Effect of Selected Organic Acids on Cadmium Sorption by Variable- and Permanent-Charge Soils*'

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Pedosphere 17(1): 117-123, 2007 ISSN 1002-0160/CN 32-1315/P PEDOSPHERE @ 2007 Soil Science Society of China Published by Elsevier Limited and Science Press www.elsevier.comAocate/pedosphere

HU Hong-Qing' , LIU Hua-Liang', HE Ji-Zheng172 and HUANG Qiao-Yun' Key Laboratory of Subtropical Agriculture Resource and Environment, Ministry of Agriculture, Huazhong Agricultural University, Wuhan 430070 (China). E-mail: [email protected] Research Center for Eco-Environment, Chinese Academy of Sciences, Beijing 100085 (China)

(Received July 6, 2006; revised September 9, 2006)

ABSTRACT Batch equilibrium experiments were conducted to investigate cadmium (Cd) sorption by two permanent-charge soils, a yellow-cinnamon soil and a yellow-brown soil, and two variable-charge soils, a red soil and a latosol, with addition of selected organic acids (acetate, tartrate, and citrate). Results showed that with an increase in acetate concentrations from 0 to 3.0 mmol L-l, Cd sorption percentage by the yellow-cinnamon soil, the yellow-brown soil, and the latosol decreased. The sorption percentage of Cd by the yellow-cinnamon soil and generally the yellow-brown soil (permanent-charge soils) decreased with an increase in tartrate concentration, but increased at low tartrate concentrations for the red soil and the latosol. Curves of percentage of Cd sorption for citrate were similar to those for tartrate. For the variable-charge soils with tartrate and citrate, there were obvious peaks in Cd sorption percentage. These peaks, where organic acids had maximum influence, changed with soil type, and were at a higher organic acid concentration for the variable-charge soils than for the permanent charge soils. Addition of cadmium after tartrate adsorption resulted in higher sorption increase for the variable-charge soils than permanent-charge soils. When tartrate and Cd solution were added together, sorption of Cd decreased with tartrate concentration for the yellow-brown soil, but increased at low tartrate concentrations and then decreased with tartrate concentration for the red soil and the latosol. Key Words: cadmium sorption, organic acids, variable- and permanent-charge soils

Citation: Hu, H. Q., Liu, H. L., He, J. Z. and Huang, Q. Y.2007. Effect of selected organic acids on cadmium sorption by variable- and permanent-charge soils. Pedoshere. 17( 1): 117-123.

INTRODUCTION

Concerns over the possible health and ecosystem effects of heavy metals in soils have increased in recent years (Gao et al., 2003; Liao, 2006; Naidu et al., 1997). For heavy metals the most concern in contaminating soil and groundwater is with cadmium (Cd), which is highly toxic and hazardous in soil environments (Naidu et al., 1997; Tran et al., 2002; Zhou et al., 2003). Due to increased industrial use, Cd pollution has increased in recent years (Robinson et al., 2001; Singh and Pandeya, 1998). Sorption of Cd in soils changes its speciation, activity, and fate in the environment (Kookana and Naidu, 1998). Laboratory studies of Cd migration generally focus on sorption characteristics of Cd onto soil or pure minerals (Floroiu et al., 2001; Fontes and Gomes, 2003; McBride et al., 1981; Naidu and Harter, 1998; Tran et al., 2002), but comparisons of sorption behavior onto different soils with varying charge properties are scare. Low molecular weight (LMW) organic acids are abundant in natural soils, particularly in the rhizosphere and regions rich in organic matter (Jones, 1998; Strobel, 2001). These acids play a key role in many rhizospheric and pedogenic processes (Hu et al., 2005a; Li et al., 2005). Concentrations of aliphatic LMW carboxylic acids of less than 1 pmol L-' to 2 mmol L-' have been reported (Hu et al., 2005b; Jones, 1998; Strobel, 2001). In general, soil solution concentrations of aliphatic di- and tricarboxylic

*'Project supported by the National Natural Sciences Foundation of China (No. 40371065) 转载中国科技论文在线 http://www.paper.edu.cn

