Effects of pH, organic acids, and competitive cations on mercury desorption in soils

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Chemosphere 69 (2007) 1662–1669 www.elsevier.com/locate/chemosphere

Effects of pH, organic acids, and competitive cations on mercury desorption in soils Y.D. Jing

a,b

, Z.L. He

a,c,*

, X.E. Yang

a

a

c

MOE Key Laboratory of Environmental, Remediation and Ecosystem Health, College of Natural Resources and Environmental Sciences, Zhejiang University, Hangzhou 310029, Zhejiang, China b Department of Resources and Planning, Qufu Normal University, Jining 273165, China University of Florida, Institute of Food and Agricultural Sciences, Indian River Research and Education Center, Fort Pierce, FL 34945, USA Received 31 January 2007; received in revised form 11 May 2007; accepted 13 May 2007 Available online 27 June 2007

Abstract The effects of pH, organic acids, and competitive cations on Hg2+ desorption were studied. Three representative soils for rice production in China, locally referred to as a yellowish red soil (YRS), purplish clayey soil (PCS), and silty loam soil (SLS) and classified as Gleyi-Stagnic Anthrosols in FAO/UNESCO nomenclature, were, respectively, collected from Jiaxin County, Deqing County, and Xiasha District of Hangzhou City, Zhejiang Province. Most of the added Hg2+ was adsorbed at low initial concentrations (<2 mg l1). Desorption of the adsorbed Hg2+ in 0.01 M KCl (simulating soil solution) was minimal, but was significantly enhanced by the change of pH, and the presence of organic acids or competitive cations. The desorption of Hg2+ in the soils decreased with pH from 3.0 to 5.0, leveled off at pH 5.0–8.0, but increased with pH from 7.0 to 9.0. The presence of organic ligands enhanced Hg2+ desorption in the soils except for YRS, in which the addition of tartaric, malic, or oxalic acid reduced Hg2+ desorption at low concentrations (<104 M), but Hg2+ desorption generally increased with organic acid concentration. Citric acid was most effective in increasing Hg2+ desorption, followed by tartaric acid and malic acid; and oxalic acid was the least effective. Desorption of adsorbed Hg2+ increased with increasing concentrations of added Cu2+ or Zn2+. Applied Cu2+ increased Hg2+ desorption more than Zn2+ at the same loading rate. Capsule: The effects of organic acids and competitive cations on Hg desorption in soil–water system are related to their concentrations, basic chemical properties, and soil properties. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Adsorption–desorption; Competitive cations; Mercury; Organic acids; Soils

1. Introduction Mercury pollution has been widely recognized as a global environmental problem because of its volatility and toxicity (Agamuthu and Mahalingam, 2005; Zahir et al., 2005). In the past two decades, a great deal of attention has been paid to understanding the pathways and mecha-

*

Corresponding author. Address: University of Florida, Institute of Food and Agricultural Sciences, Indian River Research and Education Center, Fort Pierce, FL 34945, USA. Tel.: +1 772 468 3922x109; fax: +1 772 468 5668. E-mail address: zhe@ufl.edu (Z.L. He). 0045-6535/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2007.05.033

nisms by which Hg is transformed in the environment (Jitaru and Adams, 2004). Like other trace metals, the fate of Hg2+ in the environment is largely controlled by its adsorption and desorption reactions with various adsorbents (Schindler, 1967; Thanabalasingham and Pickering, 1985). The availability of Hg2+ to organisms is determined by its activity in soil solution, which is, in turn, controlled by both the solid and solution phase characteristics of soil. As a result, Hg2+ availability to organisms may vary considerably, depending on the nature of the adsorption– desorption processes in the soils. Mercury is non-essential for plants and animals but could be readily absorbed by crops and accumulated in human body through food chains. So the fate and behavior

