Effects Of Surfactants On Environmental Chemical Behaviors Of Heavy Metals In Soil-plant Systems

Through batch experiments, the characteristics of single adsorptionand desorption of Cu²⁺, Zn²⁺, Cd²⁺, and competitive adsorption and desorption of Cu²⁺and Zn²⁺ on two typical soils (paddy soil and red soil) in Hunan province were studied,the effects of three types of surfactants (anion surfactant LAS, non-ion surfactant AEand cation surfactant CTAB) on adsorption and desorption of Cu²⁺ and Zn²⁺ on soilswere observed, effects of two types of surfactants (AE and CTAB) on heavy metalsextracting from contaminated soils were also investigated. Through solution and soilculture methods, the toxicity of surfactants to maize and soybean, and effects ofsurfactants on soil Cd fraction and bioavailability were investigated. The main resultsare summarized as follows:(1) Adsorption of Cu²⁺, Zn²⁺ and Cd²⁺ onthe soils selected for this study wasfound to follow Langmuir and Freundlich adsorption isotherms. The capacity ofpaddy soil for Cu²⁺, Zn²⁺ and Cd²⁺ was greater than that of red soil. For the same soil,the order of capacity is Cu²⁺＞Zn²⁺＞Cd²⁺. The smaller part of Cu²⁺, Zn²⁺, Cd²⁺adsorbed by soils was desorbed. For the same soil, the order of desorption ratio wasZn²⁺＞Cd²⁺＞Cu²⁺. Cu²⁺, Zn²⁺, and Cd²⁺ adsorbed by red soil were more easilydesorbed than by paddy soil.(2) The total amounts of metal adsorption were greater in competitive systemcontaining Cu²⁺ and Zn²⁺ in 1:1 molar ratios than in the single system containing Cu²⁺or Zn²⁺ at the same total concentration. Adsorption of Zn²⁺ in the competitive systemcontaining a fixed Zn²⁺ concentration was severely depressed by increasing theconcentration Cu²⁺ in the solution, but inverting the roles of Cu²⁺ and Zn²⁺ causedonly relatively minor reduction in adsorption of Cu²⁺.(3) Non-ion surfactant AE was found to enhance the Cu²⁺ and Zn²⁺ adsorptioncapacity on red soil, and to increase desorption ratios from red soil and paddy soil.There were no significant effects of AE on Cu²⁺ and Zn²⁺ on paddy soil. Anionsurfactant LAS enhanced the Cu²⁺ and Zn²⁺ adsorption capacity but decreased thedesorption ratios. Cu²⁺ and Zn²⁺ adsorption capacity increased first, then decreased with increasing concentrations of cation surfactant CTAB. The adsorption capacityreached the top value as CTAB concentration was 50 mg/L.(4) AE solution with higher concentrations and lower pH had better extractioneffects, and the extraction effects of AE on the heavy metals were Cu＞Cd＞Zn＞Pb.The concentration and pH were lower, the extraction effects of CTAB on heavymetals were better. If the extractants had the same concentration and pH value, theextraction effects of AE on Cu and Cd were better than CTAB, the extraction effectson Pb and Zn were worse than CTAB. Effects of AE and CTAB extracting heavymetals were lower, and the highest extracting ratios were 7.08 % for Cu and 2.13%for Pb.(5) In solution culture experiment, LAS could stimulate germination of maizeand soybean when the concentrations were 5 and 10 mg/L. LAS was toxic to maizeand soybean with increasing the concentrations. Within concentrations selected in thisstudy, AE and CTAB inhibited seed germination of maize and soybean, decreased rootlength and sprout height. CTAB toxicity was greater than AE In soil cultureexperiment, with increasing LAS concentrations, plant heights and chlorophyllcontents of maize and soybean showed a pattern of increasing first and decreasingafterward. Maximal values of the heights and chlorophyll contents appeared in thesoils with LAS concentrations 50 mg/L for maize and 20 mg/L for soybean.(6) Applying LAS to soil enhanced soil pH values, decreased effective Cdcontents. Applying AE and CTAB increased effective Cd contents, CTAB decreasedsoil pH values, AE did not significantly change soil pH values. Surfactants affectedthe Cd fraction distribution in soils. Three types of surfactants increased the fractionbound to organic matter. LAS decreased exchangeable fraction and increased fractionbound to Fe/Mn oxides, and CTAB increased exchangeable fraction.(7) Both LAS and CTAB application reduced Cd levels in a plant's tissues andinhibited uptake of soil's Cd to plants by reducing exchangeable Cd contents for LAS,and by competitive behavior for CTAB. AE increased Cd levels in a plant's tissues.