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Hấp phụ và giải hấp Cd trong môi trường đất

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Adsorption and desorption of cadmium by goethite

pretreated with phosphate

Department of Plant and Soil Sciences, Stockbridge Hall, University of Massachusetts, P.O Box37245,

Amherst, MA 01003-7245, USA Received 11 October 2001; received in revised form 21 March 2002; accepted 4 April 2002

Abstract

The adsorption of Cd by oxides or soils have been extensively studied, however, the desorption has received rela-tively limited attention, especially in the presence of phosphate In this study, a batch equilibration method was used to investigate Cd sorption and desorption by goethite pretreated with phosphate Phosphate not only enhanced Cd ad-sorption, but also accelerated the adsorption process Compared with Cd adsorption by goethite alone, phosphate substantially moved the adsorption curves (edges) to lower pH range, indicative of enhancement of Cd sorption The

Cd adsorption by the pretreated goethite reached apparent equilibrium within 24 h at 20°C, while such equilibrium was not observed after 4 weeks in the absence of phosphate Cadmium was more readily released from phosphate-treated goethite It is believed that phosphate blocked the pores on goethite surface, which lead to the fast adsorption kinetics and high extraction percentage These results provided strong support for the diffusion of Cd into goethite particles

Ó 2002 Elsevier Science Ltd All rights reserved

Keywords: Cadmium; Adsorption; Desorption; Phosphate; Goethite; pH

1 Introduction

Cadmium (Cd) is one of the toxic trace metals, which

can be introduced into and accumulate in soils through

agricultural application of sewage sludge and fertilizers,

and/or through land disposal of metal-contaminated

municipal and industrial wastes Chemical processes

strongly affect the fate and availability of Cd in soils It

is accepted that concentrations of heavy metals

includ-ing Cd in soil solution are most likely controlled by

sorption–desorption reactions on the surface of soil

col-loidal materials (Brummer et al., 1988; Ainsworth et al.,

1994; Backes et al., 1995; McLaren et al., 1998) As

major components of soil colloidal materials, iron and

manganese oxides play very important roles in the

sorption of heavy metals Goethite is the most wide-spread iron oxide in natural environments (Schwert-mann and Cornell, 2000), and has been well studied and used for sorption experiments (Fischer et al., 1996; Strauss et al., 1997; Manceau et al., 2000)

The sorption of heavy metals by soils or oxides has been extensively studied (Barrow, 1998; Gray et al., 1998; Eick et al., 1999; Christophi and Axe, 2000; O’Reilly et al., 2001) Results suggest that sorption ap-pears to be a multi-step process involving an initial fast adsorption followed by a slow adsorption and diffusion into solid particles Such diffusion might explain the low reversibility of heavy metal sorption to oxides (Ains-worth et al., 1994; Brummer et al., 1988; Barrow et al., 1989) However, this suggestion was not directly derived from experiments, but from the fitting of sorption data

to various diffusion equations (Barrow, 1986; Krish-namurti et al., 1999)

Previous studies (Christensen, 1984; Young et al., 1987; Swift and McLaren, 1991) indicate that desorption

www.elsevier.com/locate/chemosphere

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Corresponding author Tel.: 545-5212; fax:

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E-mail address: bx@pssci.umass.edu (B Xing).

0045-6535/02/$ - see front matter Ó 2002 Elsevier Science Ltd All rights reserved.

PII: S 0 0 4 5 - 6 5 3 5 ( 0 2) 0 0 1 6 7 - 4

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is as important as sorption, because it governs the rate

and extent of metal ions released from sorbents

Im-proved understanding of desorption characteristics may

allow us to better evaluate the bioavailability and

po-tential toxicity of trace metals in soils However,

com-pared with adsorption research, desorption of heavy

metals received relatively limited attention, especially in

the presence of phosphate (Davis and Upadhyaya, 1996;