Y.D. Jing et al. / Chemosphere 69 (2007) 1662–1669

of Hg in soil may influence its danger to human health. A detailed investigation of Hg2+ adsorption–desorption reactions is essential to understanding the fate and behavior of Hg2+ in the environment. Many environmental factors can interfere with the Hg adsorption–desorption process, which include: Hg speciation, soil pH, chloride ions, organic matter content, form and content of soil colloids, and competitive inorganic ions, etc. Although, extensive research has been devoted to Hg2+ sorption in soils, data on desorption reactions of Hg2+ in soils are limited. Among those factors, pH and Cl concentration are the key parameters in determining the speciation of Hg in soil solution. There is a direct correlation between soil pH and metal retention (Reddy and Patrick, 1977), and pH change can have a major effect on soil’s ability to retain heavy metals such as Hg (Harter, 1983). An understanding of Hg speciation and the related complex interactions is important to predict the fate and transport of metals in soil systems. Exudation of organic compounds by roots, microbial secretions, and plant and animal residues may influence ion solubility and uptake through their indirect effects on microbial activity, and root growth dynamics, and direct effects through acidification, chelation, precipitation, and oxidation–reduction reactions (Mench et al., 1988; Strom, 1997). Low molecular weight organic acids (LMWOA) are of particular importance due to metal chelating and complexing properties for mobilization of heavy metals (Mench et al., 1988; Wu et al., 2003). Different organic acids may affect differently the interaction between metal and soil. Hydrogen ions released by the carboxylic acid groups play a major role in metal dissolution. The positions and types of the functional groups were most important in determining whether an organic acid would complex metals and increase their potential leaching (Burckhard et al., 1995). Therefore, it is necessary to understand how and to what extent these organic acids may affect the behavior of heavy metals in soils. The objectives of this study were to investigate the thermodynamics of Hg adsorption–desorption in soils and the influence of pH, Cu2+, and Zn2+ on Hg desorption. The effects of organic acids including citric, tartaric, malic, and oxalic acids on desorption behavior of Hg2+ from soils were also examined at various concentrations commonly occurred in natural environments. 2. Materials and methods 2.1. Soil samples Three soils with contrasting properties were collected at 0–20 cm depth in Zhejiang Province, China. A silty loam soil (SLS) was collected from Huajiachi campus, Zhejiang University, Hangzhou, a yellowish red soil (YRS) from Deqing County, and a purplish clayey soil (PCS) from Jiaxing County. All soils belong to Gleyi-Stagnic Anthrosols in

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FAO/UNESCO nomenclature. After collection, soil samples were air-dried, ground, sieved through 2-mm mesh prior to use. Some basic physicochemical properties of the soils were determined following routine analytical methods (ACSSSC, 1983), as listed in Table 1. Labile Hg was extracted by 0.1 M HCl at a soil to solution ratio of 1:5 (SSSA, 1996) and total Hg in the soil sample was determined by aqua regia digestion method (SSSA, 1996). The concentrations of Hg in the extracts or digesters were measured by atomic fluorescence spectroscopy using atomic fluorescence spectrometer (AFS, 230E, Beijing Haiguang Instrument Co., Beijing, China). The concentrations of Hg in the extracts were determined following the method of Jones et al. (1997). In this procedure, Hg2+ in the samples is reduced to Hg0 vapor with KBH4, and transported into the detection cell of an atomic fluorescent spectrometer by ultrapure argon gas. Matrix blanks were checked to ensure no detectable background. KBH4 solutions (0.05%) were prepared before use by dissolving the required amount in 0.5% KOH solution. 2.2. Adsorption of Hg2+ Portions of 2.0 g air-dried soil were placed into 100-ml polypropylene centrifuge tubes, and 50 ml of 0.01 M KCl (pH 5.4) solution containing different levels of Hg2+ [as HgCl2] was added to each tube. The initial concentrations of added Hg2+ were 0.00, 0.12, 0.24, 0.40, 0.60, 0.80, 2.00, 8.00, 20.0, 40.0 mg l1. The suspensions were shaken at 200 rpm for 2 h at 25 °C and then equilibrated in a dark incubator for an additional 22 h, a time previously found to be sufficient for equilibration. At the end of the designated time, the suspensions were centrifuged at 2000g relative centrifugal force for 10 min and filtered. Ten milliliters of the filtrate were transferred into a 10-ml polypropylene centrifuge tube for measuring Hg2+ concentration using the AFS. Total amounts of adsorbed Hg2+ were calculated by the difference between the total applied Hg2+ and the amount of Hg2+ remaining in the equilibrium solution.

Table 1 Some physical and chemical properties of soil samples tested Item

SLS soil

YRS soil

PCS soil

pH (H2O) Total Hg (lg kg1) Avail.Hg (lg kg1) Organic C (g kg1) Avail. P (mg kg1) Total N (g kg1) CEC (cmol kg1) Free Fe oxide (Fe2O3) Size composition (g kg1) 2–0.02 (mm) 0.02–0.002 (mm) <0.002 (mm)

5.97 455 2.88 11.6 39.6 1.00 14.6 10.0

5.12 617 3.95 29.7 10.6 2.56 12.5 20.3

5.60 547 3.75 20.3 12.8 1.75 13.5 22.0

650 288 62

255 570 205

100 556 344

SLS = silty loam soil, YRS = yellowish red soil, PCS = purplish clayey soil; CEC = cation exchange capacity.