118 H. Q. HU et al.

acids are below 50 pmol L-l, but in a few cases concentrations up to 650 pmol L-' have been reported (Hees et al., 1996). Although most Cd sorption studies have been limited to soils of temperate regions where permanent- charge surfaces with net negative charge dominate soils (Naidu et al., 1997), recent research has high- lighted the sorption of Cd by various soil components (Floroiu et al., 2001; Fontes and Gomes, 2003; Naidu and Harter, 1998; Robinson et al., 2001; Tran et at., 2002; Wang and Xing, 2004). To date, differences of Cd sorption on soils with variable charge and permanent charge, which contain different clay minerals and thus have various surface properties, are not fully understood. The role of organic acids in heavy metal sorption by specific soils and components of soils has also been studied (Hu et al., 2005a); however, Cd sorption by permanent- and variable-charge soils in the presence of LMW organic acids is not well documented and comparisons between the two kinds of soils have not yet been undertaken. The purpose of this study was to determine the effects of organic acids on cadmium sorption by permanent- and variable-charge soils, in order to illustrate the processes of Cd transfer in rhizosphere soils where organic acids are present, and to provide scientific information on re-use of Cd contaminated soils.

MATERIALS AND METHODS

Samples of four soils, including two variable-charge soils, a red soil (Udult) of Hunan Province and a latosol (Oxisol) of Hainan Province] and two permanent-charge soils, a yellow-brown soil (Udalf) of Hubei Province and a yellow-cinnamon soil (Ustalf) of Hubei Province, were taken from surface horizons (0-40 cm), air-dried, and ground to pass through a 0.25-mm sieve for analysis and further experiments. Basic soil properties (Table I) were determined according to Rayment and Higginson (1992) and Xiong and Chen (1985). Measurements were as follows: pH with a pH meter at a water to soil ratio of 1:1; organic matter content by KaCr207 oxidation; clay content (< 0.002 mm) by particle analysis; clay minerals by x-ray diffraction; cation exchange capacity (CEC) by Ba2+ ion exchange; and point of zero charge (PZC) by potential titration. fiee iron and aluminum oxides were extracted by dithionite-citrate-bicarbonate solution (DCB) and determined by atomic absorption spectrometry (AAS). Acetate, tartrate, and citrate used were high-grade analytically pure reagents. Cd used was in the form'of Cd(N03)2 of analytically pure reagent.

TABLE I Basic properties of the soils tested Parent Depth pH OM Clay Clay CECs.2") CEC,") PZCd) Fede) Aid") materiala) (H2O) (< 2 pm) mineralsb) /CECs.2 cm - gkg-' - cmol kg-' - g kg-' - Yellow- Q3 0-20 7.1 9.6 482 HM (800), K (150) 22.20 0.36 2.24 12.0 NDf) cinnamon soil Yellow- Q3 0-20 5.2 16.7 320 HM (750), K (200) 16.85 0.32 2.90 21.9 2.9 brown soil Red soil Qz 0-30 4.0 26.7 480 K (450), HM (250) 18.77 0.55 3.80 36.5 5.6 Latosol Basalt 0-40 4.9 43.5 780 K (950), G 20.24 0.69 4.05 130.0 12.3

")Qz and Q3 are deposits in the middle and late Quaternary period, respectively. b)HM = hydromica (2:1), K = kaolinite (l:l), G = gibbsite. Data in the parentheses are the contents (g kg-l) of the corresponding minerals. ')CECs.z and CEC, represent the total negative charge amount at pH 8.2 and variable negative charge amount, respe- ct ively. d)Point of zero charge. ")Fed and Ald are Fe and A1 contents, respectively, extracted by dithionite-citrate-bicarbonate solution. f)Not determined.

