(V) leaching at different pH

Figure 2 shows the amounts of arsenic released at

pH 4, 7, 8.3 and 12 for two arsenic(V) initial

concentrations (0.02 and 0.2 mM). In general, the

amounts of arsenic released were small. It has

been suggested that the remarkably low reversi-

bility of arsenic(V) sorption on red mud indicates

that the mechanism governing the sorption

process involves chemisorption (GencË-Fuhrman

et al., 2004a). It has been shown that arsenate

forms inner-sphere surface complexes on both Al

and Fe oxides (Goldberg and Johnston, 2001; Sun

and Doner, 1996). Also, ligand exchange has been

proposed as the removal mechanism for adsorp-

tion of arsenate onto zeolites (Shevade and Ford,

2004). Rubinos et al. (2002) observed little ionic

strength dependence of arsenic(V) sorption on red

mud as a function of solution pH and shifts

towards higher pH values in titration curves of red

596

D. A. RUBINOS ET AL.

mud suspensions with increasing arsenate concen-

tration, suggesting an inner-sphere adsorption

mechanism for the interaction between

arsenic(V) and red mud.

Arsenic release from red mud was a pH-

dependent process. For low arsenic(V) surface

coverage, the amount of arsenic released from red

mud increased progressively with increasing pH,

with a maximum release at pH 12, where ~18% of

sorbed arsenic was released. For the high

arsenic(V) concentration, the maximum quantity

of arsenic released was also observed for the

strong alkaline pH (~33% of sorbed arsenic), but a

signi®cant amount of arsenic was also released at

pH 4 (~7% of sorbed arsenic). The ability of OHÿ

ions to displace arsenic from sorption sites is well

documented (Jackson and Miller, 2000) and

results from ligand exchange of sorbed arsenic

species with OHÿ

, with readsorption being

disfavoured by the resultant negative surface

charge at the oxide surface (Jackson and Miller,

2000). The enhanced arsenic desorption in alka-

line media has been observed for activated

BauxsolTM

and NaOH has been proposed as a

desorption agent for the regeneration of exhausted

arsenic-sorbents, including BauxsolTM

(GencË-

Fuhrman et al., 2004a). These observations can

be explained by the solubility behaviour of the

main components of red mud and the distribution

of sorbed arsenic(V) on red mud. The amounts of

Fe, Al, Si and Ca dissolved at pH 4, 7, 8.3 and 12

are listed in Table 3. The amounts of dissolved Al

and Si were much greater at pH 4 compared with

the other pH values and represent ~12% and 50%

of the total Al and Si oxide contents in red mud.

This solubility behaviour is in accordance with

the observations of Glenister and Thornber

(1985). The amounts of dissolved Fe were

FIG. 2. Arsenic(V) released from red mud (data in mg of As kgÿ1

of red mud) as a function of solution pH and As(V)

concentration. Error bars represent the standard deviation of three replicates. Different letters for the same arsenic(V)

concentration represent statistical differences (LSD test) at P <0.05 between the amounts of arsenic(V) released from

red mud at different pH values.

TABLE 3. Amounts of Fe, Al, Si and Ca dissolved from red mud as a function

of solution pH (data in mg /100 g of red mud)*.

pH Fe Al Si Ca

4 31.2 (0.9) 762.3 (2.5) 882.0 (97.9) 893.0 (79.2)

7 4.3 (0.4) 0.1 (0.0) 30.5 (5.7) 169.0 (9.9)

8.3 2.0 (0.8) 0.1 (0.1) 1.0 (1.0) 37.1 (10.6)

12 1.2 (0.4) 42.2 (18.8) 1.0 (1.0) 2.0 (0.9)

* N = 3, standard deviation in parenthesis

SPECIATION OF ADSORBED AS(V)

597

extremely small for all the pH values studied (a

maximum of only 0.12% of total Fe oxides

content at pH 4) re¯ecting the stability and poor

solubility of crystalline Fe oxides of red mud.

The increase in arsenic release observed at pH

4 for the high arsenic(V) concentration, but not

for the low arsenic(V) concentration, can be

explained considering the distribution of arsenic

in red mud. As we have seen before, at low

arsenic(V) concentrations, arsenic tends to bind

mainly to crystalline Fe oxides. Since these

compounds are not dissolved at pH 4, the

associated arsenic is not released, so the main

factor in¯uencing the arsenic release for low

concentrations will be the increasing concentra-

tion of OHÿ

ions in solution as the pH increases.

On the other hand, as arsenic(V) concentration

increases, the sorbed arsenic tends to associate

predominantly with amorphous Al compounds

(Fig. 2), that are in part soluble at pH 4 (Table 3),

thus releasing the associated arsenic. At alkaline

pH (12), the release of arsenic is again attributed

to the displacement of arsenic by OHÿ

ions.

Conclusions

The results obtained show that distribution of

sorbed arsenic(V) between the different geochem-

ical phases of red mud and the relative

contribution of the active components for

arsenic(V) sorption depend on arsenic(V)

surface concentration.

Most sorbed arsenic was associated with the

amorphous and crystalline oxide fraction as

observed from the amounts extracted in the two

acid ammonium oxalate steps, which show that

arsenic(V) is strongly sorbed by red mud.

However, it is important to note that as red mud

approaches its maximum arsenic(V)-sorption

capacity, the percentage of sorbed arsenic

associated with these two fractions diminish,

and the percentage of exchangeable (weakly

bound) arsenic increases. Moreover, for high

arsenic(V) concentrations, arsenic(V) associated

with the crystalline oxide fraction decreases and

arsenic(V) associated with the amorphous oxide

fraction increases, dominating at arsenic surface

concentrations above ~30% of maximum

arsenic(V) capacity for red mud. The dissolution

behaviour at different pH of the main components

of red mud involved in the arsenic(V) retention

process indicates that arsenic(V) associated with

the crystalline oxide fraction is less susceptible to

remobilization. Arsenic release from red mud is

in¯uenced by the pH of the release medium and

the arsenic concentration in the sorbent. Arsenic

release is enhanced in strongly alkaline media, but

for high arsenic surface concentrations arsenic

release occurs at acidic pH also, re¯ecting the

changes in the arsenic distribution in the red mud

as surface occupation proceeds. This must be

taken into account when using red mud for the

removal of arsenic(V) from heavily arsenic(V)-

contaminated systems.

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