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For thermodynamic consideration in terms of adsorption isotherms, a series of experimental points for the adsorption of Hg2+ on soil were plotted. Two commonly used mathematical expressions for describing the adsorption equilibrium, i.e., Langmuir and Freundlich isotherm models were tested with the experimental data (Table 2). The most widely used isotherm equation for modeling thermodynamic data is the simple Langmuir model, which was originally developed to represent chemisorption on a set of well defined localized adsorption sites having the same sorption energy, independent of surface coverage and no interaction between adsorbed molecules. Maximum sorption is noticed when surface of the sorbent is covered with a monolayer of adsorbate. The Freundlich equation, most widely used mathematical description of adsorption, usually fits the experimental data over a wide range of adsorbate concentration. This equation gives an empirical expression encompassing the surface heterogeneity and the exponential distribution of active sites and their energies. 2.3. Desorption of Hg2+ Batch desorption of metals was accomplished by repeated replacement with metal-free KCl solution (pH 5.4) following adsorption. The tube with Hg-enriched soil residue separated from the supernatant solution by centrifugation was weighed to measure the residual Hg2+ in the trapped solution. Fifty milliliters of 0.01 M KCl were added to each tube containing the Hg-enriched soil residue. The suspensions were shaken at 200 rpm for 2 h at 25 °C and equilibrated for an additional 22 h. The equilibrated suspensions were then centrifuged at 2000g relative centrifugal force for 10 min and filtered. Ten milliliters of the filtrate were transferred into a 10 ml polypropylene centrifuge tube for measuring Hg2+ concentration. In order to estimate the affinity of Hg2+ in soils, desorption process was repeated five times. The non-extractable fraction of the adsorbed Hg2+ was obtained by the difference between the total adsorbed Hg2+ and the total recovered Hg2+ by five successive extractions with the KCl solution (pH 5.4) (Yu et al., 2002). 2.4. pH effects The tube with Hg-enriched soil residue separated from the supernatant solution by centrifugation was weighed Table 2 Isothermal characteristics of Hg2+ adsorption in the tested soils Soils

SLS soil YRS soil PCS soil

Langmuir equation

Freundlich equation

KL (mg kg1)

b

r2

1/n

KF

r2

111 213 189

0.23 0.12 0.13

0.9997 0.9999 0.9999

0.63 0.76 0.72

17.0 19.3 19.4

0.9871 0.9789 0.9832

SLS = silty loam soil; YRS = yellowish red soil; PCS = purplish clayey soil.

to measure the residual Hg2+ in the trapped solution. Fifty milliliters of 0.01 M metal-free KCl with different pH levels was added to each tube. The initial pH of KCl solution was 3.0, 4.0, 5.0, 6.0, 7.0, 8.0 and 9.0, and the initial Hg concentration for adsorption experiment was 20.0 mg l1. The procedure of desorption was the same as described above. 2.5. Organic acid effects A pH level, at which heavy metal adsorption could be compared was selected based on the following considerations: (1) Low pH values (<4.0) were not adequate because of weak metal retention by soils; and (2) As pH increases, aqueous metal cations hydrolyze, resulting in a series of soluble meal complexes. This masking effect of precipitation precludes comparison of the relative sorption affinities under near-neutral conditions (Elliott et al., 1986). Therefore, a pH of 5.4 was selected for assessing soil’s metal selectivity preference and this pH is close to the natural pH of the tested soils. The tube with the soil residue separated from the supernatant solution by centrifugation was weighed to measure the residual Hg2+ in the solution. Fifty millilitres of 0.01 M metal-free KCl (pH 5.4) with different organic acids were added to each tube containing the Hg-enriched soil residue. Four organic acids were selected for this study: tartaric, malic, citric, and oxalic acids. The concentrations of each organic acid used were 0, 105, 104, 103, 102, and 101 M in 0.01 M KCl. The initial Hg2+ concentration for adsorption experiment was 20.0 mg l1. The initial pH of desorption solution was adjusted to 5.4 with dilute HCl or NaOH. The procedure of desorption was the same as described above. 2.6. Copper and zinc effects The tube with the soil residue separated from the supernatant solution by centrifugation was weighed to measure the residual Hg2+ in the trapped solution. Fifty milliliters of 0.01 M KCl (pH 5.4) with different concentrations of Cu2+ or Zn2+ were added to each tube containing the Hg-enriched soil residue. The initial Hg2+ concentration for adsorption experiment was 20.0 mg l1. The initial Cu2+ or Zn2+ concentrations in the desorbing solution were 0.0, 1.0, 2.5, 5.0, 10.0, 15.0 mM. The procedure of desorption was the same as described above. 2.7. Quality assurance and quality control All glassware and plastic ware used in this study were previously soaked in 14% HNO3 (v/v) and rinsed with deionized water. All reagents used were of analytical grade or better. Certified reference materials from the National Institute of Standards and Technology (NIST) were used to control the quality of our analytical system. The method detection limit (MDL) of Hg analysis by the AFS, as calculated as three times of blank standard deviation, was 0.06 ng ml1. Blanks were analyzed at the beginning,

