Effects of phthalic and salicylic acids on Cu(II) adsorption by variable charge soils

Transcription

1 iol Fertil Soils (26) 42: DOI.7/s ORIGINL PPER Renkou Xu. Shuangcheng Xiao. Dan Xie. Guoliang Ji Effects of phthalic and salicylic acids on Cu(II) adsorption by variable charge soils Received: 19 October 2 / ccepted: 21 October 2 / Published online: 2 March 26 # Springer-Verlag 26 bstract In the present study, the effect of two substituted benzoic acids on Cu(II) adsorption onto two variable charge soils was investigated, with the emphasis on the adsorption and desorption equilibrium of Cu(II). Results showed that the presence of organic acids induced an increase in Cu(II) adsorption onto the two soils. The extent of the effect was related to the initial concentrations of Cu (II) and organic acid, the system ph, and the nature of the soils. The effect of organic acids was greater for Oxisol than for Ultisol. affected Cu(II) adsorption to a greater extent than salicylic acid did. The effect of organic acids varied with ph. The adsorption of Cu(II) induced by organic acids increased with increasing ph and reached a maximum value at approximately ph 4., and then decreased. It can be assumed that the main reason for the enhanced adsorption of Cu(II) is an increase in the negative surface charge caused by the specific adsorption of organic anions on soils because the desorption of Cu(II) adsorbed in organic acid systems was greater than that for the control. The desorption of Cu(II) absorbed in both control and organic acid systems also increased with increasing ph; it reached a maximum value at ph.2 for control and salicylic acid systems and at ph.1 for a phthalic acid system, then decreased. This interesting phenomenon was caused by the characteristics of the surface charge of variable charge soils. Keywords Variable charge soils.. Phthalic acid. Cu(II) adsorption R. Xu (*). S. Xiao. D. Xie. G. Ji Institute of Soil Science, Chinese cademy of Sciences, P.O. ox 821, Nanjing, People s Republic of China rkxu@issas.ac.cn Fax: D. Xie College of Resources and Environmental Science Nanjing gricultural University, Nanjing, People s Republic of China Introduction Variable charge soils in tropical and subtropical regions carry relatively low negative surface charge and exhibit low affinities for heavy metals compared with constant charge soils (Naidu et al. 1997; Yu 1997). Low-molecularweight organic acids (LMWOs) secreted by plant roots and produced via the decomposition of plant residues exist widely in soils, particularly in the rhizosphere (Fox and Comerford 199; Jones 1998; Strobel 21). Organic acids can increase or decrease the adsorption of heavy metal ions by soils, depending on the mechanisms of interaction among the organic acid, soil, and metal ions. Harter and Naidu (199) suggested that for variable charge soils, organic acids increased the adsorption of heavy metals at ph levels under the zero point of charge (ZPC). t ph levels above the ZPC, the soil would react similarly to one dominated by permanent charge colloids. Naidu and Harter (1997) examined the influence of LMWOs on Cd 2+ adsorption by several acid soils and found that citrate and oxalate increased the adsorption at low ph and decreased the adsorption at higher ph. They attributed the increased adsorption of Cd 2+ to direct complexation of the metal by an adsorbed organic layer or the formation of a soil ligand metal complex. Results in goethite/water suspensions also indicated that Cu(II) adsorption was enhanced by chelidamic and phthalic acids at low ph through the formation of surface ternary complexes, and Cu(II) sorption in Cuphthalic acid and Cu-chelidamic acid binary systems was described well by the Generalized Two-Layer Model (li and Dzombak 1996a). On the other hand, the presence of organic acids induced a decrease in surface positive charge and an increase in surface negative charge of variable charge soils and oxides (Naidu and Harter 1997; Stumm et al. 198; Xu et al. 23, 24a). Thus, these acids should enhance the adsorption of heavy metals by variable charge soils through a change in surface charge. However, up until now, little information has been available regarding the effect of LMWOs on the adsorption of heavy metals by variable charge soils through electrostatic attraction (Violante et al. 23).