For Cd sorption isotherms, Cd sorption was determined using a batch equilibration technique. A series of solutions containing 0-0.2 mmol L-l Cd(N03)~and 1 mmol L-' KN03 (pH 5.0) were added to soils at 100 mL g-'. The suspensions were shaken for 4 h (Chen and Chen (2002) reported 2 h required 中国科技论文在线 http://www.paper.edu.cn

ORGANIC ACID EFFECT ON SOIL CD SORPTION 119

to reach equilibrium for Cd sorption by soils) at 25 "C and then centrifuged for 10 min at 5 000 r min-l. The concentrations of Cd in the solutions were determined by AAS. The amount of Cd sorption was calculated from the difference of Cd concentration between the initial and equilibrium solutions. The amounts of Cd sorption by the soils over the range of Cd concentrations added were fit to the Langmuir equation:

Q = KCb/(l + KC) (1)

where Q is the Cd sorption quantity; K is a constant relevant to affinity power; C is Cd concentration in the equilibrium solution; and b is the maximum sorption quantity. For Cd sorption in the presence of different concentrations of organic ligands, a mixed solution (pH 5.0) of 0.2 mmol L-' Cd(N03)Z and 0-3.0 mmol L-' acetic, tartaric, or citric acid was added to soils at 100 mL g-'. Cd sorption was conducted at 25 "C by shaking for 4 h. Suspensions were then centrifuged for 10 min at 5000 r min-', and the concentration of Cd in the equilibrium solution was measured by AAS with the amount of Cd sorption calculated as described above. The Cd sorption with addition of tartrate and Cd together was called competitive sorption contrasting to the Cd secondary sorption described below. For Cd secondary sorption after pre-sorption of organic acids, one g of each soil was weighed in a centrifuge tube, and 100 mL organic acid solution of 0-3.0 mmol L-l concentrations were added. The suspensions were shaken for 8 h at 25 OC, centrifuged, and the supernatant solution was discarded. Another 100 mL Cd solution of 0.2 mmol L-' was added, and then the suspensions were shaken for 4 h at 25 "C. The concentrations of Cd in equilibrium solutions were measured as described above and the amount of Cd sorption was determined. All data were the mean of two replications and the standard deviations were tested as negligible in the experiments.

RESULTS

Sorption of Cd in the absence of organic acids

Cadmium sorption increased with an increase in Cd concentrations in the equilibrium solution (Fig. 1). At a Cd addition concentration of 0.2 mmol L-', the order for Cd sorption in mmol kg-' was as follows: yellow-cinnamon soil (16.5) > yellow-brown soil (13.8) > latosol (7.2) > red soil (2.6). This order corresponded with the constituents of the soils including clay minerals, clay content, and pH.

-1 8 216 + YCS -1 0 4 0 YBS €12 -1E 0 x LS - 50 + YCS 0 YES C 8 .s ._0 CI E 6 0 4 v) 0 20 -0 2 0 0 .I 10 0.00 0.03 0.06 0.09 0.12 0.15 0 18 0' Concentration of Cd in equilibrium 0.0 0.5 1.0 1.5 2.0 2.5 3.0 solution (rnmol L") Concentration of acetate (mmol L.')

Fig. 1 Isotherms of Cd sorption by two permanent-charge soils, a yellow-cinnamon soil (YCS) and a yellow-brown soil (YBS), and two variable-charge soils, a red soil (RS) and a latosol (LS).

Fig. 2 Percentage of Cd sorption by two permanent-charge soils, a yellow-cinnamon soil (YCS) and a yellow-brown soil (YBS), and two variable-charge soils, a red soil (RS) and a latosol (LS) in the presence of 0 to 3.0 mmol L-l acetate. 中国科技论文在线 http://www.paper.edu.cn

120 H. Q. HU et al.

Based on the Langmuir equation (Equation l), the calculated maximum sorption (b) and affinity constants (K)were 22.35 mmol kg-' and 65.76 L mol-' for the yellow-cinnamon soil (r = 0.98); 23.41 mmol kg-' and 22.91 L mol-' for the yellow-brown soil (r = 0.906); 10.28 mmol kg-' and 18.75 L mol-' for the latosol (r = 0.998); and 7.24 mmol kg-' and 3.78 L mol-' for the red soil (r = 0.937). The affinity constants for Cd sorption of the permanent-charge soils (yellow-cinnamon and yellow-brown soils) were greater than those of the variable-charge soils (latosol and red soil). At the Cd addition concentration of 0.2 mmol L-', sorption may reach maximum for the latosol and red soil, whereas it had not been attained for the yellow-cinnamon and yellow-brown soils.