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middle, and end of sample batch to ensure no background was above the method detection limit. The recovery of spiked samples ranged from 96% to 106%, with a RSD of 1.86% of the mean. 2.8. Statistical analysis All experiments were replicated three times and only mean values were presented. All data were processed by Microsoft Excel, and the regression and other statistical analyses were conducted using the statistical programs of SPSS (V10.1). 3. Results and discussion 3.1. Adsorption and desorption of Hg2+ Adsorption isotherms of Hg2+ were constructed to compare metal adsorption capacity among the three tested soils. In all cases, Hg2+ adsorption decreased in the order: yellowish red soil > purplish clayey soil > silty loam soil, with YRS soil having the highest sorption capacity for Hg and SLS soil the lowest. Mercury applied at low concentrations (0.0–2.0 mg l1) was mostly adsorbed by the soils. Adsorption of Hg2+ increased steeply with Hg2+concentration in the equilibrium solution at the low concentration range (<15 mg l1) for the three soils, and the increase diminished at the equilibrium solution of Hg2+ concentrations >15.0 mg l1 (Fig. 1). At the highest level of added Hg2+ (40.0 mg l1), YRS soil adsorbed 18.7% of the applied Hg2+, as compared with 18.4% for PCS soil and 13.7% for SLS soil based on experiments, probably due to its higher total organic carbon and clay content (Table 1). Other soil properties such as pH, and the presence of chlorite and illite minerals that are associated with Hg

The amount of Hg2+ adsorbed (mg kg-1)

250

200

150

100 SLS soil YRS soil PCS soil

50

10

Hg2+

20

30

concentration in equilibriun solution (mg

adsorption may also influence metal adsorption through their relation to CEC (Gillis and Miller, 2000). The isotherms of Hg2+adsorption in the soils can be well described by the simple Langmuir and Freudlich model (Table 2). The simple Langmuir model gave a relatively better representation of the experimental data based on the fitting correlation coefficients (r2). The Freundlich parameters KF and 1/n, which, respectively, measure the capacity and intensity of adsorption (Bhattackarya and Venkobachar, 1984), are listed in Table 2. The values of 1/n, between 0 and 1, confirm the heterogeneity of the adsorbent (Mishra et al., 1998). The values of 1/n for SLS soil, YRS soil, and PCS soil were 0.63, 0.76 and 0.72, respectively, which confirms the heterogeneity of soil mass. This would support the theory that Hg2+ is adsorbed to strong sites first (at low solution Hg2+ concentrations), and occupies weaker sites when additional Hg2+ is loaded into the system. The KF values of YRS soil (19.3) and PCS soil (19.4) were similar, but greater than that of SLS soil (17.0), which suggests that the YRS and PCS soil have a greater adsorptive capacity for Hg2+ than the SLS soil. Langmuir equation was widely used because it had two constants with definite physical meaning. The monolayer maximum adsorption (KL) from the Langmuir equation is usually used for comparing potential adsorption capacity of different soils and soil components. The maximum adsorption (KL) value was 111 mg kg1 for SLS soil, 213 mg kg1 for YRS soil and 189 mg kg1 for PCS soil. The b is a constant related to the binding energy and partly reflects adsorption energy level. If b is a positive value, adsorptive reaction could spontaneously proceed at ordinary temperature. The larger the value of b is, the stronger the degree of spontaneous reaction is. The b value was 0.23 for SLS soil, 0.12 for YRS soil and 0.13 for PCS soil, respectively. The SLS soil had the strongest adsorptive intensity but the smallest adsorptive capacity as indicated by the values of KL and b (Table 2). Desorption of the adsorbed Hg2+ was very small in the 0.01 M KCl. Mercury desorption increased with increasing Hg2+ adsorption saturation for all the three soils, though with different rates. The SLS soil desorbed the most Hg2+, followed by PCS soil, and the YRS soil desorbed the least at the same Hg2+ load. After five successive desorptions, the accumulative amounts of Hg2+ desorbed accounted for only 24.4% of the adsorbed Hg2+ for SLS soil, 14.4% for YRS soil and 18.4% for PCS soil. 3.2. pH effects