2 444 The effect of some aliphatic LMW carboxylic acids on Cu(II) adsorption by variable charge soils has been studied in a previous paper (Xu et al. 24b). The objective of this investigation was to evaluate the effect of two substituted benzoic acids, salicylic acid, and phthalic acids, on Cu(II) adsorption by two variable charge soils. The adsorption and desorption of Cu(II), and the adsorption of organic acid were examined in the same system and the mechanisms of the effect of organic acids on the adsorption of Cu(II) are discussed. Materials and methods Soils and organic acids Two variable charge soils, an Ultisol (located at E, N) and an Oxisol (located at 1 E, 2 2 N) collected from subtropical regions of south China, were used. Some properties of the soils are given in Table 1. Two substituted benzoic acids, salicylic acid, and phthalic acid were selected for the study; both were chemically pure reagents. Experimental procedure stock solution containing. mol l 1 Cu(NO 3 ) 2 was prepared using reagent-grade Cu(NO 3 ) 2 3H 2 O. ppropriate quantities of Cu(II) solution and organic acid solution were added into -ml flasks to obtain mixed solutions containing 1. mmol l 1 organic acid and various concentrations of Cu(II) (.2,.,.8, 1., 2., and 3. mmol l 1 ) for adsorption isotherm experiments. The mixed solutions with 1. mmol l 1 Cu(II) and 1. mmol l 1 organic acid were prepared for ph effect experiments using a similar method, and mixed solutions with 1. mmol l 1 Cu(II) and various concentrations of salicylic acid (.1,.,.8, 1., and 1. mmol l 1 ) were prepared for experiments on the effect of organic acid concentration. ll solutions contained 1. mmol l 1 NaNO 3 as the supporting electrolyte. Finally, the solutions were adjusted to different ph values with 1:1(v/v) HNO 3 or. mol l 1 NaOH solutions. Samples of 1. g of soil were weighed into centrifuge bottles in two replicates. The bottle with soil was weighed together as W 1. Then, 2 ml of the mixed solution was added into each of the bottles. The suspensions were shaken in a constant-temperature water bath at 28 C (±1 C) for 2 h. fter standing overnight, the solution was separated from the solid phase by centrifugation at 3, rpm (3,38 g) for min. Copper in solution was determined using a spectrophotometric method (Rohde 1966), and salicylic acid and phthalic acid were determined by ultraviolet (UV) spectroscopy (li and Dzombak 1996b). The adsorption of Cu(II) and organic acids was calculated from the difference between the total amount added and the amount remaining in solution. Then, the bottle with its content was weighed again as W 2. To desorb the adsorbed Cu(II), 2 ml of.1 mol l 1 KNO 3 solution was added. The suspension was shaken for 1 h, and the solution was then separated by centrifugation at 3, rpm (3,38 g) for min. Copper in solution was determined and the amount of Cu(II) desorbed by KNO 3 was calculated using the following equation: Cu des1 mmol kg 1 ¼ ½CuŠ K1 ð2 þ W 2 W 1 Þ ½CuŠ ad ðw 2 W 1 Þ where [Cu] K1 is the concentration of Cu(II) in.1 mol l 1 KNO 3 solution after desorption experiments (mmol l 1 ), [Cu] ad is the concentration of Cu(II) in equilibrium solution after adsorption experiments (mmol l 1 ), W 1 is the total weight of soil sample and bottle (g), and W 2 is the total weight of soil sample and bottle together with the residual solution (g). In multiple desorption experiments, the adsorption step and initial desorption were conducted using the same method mentioned above. Then, the bottle together with the soil sample and residual solution was weighed again as W 3 (g). further 2 ml of.1 mol l 1 KNO 3 solution was added. The suspension was shaken for 1 h, and the solution was then separated by centrifugation at 3, rpm (3,38 g) for min. Copper in solution was determined as [Cu] K2 (mmol l 1 ) and the amount of Cu(II) desorbed by KNO 3 in the second step was calculated using the following equation: Cu des2 mmol kg 1 ¼ ½CuŠ K2 ð2 þ W 3 W 1 Þ ½CuŠ K1 ðw 3 W 1 Þ: This desorption step was repeated for another three times and the amount of Cu(II) desorbed in these further three steps (Cu des3,cu des4,cu des ) was calculated using the same Table 1 Properties of the soil samples Soil Parent material ph a CEC b (cmol kg 1 ) OM c (g kg 1 ) Fe 2 O d 3 (g kg 1 ) Dominant clay mineral e Oxisol asalt K, G, H (Go) Ultisol Quaternary K, I (V) a Soil/water ratio 1:2. b Cation exchange capacity, amonium acetate method c Organic matter, dichromate method d Dithionite citrate bicarbonate (DC) method e K kaolinite; G gibbsite; H hematite; V vermiculite; Go goethite; I hydrous mica