Cd sorption in the presence of organic acids

Cd sorption decreased with an increase in acetate concentration in the yellow-cinnamon soil, yellow- brown soil, and latosol, but had little influence in the red soil (Fig.2). When acetate concentration increased from 0 to 3.0 mmol L-l, the sorption percentages of Cd decreased from 83.1% to 67.3% for the yellow-cinnamon soil, 68.7% to 55.2% for the yellow-brown soil, and 38% to 27.6% for the latosol. Tartrate with increasing concentrations reduced the percentage of Cd sorption (Fig. 3) by the yellow- cinnamon soil. In the yellow-brown soil, Cd sorption percentage showed a slight increase at 0.05 mmol L-' tartrate and then a reduction at higher concentrations. For the two variable-charge soils, the red soil and latosol, however, the Cd sorption percentage versus tartrate concentration curves had distinct peaks: Cd sorption increased with tartrate concentration to a peak at 0.6 and 0.4 mmol L-l tartrate, respectively, and then decreased at higher concentrations. The tartrate concentrations, corresponding to peaks of Cd sorption percentage, were greater for soils having more variable charge properties (red soil and latosol) than for the permanent charge soil (yellow-cinnamon soil).

90 r 80 80 g 70 $ 70 + YCS 0 YBS v v c 60 60 A RS x LS .=0 50 .= 50 p 40 p 40 0 g 30 30 ;(I, 20 + YCS 0 YBS ;20 10 A RS x LS 10 0 I v- 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Concentration of tartrate (mrnol I-.') Concentration of citrate (mmol I-.')

Fig. 3 Percentage of Cd sorption by two permanent-charge soils, a yellow-cinnamon soil (YCS) and a yellow-brown soil (YBS), and two variable-charge soils, a red soil (RS) and a latosol (LS) in the presence of 0 to 3.0 mmol L-' tartrate.

Fig. 4 Percentage of Cd sorption by two permanent-charge soils, a yellow-cinnamon soil (YCS) and a yellow-brown soil (YBS), and two variable-charge soils, a red soil (RS) and a latosol (LS) in the presence of 0 to 3.0 mmol L-' citrate.

Cd sorption with citrate addition (Fig. 4) was similar to that with tartrate. With citrate, the magnitude of reduction was smaller, the peak shape of the curves was narrower and the peak tended to have a lower citrate concentration than with tartrate for each soil. The peak citrate concentrations in the yellow-brown soil, red soil, and latosol were 0.04, 0.40, and 0.30 mmol L-l, respectively.

Cd secondary sorption and competitive sorption with tartrate

In the yellow-cinnamon soil, Cd secondary sorption in the presence of tartrate (addition of cadmium after tartrate adsorption) was always greater than its competitive sorption with tartrate (tartrate and Cd solution added together) (Fig. 5). When tartrate concentrations increased from 0 to 0.1 mmol L-', the percentage of Cd secondary sorption increased from 67.7% to 71.6%, whereas for 0.1-3.0 mmol L-' 中国科技论文在线 http://www.paper.edu.cn

ORGANIC ACID EFFECT ON SOIL CD SORPTION 121

tartrate, the magnitude of increase was reduced. However, for competitive sorption with an increase in tartrate concentration the percentage of Cd sorption decreased.

h YES-SA 60 + C 0 YBS-CA .-0 A RS-SA 40 p A RS-CA 0 u) H LS-SA 20 0 LS-CA

0' I 0.0 0.5 1 .o 1.5 2.0 2.5 3.0 Concentration of tartrate (rnrnol L-')

Fig. 5 Comparison between Cd secondary sorption (SA) (addition of cadmium after tartrate adsorption) and competitive sorption (CA) (tartrate and Cd solution added together) by a permanent-charge soil, a yellow-brown soil (YBS), and two variable-charge soils, a red soil (RS) and a latosol (LS).

In the red soil and latosol, Cd secondary sorption increased with an increase in tartrate concentration, and the increase was greater than in the permanent-charge soil (yellow-brown soil). Moreover, Cd competitive sorption increased by tartrate added at low concentrations. With an increase in the concentration of tartrate to 3.0 mmol L-' , the percentage of competitive sorption declined, although it was still higher than that at the 0 tartrate level. When the tartrate concentrations were lower than 0.7 mmol L-' , competitive sorption for Cd was greater than secondary sorption.