0 0

1665

40

l-1)

Fig. 1. Isotherms of Hg2+ adsorption in the three soils. Data are means of three replications. The error bar represents the standard deviation of duplicate results.

The amounts of Hg2+ desorbed from the three soils were greatly affected by pH (Fig. 2). The overall sequence of Hg2+ desorbability (%) (the percentage of desorbed Hg in the total adsorbed) was: SLS soil > PCS soil > YRS soil at the pH range of 4–9. The three tested soils behaved similarly in Hg2+ desorption in relation to pH change, although the Hg2+ desorbability changed more or less with pH.

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Y.D. Jing et al. / Chemosphere 69 (2007) 1662–1669 35 SLS soil YRS soil PCS soil

Hg2+ desorbability (%)

30

25

20

15

10

5

0 3

4

5

6

7

8

9

10

pH Fig. 2. The desorption of Hg2+ in the three soils as a function of pH. Data are means of three replications. The error bar represents the standard deviation of duplicate results.

Mercury desorption with increasing solution pH was characteristic of a ‘‘U’’ pattern (Fig. 2). The desorption curve can be divided into three stages: (1) the desorption decreasing stage at pH 3.0–5.0, at which Hg2+ desorbability decreased from 22.7% to 19.5% for SLS soil, from 13.9% to 9.3% for YRS soil, and from 25.8% to 14.1% for PCS soil; (2) the minimal desorption stage at pH 5.0– 7.0, at which the desorption rate leveled off; (3) the desorption increasing stage at pH 7.0–9.0, at which Hg2+ desorbability increased from 20.5% to 28.9% for SLS soil, from 9.2% to 11.9% for YRS soil, and from 14.1% to 19.0% for PCS soil. These results support the previous findings that an increase in Hg2+ adsorption occurs from pH 2 to 4, with adsorption maxima between pH 4 and 5 (Barrow and Cox, 1992; Yin et al., 1996). The results also indicate that the magnitude of Hg2+ desorption varies with pH. There is no general agreement on a single mechanism responsible for this behavior, but strong adsorption of 0 metal hydroxo complexes (HgOH+ and HgðOHÞ2 ), hydrolysis of Al on exchange sites, competition of protons for adsorbing sites, and acid catalyzed dissolution of reactive oxide sites may be involved. In the past, a large number of Hg2+ adsorption studies that involved a variety of adsorbents have been performed in Cl systems. In the presence of Cl, Hg2+ adsorption edge shifts to higher pH values, irrespective of adsorbents (Thanabalasingham and Pickering, 1985; Yin et al., 1996). Chloride reduces Hg2+ retention through the formation of soluble Hg2+–Cl species that have minimal affinity to the variable-charge surfaces. 3.3. Organic acid effects The batch desorption behavior of Hg2+ was similar for the three soils (Fig. 3). However, Hg2+ desorption was