3 44 equation as for Cu des2. Then, the total desorption of Cu(II) was calculated using the following equation: Cu total mmol kg 1 ¼ Cu des1 þ Cu des2 þ Cu des3 þ Cu des4 þ Cu des : fter adsorption experiments, the ph values of equilibrium solutions in all treatments were determined. The resulting ph values were used in constructing figures and tables. Results Effect of organic acids on adsorption isotherms of Cu(II) Figure 1 shows that the presence of organic acids induced an increase in the adsorption of Cu(II) by two variable charge soils. Cu(II) adsorption in Oxisol was affected to a greater extent by phthalic acid than by salicylic acid (Fig. 1a). This was consistent with the quantity of the two organic acids adsorbed by the soil, as shown in Fig. 2. The Cu(II) adsorption (mmol kg ) Cu adsorption (mmol kg ) Equilibrium concentration of Cu (II) (mmol L ) ph4.6 ph4.9 ph Equilibrium Cu concentration (mmol L ) Fig. 1 Effect of organic acids on Cu(II) adsorption by Oxisol at ph 4.8 (a) and Ultisol (b) (initial concentration of organic acid was 1. mmol l 1 ) 2. dsorption (mmol kg ) dsorption (mmol kg ) Concentration of Cu (II) added (mmol L ) Concentration of Cu added (mmol L ) Fig. 2 dsorption of the two organic acids by Oxisol (a) and Ultisol (b) in the same Cu(II) adsorption experiments as for Fig. 1 Oxisol adsorbed more phthalic acid than salicylic acid. Greater adsorption of organic acid induced more adsorption of Cu(II) by the soil (Fig. 1a). lthough the same trend of Cu(II) adsorption in organic acid systems was observed for Ultisol, there was no difference between the effects of phthalic and salicylic acids due to the higher ph of the salicylic acid system, as shown in Fig. 1b. Figure 2 also shows that the Oxisol had higher adsorption capacity for organic acids than the Ultisol. The quantity of organic acids adsorbed by the two soils slightly decreased with an increase in the initial concentration of Cu(II) (Fig. 2b). This is because the equilibrium concentration of Cu(II) increased with its initial concentration and Cu(II) in solution formed soluble organic Cu complexes with organic acids, which depressed the adsorption of organic acids to some extent. Some authors have proposed a variety of adsorption equations to describe the adsorption of ions by soils as a function of ion concentration in equilibrium solution. The most commonly used equations are as follows: Langmuir equation : C ðx=mþ ¼ 1 ðkbþþc=b (1) Freundlich equation : x=m ¼ KC 1=n (2) where C is the equilibrium concentration of adsorbate in solution, x is the amount of adsorbate adsorbed, m is the

4 446 amount of adsorbent, and K is a constant related to binding strength. oth Langmuir and Freundlich equations were used to fit the adsorption data shown in Fig. 1. It was found that these two equations fitted the data well. The parameters calculated for the equations for different experiment systems are listed in Table 2. It is evident from the table that most of the correlation coefficients (R 2 ) are greater than.9 and that the data fitted the Langmuir equation better. Therefore, it can be considered that both Langmuir and Freundlich equations can be used to describe adsorption of Cu(II) by variable charge soils in the presence of organic acids. Cu(II) adsorption (mmol kg ) dsorption equilibrium ph Effect of ph on Cu(II) adsorption in the presence of organic acids Figure 3 shows that the adsorption of Cu(II) increased with increasing system ph for all treatments. The presence of organic acids induced an increase in Cu(II) adsorption by the two soils. showed a greater effect on Cu (II) adsorption than salicylic acid, especially in the ph range from 4.2 to.. The effect of phthalic acid in the two soils increased with increasing ph and reached a maximum effect at ph 4., and then decreased. The effect of salicylic acid also increased with increasing ph and reached a maximum effect at ph 4.3 in Oxisol and ph 4.1 in Ultisol, and then decreased. The adsorption of organic acids by the two soils in Cu(II) adsorption experiments is shown in Fig. 4. The adsorption of phthalic acid by the two soils increased slightly with increasing ph and reached a maximum value at ph 4., and then decreased, which is consistent with its effect on