In order to investigate the effects of citric acid on isothermal adsorption of Zn(II) and Cd(II) under conditions of single element

coexistence of two elements

and adding citric acid into the competitive system (a form of shorthand：the first

second

and third condition) by the purple bulk soil and micro-aggregates of different particle sizes in a tea plantation. The experiment was carried out by using methods of siphon settlement and centrifugal freezing thawing. The results showed that: (1) The multilayer adsorption dominated by specific adsorption was saturated gradually with increase of the Zn2+ and Cd2+ concentration. The adsorptive amount order was <0.002 mm > 0.05-0.002 mm > bulk soil. The Zn and Cd accumulating ability of the micro-aggregates of 0.25-0.05 mm and 2-0.25mm particle sizes were relatively lower. Under different conditions

Cd was easier to accumulate than Zn. The adsorption of Zn and Cd on soil surface was spontaneous and exothermic. The fitting results of the Freundlich equations were better than those of the Langmiur equation. At the same time

there was a significant correlation between the maximum adsorption capacity and organic matter content. (2) Under a single processing mode

the holding effects of Zn and Cd were obvious. When Zn and Cd coexisted

their competitive behaviors weaken each other

with Zn having a stronger inhibitory effect on Cd. (3) Citric acid promoted the adsorption of Zn and Cd when the concentration was 0.1-1 mmol/L

but accelerated leaching when the concentration was higher than 10 mmol/L. The retention of Zn and Cd by micro-aggregates (<0.002mm) was further increased after adding citric acid at different concentrations. The results showed that citric acid could be used as a regulator to maintain the supply of Zn in the soil appropriately while weakening the migration of Cd

in order to alleviate the compound pollution in the soil in tea plantations.