higher in the presence of citric or tartaric acids than malic or oxalic acid at increased concentrations. This implies that desorption behavior of Hg2+ is related to the type and concentration of organic acids. The desorption of Hg2+ in the three soils increased with an increase in citric, tartaric, malic, or oxalic acid concentration from 105 to 0.1 M. The amount of Hg2+ desorbed without organic acid was 37.5 mg kg1for SLS soils, 51.5 mg kg1 for YRS soils and 41.1 mg kg1 for PCS, respectively. When citrate concentration in the desorption solution increased from 105 to 101 M, the amount of Hg2+ desorbed increased from 91.8 to 179 mg kg1 for SLS soil, from 159 to 255 mg kg1 for YRS soil, and from133 to 243 mg kg1 for PCS soil. Similarly, Hg2+ desorption increased from 26.4, 108.9 and 57.5 mg kg1 to 119, 213 and 158 mg kg1, respectively, for SLS soil, YRS soil, and PCS soil when tartaric acid concentration in the desorption solution increased from 105 to 101 M. Malic acid was less effective than citric or tartaric acid, and the desorption of Hg2+ in the presence of 101 M malic acid was only 100, 133 and 108 mg kg1 for SLS soil, YRS soil, and PCS soils, respectively. As for oxalic acid, the amount of Hg2+ desorbed was the lowest, ranging from 22.0 to 54.8 mg kg1 for SLS soil, from 60.0 to 130 mg kg1 for YRS soil, and from 51.1 to 92.7 mg kg1 for PCS soil when oxalic acid concentration was raised from 105 to 101 M. The desorption behavior of Hg2+in SLS soil was slightly different from that in the other two soils in the presence of oxalic, malic, or tartaric acid. The amount of Hg2+ desorbed at the presence of organic acid at low concentrations (105–104 M) was less than that of the control (without organic acid), and increased at higher concentration of organic acid (104 M or higher). The PCS and YRS soil did not have this phenomenon. Similar results were reported regarding the effects of organic acids on Pb desorption (Yang et al., 2006).These results indicate that the presence of oxalic, malic, or tartaric acids at low concentrations (<103 M for oxalic, malic or tartaric acids for the three soils) inhibits Hg2+ desorption in some soils, though enhances Hg2+ desorption for all the tested soils at higher concentrations (>103 M for oxalic, malic or tartaric acids). This can be explained by the fact that small amounts of organic acids added to soil are mostly adsorbed by organic and inorganic components in the soil, which may increase negative charge or CEC of the soil. Moreover, Hg2+ in desorbing solution may be bound to the organic ligands that was adsorbed on the surfaces of the soil (Huang and Bethelin, 1995), thus reducing Hg2+ desorption. However, at higher organic acid concentrations in desorption solution, the ratio of organic ligands remaining in solution to the organic ligand adsorbed by the soil rapidly increases, and consequently increased the competitive ability of the organic ligands for adsorbing sites with Hg2+, thus resulting in enhanced desorption with increasing organic acid concentrations above a certain level. The net effect of organic acids on Hg2+ desorption is controlled by the relative binding strengths between the soil surface

Y.D. Jing et al. / Chemosphere 69 (2007) 1662–1669

c

300.00 250.00 200.00 150.00 100.00 50.00

SLS YRS PCS

Amount of Hg2+ desorbed (mg kg-1)

Amount of Hg2+ desorbed (mg kg-1)

a

160.00 140.00 120.00 100.00 80.00 60.00 40.00 20.00 0.00

0.00 0.00E+00 1.00E-05 1.00E-04 1.00E-03 1.00E-02 1.00E-01

0.00E+00 1.00E-05 1.00E-04 1.00E-03 1.00E-02 1.00E-01

Malic acid concentration added (M)

Citric acid concentration added (M)

d

300

Amount of Hg2+ desorbed (mg kg-1)

Amount of Hg2+ desorbed (mg kg-1)

b

1667

250 200 150 100 50

160 140 120 100 80 60 40 20 0

0 0.00E+00 1.00E-05 1.00E-04 1.00E-03 1.00E-02 1.00E-01

Tartaric acid concentration added (M)

0.00E+00 1.0E-05

1.0E-04

1.0E-03

1.0E-02

1.0E-01

Oxalic acid concentration added (M)

Fig. 3. The desorption of adsorbed Hg2+ in the three soils as a function of organic acid concentrations. (a – citric acid; b – tartaric acid; c – malic acid; d – oxalic acid). Data are means of three replications. The error bar represents the standard deviation of duplicate results.

sites and the metal, and the complex reactions between the organic acid and the metal (Wu et al., 2003). Krishnamurti et al. (1997) demonstrated that various LMWOAs (low molecular weight organic acids) were able to influence the release of heavy metals from different soils and increase their solubility in bulk soil through the formation of soluble metal–LMWOA complexes. The organic ligands are not only adsorbed on the external surface, but also enter the interlayers of layer silicate clay minerals. All of these factors affected the characteristics of Hg2+ adsorption in soils. Hence, addition of organic acids, which chelate with Hg2+ and HgOH+ may result in smaller amounts of Hg2+ adsorbed onto soil surface. The stronger the chelator, the less Hg2+ is adsorbed, and correspondingly, more Hg2+ is desorbed. In this study, citric acid at high concentrations (>103 M) had the greatest improvement of Hg2+ desorption, followed by tartaric and malic acid, and the smallest improvement of Hg2+ desorption was with oxalic acid. The difference of organic acids in enhancing Hg2+ desorption is likely related to their molecular weight and structure. Organic acid that has a bigger molecular weight such as citric acid can attract and/or chelate more metals because it carries more negative charge and has more surface area than a smaller organic acid such as oxalic acid. The molecular weight of the tested acids was in the order: citric > tartaric > malic > oxalic acid, which agrees with