Cu(II) adsorption, as shown in Fig. 3. The adsorption of salicylic acid also increased with increasing ph and reached a maximum value at ph 4.3, and then decreased. oth soils adsorbed more phthalic acid than salicylic acid. The Oxisol showed greater adsorption capacity for the two organic acids than the Ultisol due to higher content of free iron oxides in the former, as shown in Table 1. Cu(II) adsorption (mmol kg ) dsorption equilibrium ph Fig. 3 Effect of ph on Cu(II) adsorption by Oxisol (a) and Ultisol (b) in the presence of organic acids (initial concentration of Cu(II) and organic acid was 1. mmol l 1 ) adsorption experiment systems and reached a maximum value at approximately ph.2 for salicylic acid in both soils, at ph 4.9 for phthalic acid in Oxisol, and at ph.1 for this acid in Ultisol, and then decreased. The desorption of Cu(II) adsorbed in the salicylic acid system was greater than that in the control system. The desorption trend for Cu (II) adsorbed in the phthalic acid system was similar to that in the salicylic acid system at ph values less than.2, while above this ph level desorption was lower than that in the control system. Desorption of Cu(II) adsorbed at different ph values The results in Fig. show that the desorption of Cu(II) adsorbed in all treatments increased with increasing ph of Table 2 Parameters of Freundlich and Langmuir equations for different treatments Soil Treatment Langmuir equation Freundlich equation K b R 2 K n R 2 Oxisol Ultisol

5 dsorption (mmol kg ) ph Cu ( I I ) desorption (mmol kg ) dsorption equilibrium ph 447 dsorption (mmol kg ) Effect of salicylic acid concentration on adsorption of Cu(II) Figure 6 shows that Cu(II) adsorption increased with increasing concentration of salicylic acid in Oxisol. This is consistent with the changing trend for salicylic acid adsorbed by the soil, as shown in Fig. 6. taninitial acid concentration of 1. mmol l 1, the quantity of salicylic acid adsorbed by the soil was 16.8 mmol kg 1, and the acid induced an increase in Cu(II) adsorption of 3.87 mmol kg 1. The percentage increase in Cu(II) adsorption due to the adsorption of salicylic acid was 23.%. t initial concentrations of.,.8 and 1. mmol l 1, the percentage increase was., 11., and 16.3%, respectively, which shows an increase with the amount of salicylic acid added. Comparison between adsorption and desorption of Cu(II) The results of Cu(II) adsorption, initial desorption of Cu(II) and total desorption of Cu(II) are shown in Table 3. In the control system, Oxisol adsorbed more Cu(II) than Ultisol, while the desorption of adsorbed Cu(II) was higher in Ultisol than that in Oxisol, which resulted in a higher ph Fig. 4 dsorption of organic acids by Oxisol (a) and Ultisol (b) at different ph values in the same Cu(II) adsorption experiments as for Fig. 3 Cu (II ) desorption (mmol kg ) dsorption equilibrium ph Fig. Desorption of Cu(II) adsorbed by Oxisol (a) and Ultisol (b) at different ph values in the same systems as for Fig. 3 desorption rate in the Ultisol. The desorption rate (total desorption/adsorption) was 8.1% for Ultisol and 62.% for Oxisol. Initial Cu(II) desorption as a percentage of total desorption was also greater in Ultisol (88.8%) than in Oxisol (68.%). The presence of organic acids resulted in an increase in adsorption, desorption, and the desorption rate of Cu(II). From the results shown in Table 3, the dsorption / desorption (mmol kg ) 2 1 Cu (II) adsorption Total Cu (II) desorption adsorption added (mmol L ) Fig. 6 Effect of amount of salicylic acid added on adsorption and desorption of Cu(II) on Oxisol at ph 4.36 (initial concentration of Cu(II) was 1. mmol l 1 )

6 448 Table 3 Cu(II) adsorption, initial desorption, and total Cu(II) desorption in the two soils as influenced by organic acids Soil Treatment ph dsorption (mmol kg 1 ) Desorption (mmol kg 1 ) Desorption rate (%) Initial Total Ultisol ±.8 8.7±.1 9.8± ±.4.