McLaren et al., 1998) Phosphorus, one of the major

nutrients for plants, is widely distributed in soils and has

a high affinity for iron hydroxide surfaces Several

in-vestigators have shown that the adsorption of phosphate

on iron oxides may enhance cation adsorption (Kuo

and McNeal, 1984; Diaz-Barrientos et al., 1990; Venema

et al., 1997) However, the reduction of Cd adsorption

in the presence of phosphate in soils was reported by

Krishnamurti et al (1999); they attributed this reduction

to the formation of Cd–phosphate complexes in

solu-tion We want to further examine Cd release in the

presence of phosphate Therefore, the objectives of this

research were to study the Cd sorption by goethite

pretreated with phosphate, and to determine the effect of

phosphate treatment on Cd desorption kinetics from

goethite

2 Materials and methods

2.1 Goethite preparation

Goethite was prepared using the method of

Schw-ertmann et al (1985) Briefly, ferrihydrite was

precipi-tated by rapidly adding 400 ml of a 1 M FeNO3solution

to 3600 ml of KOH solution to yield a final OH

con-centration of 0.7 M The suspension was then stored in

polyethylene bottles at 10 °C for 2months After

syn-thesis, the goethite was washed thoroughly with

de-ionized water to remove NO

3 and Kþ, and finally dried

at 60°C and ground in an agate mortar The prepared

sample showed a typical X-ray diffraction pattern of

goethite, and only 0.8% of total iron remained soluble in

oxalate solution, indicative of nearly complete

crystal-lization from ferrihydrite Our goethite was the same

sample as used in a surface topographic study by Fischer

et al (1996) The goethite had a BET surface area of 75

m2/g measured by the method described by Kruse and

Lagaly (1988)

2.2 Adsorption measurement

Goethite suspensions were prepared in 0.01 M

Ca(NO3)2 with 20 mg oxide per milliliter After

ultra-sonic dispersion, 1 ml aliquots were dispensed into 15 ml

polycarbonate tubes (20 mg/tube) Small amounts of

HNO3 and NaOH were gradually applied to the

sus-pensions to achieve a pH range between 3 and 8 The

tubes were then made up to 8 ml with 0.01 M Ca(NO3)2 and shaken on an rotary shaker for 14 days at 20°C to reach constant pH Because it was extremely difficult to achieve the same pH for replicates, a large number of samples were used at various pH values rather than employing duplicates or triplicates at a same pH Phosphate treatment was carried out as follows One milliliter 0.01 M phosphate [Na2HPO4 in 0.01 M Ca(NO3)2] was added to the goethite suspension After the samples were thoroughly mixed on a rotary shaker

at 30 rpm for 7 days at 40°C, 1 ml 104M Cd [Cd(NO3)2

in 0.01 M Ca(NO3)2] was added to the tubes (i.e., the goethite suspensions) to make up to the 10 ml final volume Then, the suspensions were ultrasonically dis-persed and shaken for various reaction times of 15 min,

24 h, 7 days, and 4 weeks at 20 °C At the end of each reaction time, the suspensions were centrifuged and the supernatants were collected for measurements of

pH and Cd concentration The Cd adsorption in the absence of phosphate was also prepared following the same procedure at the same time The Varian atomic absorption spectrometer (AAS) with graphite furnace (SpectrAA 55) was used to determine the Cd concen-tration

2.3 Desorption measurement

Cadmium extractions were carried out immediately after Cd adsorption The 5 M HCl solution was used as

an extractant Briefly, samples were centrifuged at 7840

g at the end of adsorption After the supernatants were removed carefully, fresh 5 M HCl (10 ml) was added and mixed with solid particles by ultrasonic dispersion The new suspensions were again mixed on the rotary shaker for a predetermined extraction time Due to the disso-lution of goethite in 5 M HCl (Cornell et al., 1976), extraction times were designed to be 5 min, 30 min, 2h, and 4 h for each tube, that is, each tube was sequentially extracted four times At the end of each extraction, the samples were centrifuged at 7840 g for 10 min The su-pernatants were decanted from the tubes and the con-centration of Cd was measured by AAS

3 Results and discussion

Goethite and phosphate are commonly found in soils Under natural conditions, the interaction between phosphate and goethite is usually in equilibrium before any addition of exogenous heavy metals into soils In this respect, the sorption characteristics of Cd by goethite pretreated with phosphate may resemble the sorption– desorption reactions occurring in the natural environ-ment

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3.1 Cadmium adsorption

Cadmium adsorption by goethite was highly

pH-dependent Almost all Cd was adsorbed at high pHs

for the initial concentration of 105 M After 15 min

reaction, nearly all Cd were adsorbed at pH P 7:2in

the absence of phosphate, and at pH P 5:5 in the

pres-ence of phosphate Fig 1 shows that Cd adsorption

was greatly enhanced in the presence of phosphate

Al-though phosphate did not change the shape and slope of

adsorption curves, the adsorption edge shifted distinctly

to lower pH, indicating the substantial enhancement of

Cd adsorption The pH50(the pH value at which 50% of

heavy metal ions were adsorbed) was reduced by up to

1.7 pH units (Table 1) At the reaction time of 15 min,

for example, about 75% at pH 5 and 100% at pH 6 of Cd

ions were adsorbed by the goethite pretreated with

phosphate, while only 1.5% and 23% were adsorbed at

the same pHs in the absence of phosphate The

en-hanced Cd adsorption could be attributed to the

re-duction of the pHzpc (zero point of charge) of goethite

and surface potential after the phosphate treatment

(Kuo and McNeal, 1984) Phosphate sorption increases

surface negative charge and decreases the electrostatic potential near the solid surface, which might cause an increase in the Cd surface loading This result is con-sistent with that reported by Venema et al (1997)