the sequence of their effect on Hg2+ desorption, i.e., citric acid being the most effective and oxalic acid being the least effective. The functional groups of various acids such as carboxylic and hydroxyl groups are important binding sites for both free metal ions in solution and metals on soil surface as they can form outer-sphere or inner-sphere complexes with the metals (Jelinek et al., 1999; Ponizovskii and Mironenko, 2001). It is evident that an organic acid with more carboxylic and hydroxyl groups has a greater influence on Hg2+ desorption, owing to its higher complex capability. For instance, critic and tartaric acids have more functional groups than malic and oxalic acids, and accordingly they desorbed more Hg2+. The type of functional group seems more important than the total number of functional groups in affecting Hg2+ desorption. For instance, critic acid has less number of functional groups but more carboxylic groups than tartaric acid, and as a result, citric acids is more effective in increasing Hg2+ desorption. In addition to a greater molecular weight, the higher complexing capability of a carboxylic group than a hydroxyl group appeared to be an important factor. 3.4. Copper and zinc effects Some common trends were observed regarding Hg2+ desorption in the presence of Cu2+ or Zn2+ as a competitive cation. The amounts of desorbed Hg2+ increased with

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increasing Cu2+ or Zn2+ concentration in the desorbing solution except for YRS soil and PCS soil at low Zn concentrations (Fig. 4). For instance, the desorbed Hg2+ increased from 37.5, 51.5, and 41.1 to 136, 67.0, and 112 mg kg1, respectively, for SLS soil, YRS soil, and PCS soil when Cu2+ concentration was raised from 0 to 15.0 mM (Fig. 4). The desorption of Hg2+ was increased to 113, 63.9 and 98.9 mg kg1, respectively, for SLS soil, YRS soil, and PCS soil in the presence of Zn2+ at 15.0 mM. The presence of Cu2+ increased Hg2+ desorption

Amount of Hg2+ desorbed (mg kg-1)

a

160 Zn

SLS soil

140

Cu

120 100 80 60 40 20 0 0

1

2.5

5

10

15

Concentration of Zn2+ and Cu2+ (mM) Amount of Hg2+ desorbed (mg kg-1)

b

90 80

Zn

YRS soil

Cu

70 60 50 40 30 20 10 0 0

1

2.5

5

10

15

Concentration of Zn2+ and Cu2+ (mM) Amount of Hg2+ desorbed (mg kg-1)

c

140 120

Zn

PCS soil

Cu

100 80 60 40 20 0 0

1

2.5

5

10

15

Concentration of Zn2+ and Cu2+ (mM) Fig. 4. The desorption of adsorbed Hg2+ in the soils as a function of competitive ion concentrations. (a – SLS soil; b – YRS soil; c – PCS soil). Data are means of three replications. The error bar represents the standard deviation of duplicate results.