± ± ± ±.4.6± Oxisol acid ±.4.4± ± ± ± ± ± ±.1 9.8± In the adsorption experiments of Cu(II), initial concentration of organic acid and Cu(II) was 1. mmol l 1 difference in Cu(II) adsorption between control and organic acid systems (Δadsorption) and the difference in desorption between control and organic acid systems (Δdesorption) can be calculated. The results indicate that Δadsorption in Ultisol was equal to the Δdesorption and suggest that the adsorbed Cu(II) increased by organic acids in Ultisol can be desorbed by KNO 3 completely at approximately ph 4.. On the other hand, in Oxisol, Δdesorption was less than Δadsorption, and thus only part of the adsorbed Cu(II) increased by organic acids in Oxisol can be desorbed by KNO 3 atthesameph.therate of Δdesorption/Δadsorption in Oxisol was 77.4% for the salicylic acid system and 84.8% for the phthalic acid system. Discussion There are two possible mechanisms responsible for the increase in Cu(II) adsorption due to the presence of organic acids: the formation of soil organic acid cation ternary surface complexes and an increase in soil negative surface charge due to the specific adsorption of organic anions by the soil, results in an increase in Cu(II) adsorption through electrostatic attraction. The formation of ternary surface complexes has been widely proposed as being responsible for enhanced sorption of metal ions by oxides and soils in the presence of complexing ligands (enjamin and Leckie 1982; Davies and Leckie 1978; Harter and Naidu 199; Mcride 198; Naidu and Harter 1997; Rudin and Motschi 1984). li and Dzombak (1996a) reported that ternary surface complexation models could be used to describe sorption in complex metal ligand oxide systems and suggested that the formation of ternary surface complexes was a major mechanism. However, the results in our study suggest that an increase in the net negative surface charge of soils was also an important mechanism for enhanced Cu(II) adsorption by organic acids. If enhanced adsorption is caused by the formation of ternary surface complexes, the Cu(II) adsorbed by soils should not be desorbed by a neutral salt, such as KNO 3. Our results show that the presence of organic acids induced an increase in Cu(II) adsorption by the two soils, and the amount of desorption of Cu(II) adsorbed in the organic acid systems was greater than in the control system. Results from a separate experiment showed that the presence of organic acids increased the surface negative charge and decreased the surface positive charge of variable charge soils (Xu et al. 23). The increase in net negative surface charge led to an increase in Cu(II) adsorption by the soils. s can be observed from the results in Table 3, Cu(II) adsorbed by Ultisol and enhanced by organic acids could be completely desorbed by KNO 3, and most of the Cu(II) adsorbed by Oxisol and enhanced by organic acids could also be desorbed. It is possible for ternary surface complexes to form in the Oxisol system, because there was % of adsorbed Cu(II) enhanced by organic acids that could not be desorbed by KNO 3 (Table 3). ased on the data shown in Fig. 6, it is possible to calculate the increment in adsorption [Δadsorption of Cu(II)] and desorption of Cu (II) [Δdesorption of Cu(II)] for different amounts of salicylic acid added. The results are given in Table 4. oth Δadsorption and Δdesorption increased with increasing concentration of salicylic acid, while the percentage of Δdesorption in Δadsorption decreased from 88.9% in.8 mmol l 1 to 77.3% in 1. mmol l 1 added salicylic acid. This means that the formation of ternary soil Cu organic Table 4 Differences in adsorption and desorption of Cu(II) between a salicylic acid system and a control system cid concentration (mmol l 1 ) Cu(II) (mmol kg 1 ) Δadsorption a Δdesorption b Δdesorption as a percentage of Δadsorption (%) a Difference between the Cu(II) adsorption in salicylic acid system and in control b Difference between the desorption of Cu(II) adsorbed in salicylic acid system and in control

7 acid surface complexes increased with increasing concentration of organic acids. This was because more organic acid adsorbed to the soil surface at higher organic acid concentrations. The enhancement of Cu(II) adsorption by organic acids was dependent on the nature of the acid and the chemical and mineralogical composition of the soils. The extent of the organic acid effect was closely related to the amount of acid adsorbed: the greater the amount, the greater the effect (Figs. 1 and 2). The higher content of iron oxides in Oxisol (Table 1) resulted in adsorption of a greater amount of organic acid by the soil and thus a greater organic acid effect on Cu(II) adsorption by the soil compared with the Ultisol. This is because the iron oxides in soils are the principal absorbents for anions (Yu 1997). There are two reasons for the increase in Cu(II) adsorption and desorption with increasing ph: the increase