Phosphate also altered the adsorption kinetics of

Cd by goethite The continuous decrease of pH50 as a function of reaction time implies that the Cd adsorption

on the untreated goethite did not reach equilibrium after

4 weeks reaction (Table 1) At pH 6, for example, 23% of

Cd was adsorbed at 15 min, while 32%, 37%, and 55% of

Cd was adsorbed at reaction times of 24 h, 7 days, and 4

Fig 1 Percent Cd adsorption by the phosphate-treated goethite () and untreated goethite () as a function of pH at different reaction times and a constant temperature of 20 °C.

Table 1

pH 50avalues of Cd at initial concentration 10 5 M and 20 °C Reaction time

Phosphate-trea-ted goethite

Untreated goethite

Difference

a pH 50 is defined as such a pH value at which 50% of initial metal ions are adsorbed.

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weeks, respectively However, except for 15 min contact

time, little change in the amount of Cd adsorbed was

observed between times in the presence of phosphate

These results indicate that the Cd adsorption on goethite

in the presence of phosphate needed 6 24 h to reach

apparent equilibrium at 20°C The acceleration of Cd

adsorption may be caused by blocking and/or

occupy-ing of meso- and micro-pores on goethite surface or

between goethite domains by phosphate (Madrid and de

Arambarri, 1985; Torrent et al., 1990; Strauss et al.,

1997) As a result, Cd ions might not be able to diffuse

further into the pores and the overall adsorption process

was faster

These meso- and micro-pores have been proposed as

pathways for diffusion of metal ions into oxide matrix

following initial fast adsorption (Brummer et al., 1988;

Bibak et al., 1995) Fischer et al (1996) reported that the

pores on goethite surface were 20–30 nm wide and

be-came narrower towards the interior of the crystals to 2

nm or less The diffusion of ions may be not limited from

the pore entrances (20–30 nm), but with increasing pore

depth and decreasing pore size, only small ions can

penetrate into the narrow pores Because phosphate ions

are bigger than Cd ions (0.22 nm in radius for

phos-phate, 0.097 nm for Cd2þ), it is possible for phosphate

ions to diffuse into and block such pores during

pre-treatment For some micro-pores, phosphate may only

diffuse partway into the pores or block the entrance,

resulting in rapid sorption kinetics

3.2 Cd desorption

Several authors have demonstrated that the sorption

process of trace metals is not completely reversible, and

explanations have been proposed for such observations,

including diffusion of trace metals within oxide particles

or into micro-pores (Brummer et al., 1988; Backes et al.,

1995; Gray et al., 1998), precipitation (Farrah and

Pickering, 1978), incorporation of metals into oxides

(McKenzie, 1970; Ainsworth et al., 1994), and

re-adsorption (Davis and Upadhyaya, 1996) A wide range

of solutions were used as extractants in these studies,

including Ca(NO3)2, HNO3, NaCl, EDTA, and NTA

These solutions exhibit differences in their extraction

ability In our study, 5 M HCl was used as an extractant

because such a strong acid would effectively release Cd

from the goethite particle surface and prevent

re-adsorption of Cd (Farrah and Pickering, 1978) In the

present work, we focused on the samples with nearly

100% Cd adsorption; in this case, the pHs of initial

sorption reaction ranged from 5 to 8 Our results showed

that most Cd was released from goethite after a 400 min

extraction (cumulative time)

Fig 2shows the cumulative average percentage of Cd

released as a function of extraction time for the

phos-phate-treated goethite Because of a slight difference in

Cd recovery between pHs from 5 to 8, the average values were used to demonstrate the overall effect of contact time As one may see, there was no substantial difference

in Cd adsorption for reaction times beyond 24 h (Fig 1), but the Cd recovery decreased with increasing contact time (Fig 2) For 15 min adsorption, about 95% and nearly all of Cd adsorbed were released into solution after 5 and 400 min extraction, respectively But for 4 weeks adsorption, percentages were 82% at 5 min and 93% at 400 min One possible reason is that not all the pores were blocked by phosphate, which might be still available for Cd diffusion Another possibility is that the sorption of phosphate by goethite might not reach equilibrium during pretreatment As a consequence, phosphate could diffuse further into the pores during Cd sorption and Cd diffusion would be extended with in-creasing contact time Also, the dissolution rate of go-ethite–phosphate complex may become slower with time, which could cause lower Cd extraction

