有机酸对Cu Pb Cd在土壤表面竞争吸附的影响

Effect of Organic Acids on Competitive Adsorption of Cu(Ⅱ), Pb(Ⅱ) and Cd(Ⅱ) by Variable Charge Soils

选择一种红壤和一种水稻土,研究低分子量有机酸对Cu(Ⅱ)、Pb(Ⅱ)和Cd(Ⅱ)在这2种土壤中竞争吸附的影响,并与单一体系中的结果进行比较。结果表明,由于柠檬酸和酒石酸与Cd(Ⅱ)形成络合物的稳定常数较小,2种有机酸对2种土壤中Cd(Ⅱ)的吸附均表现出促进作用,而且这种促进作用随有机酸初始浓度的增加而增大。酒石酸促进了红壤对Cu(Ⅱ)和Pb(Ⅱ)的吸附,而且低pH下的促进作用大于高pH下的作用。pH4.2时酒石酸对水稻土吸附Cu(Ⅱ)和Pb(Ⅱ)有一定的促进作用;在pH5.2时酒石酸使水稻土对Cu(Ⅱ)的吸附量减小,但对Pb的吸附基本无影响。柠檬酸对Cu(Ⅱ)和Pb(Ⅱ)的吸附的影响表现为峰形曲线,即吸附量随有机酸浓度的增加而增大,在某一浓度时达最大,然后逐渐减小。在较高浓度和较高pH下,柠檬酸对Cu(Ⅱ)和Pb(Ⅱ)的吸附表现出一定程度的抑制作用,特别在水稻土中。由于红壤游离氧化铁的含量比水稻土高得多,有机酸对红壤吸附重金属离子的促进作用大于水稻土中的作用,相反有机酸对水稻土吸附重金属离子的抑制作用大于红壤中的作用。虽然在竞争条件下有机酸对某一重金属离子吸附的促进作用小于单一体系中的,但竞争条件下所有重金属离子吸附增量的总和大于单一体系中单个离子的吸附增量。在柠檬酸体系中,低pH下Cu(Ⅱ)和Pb(Ⅱ)在单一体系中的吸附量高于竞争体系中的,但在高pH下结果相反,这主要与柠檬酸在不同pH下对重金属离子吸附的影响机理不同有关。

A red soil and a paddy soil were chosen to investigate the effect of low-molecular-weight organic acids on competitive adsorption of Cu （ Ⅱ ）, Pb（ Ⅱ） and Cd（ Ⅱ ） and the results from multi-heavy metal system were compared with that from single metal system. The results showed that citric acid and tartaric acid induced an increase in Cd（ Ⅱ ） adsorption in red soil and paddy soil both at pH4.2 and pH5.2 due to the small stability constant of organic-Cd complexes, and the promotion increased with the increase of the initial concentration of these two acids. Tartaric acid increased the adsorption of Cu （ Ⅱ ） and Pb（ Ⅱ ） by red soil, and increased more at pH4.2 than that at pH5.2. Tartaric acid also increased the adsorption of Cu （ Ⅱ） and Pb（ Ⅱ ） by paddy soil to some extent at pH4.2, but it decreased the Cu（Ⅱ） adsorption in this soil at pH5.2 and did not affected Pb（ Ⅱ） adsorption appreciably at this pH. The adsorption of Cu （ Ⅱ ） and Pb（ Ⅱ） in citric acid system increased with the increase of the initial concentration, and reached a maximum at a certain concentration, and then decreased gradually. At pH5.2 and under higher concentration of citric acid, the adsorption of Cu （ Ⅱ ） and Pb（Ⅱ ） decreased, especially in paddy soil. Due to the higher content of free iron oxides in red soil than that in paddy soil, the promoting effect of organic acids on adsorption of heavy metals was greater in red soil than that in paddy soil; whilst the inhibiting effect of organic acids on the adsorption was larger in paddy soil than that in red soil. The increment of heavy metal adsorption by organic acid in competitive condition was smaller than that in single metal system, but the total increment of adsorption for all heavy metals in multi-heavy metal system was greater than the adsorption increment for one of heavy metals in single metal system. In citric acid system, the adsorption of Cu （ Ⅱ ） and Pb（ Ⅱ ） in single metal system was higher than that in binary-metal system at low pH, whilst an opposite trend was observed at higher pH, which related to the different effects of citric acid on adsorption of heavy metals at different pH values.