more than Zn2+ at the same concentration levels, implying that ion characteristic is a factor affecting metal adsorption and desorption. The effects of Zn2+ presence at low concentrations (up to 2.5 mM) on Hg2+ desorption varied considerably among the three different soils, increasing Hg2+desorption in SLS soil, no significant change in PCS soil, but decreasing Hg2+ desorption in YRS soil (Fig. 4). The mechanisms of this variation are unclear. It appeared to relate soil organic matter content, and Zn2+ was more competitive with Hg2+ in the low organic matter soil (SLS soil) than soils with relatively higher organic matter (YRS soil) (Table 1). The competition between Hg2+ and Cu2+ or Zn2+ for adsorbing sites seems to depend on several ionic characteristics. The more electronegative a metal ion is, the more weakly the metal ion is attracted to soil surface. Copper (II) has a greater electronegativity (1.9), and subsequently causes more desorption of Hg2+ than Zn2+ (electronegativity 1.65). The ratio of ionisation potential to ionic radius was considered as a useful criterion for predicting relative desorption capacity of various metals with Hg2+ (Ong and Swanson, 1966). This trend is also observed in this study. Another factor is the ionic radius itself. A smaller ionic radius means more molecules being able to adsorb onto a fixed surface area of an adsorbent. As Cu2+ and Zn2+ are bivalent cations at the same concentration levels, a correlation between ionic size and adsorption selectivity may be expected. In the ion exchange process, the strength with which cations of equal charge are held is generally inversely proportional to the unhydrated radii. Thus, the predicted order of selectivity based on unhydrated radii is Hg2+ (0.110 nm) > Zn2+ (0.074 nm) > Cu2+ (0.072 nm). Although the unhydrated radii of Cu2+ and Zn2+ are similar, the chelation of Cu2+ with soil components is more stable than Zn2+. Therefore, Cu2+ is more readily adsorbed onto soil surface and occupies more adsorption sites than Zn2+, and subsequently, more Hg2+ was desorbed by Cu2+. The selectivity sequences observed for these soils support previous finding that the unhydrated radius may serve as a predictive index of metal adsorption (McKay and Poter, 1997). The standard reduction potentials also decrease with observed sorption capacity. To sum up, the experimental results (Fig. 4) confirms that the relative orders of the ionic properties such as electronegativity, ionisation potential, ionic radius, and reduction potential can serve as good indicators of the relative competitive adsorption capacities of various metal ions with Hg2+on a qualitative basis. The experimental results shown in Fig. 4 can be further illustrated by the pK of the first hydrolysis product of the particular metal. The studied metals are in the order of increasing pK values: Cu (8:0) < Zn (9:0), suggesting that at the same pH, there is more CuOH+ than ZnOH+. This agreed with the previous finding that metal ion characteristic is one of the major factors that determine the metal’s affinity to soil surface, and the sequence was Cu2+ > Zn2+ (Pagnanelli et al., 2003).

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4. Conclusion The soils tested in this study had high adsorption capacity for Hg2+, and most of the added Hg2+ was adsorbed at low initial concentrations (<2 mg l1). The adsorption characteristics of Hg2+in the soils (SLS soil, YRS soil, and PCS soil) were better described by a simple Langmuir than Freudlich equation. Only a small percent of the adsorbed Hg2+ was desorbed in a dilute indifferent electrolyte solution. At the highest Hg2+ loading (initial concentration at 40.0 mg l1), only 24.4, 14.4, and 18.4% of the total adsorbed Hg2+ was desorbed in SLS soil, YRS soil, and PCS soil, respectively. pH was an important factor controlling Hg2+ desorption. Acidic environment (pH <5.4) was favorable for Hg2+ desorption, and thus, soil acidification can increase the release of Hg from the soil, and subsequently to the surrounding environment. Organic acids remarkably influenced the desorption behavior of Hg2+ in the soils. The presence of organic acids at low concentrations (<104 M) tended to inhibit Hg2+ desorption, but enhanced Hg2+ desorption at higher concentrations (>104 M). Citric acid at high concentrations (>103 M) was the most effective in increasing Hg2+ desorption, followed by tartaric acid and malic acid, and oxalic acid was the least effective. The desorption of Hg2+ was also affected by the presence of competitive cations (Cu2+ or Zn2+). The desorption of the adsorbed Hg2+ increased with increasing concentrations of added Cu2+ or Zn2+, and Cu2+ was more effective than Zn2+in competing with Hg2+ for adsorbing sites at the same concentration levels. All the factors studied in this study merit attention in the management of Hg polluted soils. Acknowledgements This study was, in part, supported by a grant from the Natural Science Foundation of China (# 20577044) and a grant from the Science and Technology Ministry of China (# 2002CB410800), and by a Program Funding for Changjiang Scholars and Innovative Research Team of Higher Education of China (#IRT0536). References Agamuthu, P., Mahalingam, R., 2005. Mercury emissions: is there a global problem? Waste Manage. Res. 23, 485–486. Agrochemistry Commission, Soil Science Society of China (ACSSSC), 1983. Routine Methods for Soil and Agrochemical Analysis. Science Press, Beijing. Barrow, N.J., Cox, V.C., 1992. The effects of pH and chloride concentration on mercury sorption: II. By a soil. J. Soil Sci. 43, 305–312. Bhattackarya, A.K., Venkobachar, C., 1984. Removal of cadmium(II) by low cost adsorbents. J. Environ. Eng. 110, 110–112. Burckhard, S.R., Schwab, A.P., Banks, M.K., 1995. The effects of organic acids on the leaching of heavy metals from mine tailings. J. Hazard. Mater. 41, 135–145.

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