In an attempt to examine the effect of phosphate pretreatment on Cd desorption, the Cd extraction from goethite in the absence of phosphate was carried out for the 4 weeks sorption experiments (Fig 3) It is clear that the phosphate pretreatment greatly enhanced Cd de-sorption Moreover, we might have underestimated the difference in Cd desorption between the two systems because goethite treated with phosphate dissolves more slowly in 5 M HCl than the phosphate-free goethite (Strauss et al., 1997); more Cd would be released from phosphate-treated goethite assuming its dissolution rate

is the same as that of phosphate-free goethite and, thus, the difference on the percentage of Cd extraction would

be even greater than that we reported here Nevertheless,

Fig 2 Effect of contact time on the average percent Cd re-leased (on a cumulative basis for both time and Cd recovery) from the phosphate-treated goethite Some error bars are in-visible because the symbols are greater than the error bar size (error bar represents one standard deviation).

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results support our hypothesis that phosphate can block

pores of goethite, which hinders Cd diffusion into the

inner surface binding sites as Cd ions did in the absence

of phosphate As a result, more Cd accumulated on the

particle surface or near surface in the presence of

phosphate Such Cd distribution would be easier for

extraction

In this study, the samples were thoroughly mixed by

ultrasonic dispersion It is reasonable to assume that the

goethite particles were evenly dissolved from surface in

HCl solution Under this assumption, our results

sup-port the diffusion of Cd into solid phase or the

meso-and micro-pores of goethite particles with increasing

contact time (Brummer et al., 1988; Backes et al., 1995)

One may try to use re-adsorption or precipitation to

explain the results reported here However, we disagree

for the following reasons First of all, 5 M HCl is such a

strong extractant that the possibility of re-adsorption

would be ruled out Secondly, HCl would be able to

dissolve any precipitated hydrous oxide species of heavy

metals (Farrah and Pickering, 1978) If precipitation was

involved in the Cd sorption on goethite, more Cd would

be released from goethite at higher pHs of initial

sorp-tion reacsorp-tion due to the higher Cd sorpsorp-tion In fact, a

slightly decreasing Cd recovery was observed with

in-creasing pH from 5 to 8 for a given adsorption period

(Table 2), which can be explained well by diffusion In

the pH range from 5 to 8, the adsorption of phosphate

on goethite decreased from about 190 to about 165

lmol/g after 7 day sorption (Fig 4) Therefore, more

pores would be blocked by phosphate at lower pHs,

which would result in the increase of Cd extraction The

opposite would be true for high pHs

4 Conclusion

Phosphate treatment greatly influenced Cd sorption Not only did phosphate enhance Cd adsorption, but also accelerated the adsorption kinetics The adsorption edges shifted to lower pH substantially in the presence of phosphate, indicative of enhanced Cd adsorption The adsorption of Cd did not reach equilibrium after 4 weeks

of reaction in the absence of phosphate; however, for the

Fig 3 Average percent Cd released (on a cumulative basis for

both time and Cd recovery) from the phosphate-treated

go-ethite () and untreated goethite () after 4 weeks adsorption at

20 °C (error bar represents one standard deviation).

Table 2 Percent Cd released from the phosphate-treated goethite after 5 min extraction by 5 M HCl

released (%)

6.9294.0

6.5279.4

Fig 4 Amount of phosphate adsorbed by goethite after 7 days

at 10 3 M initial concentration and 40 °C, showing a gradual decrease of sorption with increasing pH.

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treated goethite, Cd adsorption reached apparent

equi-librium within 24 h, as indicated by a relatively constant

pH50 Cadmium was more readily released from the

phosphate-treated goethite Both adsorption and

extrac-tion results support the hypothesis that phosphate can

block/occupy meso- and micro-pores of goethite

parti-cles during the pretreatment, causing fast sorption and

desorption processes Therefore, these results provided

strong support for the diffusion of Cd into goethite

particles during sorption

Acknowledgements

Authors thank Dr G.W Brummer for his helpful

discussion and for providing the goethite material

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