in negative surface charge of variable charge soils with increasing ph and hydrolysis of Cu(II) to form Cu(OH) + ions at higher ph. Soil surfaces have stronger affinity for Cu(OH) + than for Cu(II) (Sposito 1989) and Cu(OH) + adsorbed by soil is difficult to desorb using a neutral salt; this resulted in a decrease in the desorption of Cu(II) adsorbed at ph levels above.2 for the control and salicylic acid systems (Fig. ). The effect of organic acids varied with ph. t higher ph, the enhancement of Cu(II) adsorption by organic acids was lower due to a decrease in adsorption of organic acids by the soils (Fig. 4). When adsorption experiments were conducted above ph.2, desorption of adsorbed Cu(II) was lower in the phthalic acid system than in the control system. The mechanism for this phenomenon is not exactly known. Presumably, it may involve coprecipitation or the formation of strongly bonded surface complexes in the soil. Conclusions The presence of salicylic and phthalic acids can lead to an increase in adsorption of Cu(II) by variable charge soils through a change in surface charge induced by specific adsorption of organic anions, and surface ternary complexes of soil organic anion Cu(II). Due to the increase in electrostatic adsorption of Cu(II) caused by changes in the surface charge, the presence of organic acids can also lead to an increase in desorption of adsorbed Cu(II). The extent of the effect of organic acids is dependent on the system ph, the initial concentrations of organic acids and Cu(II), and the nature of the soil. References 449 li M, Dzombak D (1996a) Effects of simple organic acids on sorption of Cu 2+ and Ca 2+ on goethite. Geochim Cosmochim cta 6: li M, Dzombak D (1996b) Competitive sorption of simple organic acids and sulfate on goethite. Environ Sci Technol 3:61 71 enjamin MM, Leckie JO (1982) Effects of complexation by Cl, SO 4, and S 2 O 3 on adsorption behavior of Cd on oxide surface. Environ Sci Technol 16: Davies J, Leckie JO (1978) Effects of adsorbed complexing ligands on trace metal uptake by hydrous oxides. Environ Sci Technol 12: Fox TR, Comerford N (199) Low-molecular-weight organic acids in selected forest soils of the south-eastern US. Soil Sci Soc m J 4: Harter RD, Naidu R (199) Role of metal-organic complexation in metal sorption by soils. dv gron : Jones DL (1998) Organic acids in the rhizosphere a critical review. Plant Soil 2:2 44 Mcride M (198) Influence of glycine on Cu 2+ adsorption by microcrystalline gibbsite and boehmite. Clays Clay Miner 33: Naidu R, Harter RD (1997) Effect of different organic ligands on cadmium sorption by and extractability from soils. Soil Sci Soc m J 62:644 6 Naidu R, Kookana RS, Sumner ME, Harter RD, Tiller KG (1997) Cadmium sorption and transport in variable charge soils: a review. J Environ Qual 26: Rohde RK (1966) Spectrophotometric determination of copper in lead, tin, aluminum, zinc, and their alloys with biscyclohexanone oxalyldihdrazone. nal Chem 38: Rudin M, Motschi H (1984) molecular model for the structure of Cu complexes on hydrous oxide surfaces: n ENDOR study of ternary Cu(II) complex on δ-alumina. J Colloid Interface Sci 98: Sposito G (1989) The Chemistry of Soils. Oxford Univ. Press, New York, p 277 Strobel W (21) Influence of vegetation on low-molecularweight carboxylic acids in soil solution a review. Geoderma 99: Stumm W, Kummert R, Sigg L (198) ligand exchange model for the adsorption of inorganic and organic ligands at hydrous oxide surfaces. Croat Chem cta 3: Violante, Ricciardella M, Pigna M (23) dsorption of heavy metals on mixed Fe l oxides in the absence or presence of organic ligands. Water ir Soil Pollut 14: Xu RK, Zhao Z, Ji GL (23) Effect of low molecular weight organic anions on surface charge of variable charge soils. J Colloid Interface Sci 264: Xu RK, Li C, Ji GL (24a) Effect of low-molecular-weight organic anions on electrokinetic properties of variable charge soils. J Colloid Interface Sci 277: Xu RK, Xiao SC, Li JY (24b) Effect of low molecular weight organic carboxylic acids on adsorption of copper by variable charge soils. J gro-environ Sci 23:34 37 (in Chinese with English abstract) Yu TR (1997) Chemistry of variable charge soils. Oxford Univ. Press, New York, p cknowledgements This study was supported by the Knowledge Innovation Program Foundation of the Chinese cademy of Sciences (ISSSIP22) and the National Natural Science Foundation of China (42762).