Effect of Selected Organic Acids on Cadmium Sorption by Variable- and Permanent-Charge Soils*'
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http://www.paper.edu.cn 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 Effect of Selected Organic Acids on Cadmium Sorption by Variable- and Permanent-Charge Soils*' 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 rhizo- sphere 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, dif- ferences 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.