DISCUSSION

Permanent-charge soils and variable-charge soils are different in clay mineral constituents and surface chemistry. Permanent-charge soils are dominated by a number of 2:l type layer silicates, such as hydromica; however, variable-charge soils contain relatively abundant 1: 1 type minerals, such as kaolinite, and iron and aluminum oxides. In general, the net negative surface charge in variable-charge soils increases with increasing soil pH and organic matter content , thereby inducing increased sorption of metal ions by variable-charge soils and minerals (Hu et al., 2005a). The order for Cd sorption of yellow- cinnamon soil > yellow-brown soil > latosol > red soil (Fig. 1) showed that the soils with a permanent charge (yellow-cinnamon and yellow-brown soils) had greater sorption capacity for Cd than those with a variable charge. The existence of a number of 2:l type clay minerals, higher pH, and higher clay content, which were characteristic of permanent-charge soils, favored greater Cd sorption. Cadmium sorption by the two types of charged soils in the presence of organic acids showed notice- able differences. However, the effects of acetate, tartrate, and citrate on Cd sorption were similar. In the presence of lower organic acid concentrations, variable-charge soils had a Cd sorption peak, and then the percentage of Cd sorption decreased with further increases in organic acid concentration; whereas permanent-charge soils showed no peak or the peak appeared at lower concentrations with the curve having a smaller concentration range. Organic acid ligands may occupy the sorption sites ofsoil mineral surfaces that heavy metals may do (Hu et al., 2005a). They may also form complexes with Cd in solution. Permanent-charge soils had greater Cd sorption capacity because of the more plentiful 2:l type minerals. With organic acid addition, organic ligand-chelated Cd could increase the negative charge of Cd ions in solution, leading to repulsion at the soil surface. In this case for permanent-charge soils, organic acids could only decrease Cd sorption capacity. Variable-charge soils, however, had both positively and negatively charged surfaces, depending on the pH of the solution. The surface of positive-charge minerals was unfavorable for Cd sorption, but in the presence of lower concentrations of organic acids, sorption of organic acids may produce new sorption sites for Cd and increase the negative charge (Hu et 中国科技论文在线 http://www.paper.edu.cn

122 H. Q. HU et al.

al., 2005a), all of which would increase sorption of Cd by the soils. At higher concentrations of organic acids, however, the organic acids remaining in solution might compete with Cd for soil surface sites, thus explaining the decline in sorption percentage of Cd. The difference in secondary sorption and competitive sorption could result from organic acid chelation with Cd in solution, sorption of lower concentration organic acids, generation of new sorption sites on soils, or formation of organic acid-Cd complexes that would reduce the Cd positive charge. All of these would depend on the concentration of organic acids added and soil properties. Presence of tartrate increased the Cd secondary sorption by the yellow-cinnamon soil, red soil, and latosol. This could be due to tartrate sorption in the soils, which would increase the negative charge of the soil surface, or produce sorption sites for Cd. However, due to reduced organic acid concentrations in the solution, competition for Cd sorption sites was not evident. For the permanent-charge soil (yellow-cinnamon soil), Cd competitive sorption was always lower than secondary sorption. Thus, on permanent-charge soil (yellow-cinnamon soil) pre-sorption of tartrate could generate more sorption sites for Cd, or it could act as a complexing reagent to Cd. By contrast, if higher concentrations of tartrate were present in the system, tartrate would compete with Cd for sorption sites on soil or form chelation with Cd in solution and thus reduce Cd competitive sorption. However, for the variable-charge soils (red soil and latosol), at lower tartrate concentrations (below 0.7 mmol L-'), competitive sorption of Cd was greater than secondary sorption. Thus, for the variable-charge soils at lower tartrate concentrations, both new site production and a decrease in surface charge density of Cd would influence the competitive sorption of Cd, but only new site production influenced secondary sorption. Therefore, the surface properties of soils controlled the impacts of organic acid ligands.

ACKNOWLEDGEMENT

The authors appreciate the critical comments of the two reviewers and Dr. David Hamilton for his language revision.

REFERENCES

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