Effects of insoluble decomposition products of manures on adsorption and desorption of copper and cadmium

The effects of the insoluble decomposition products of three manures ( rice straw , pig manure and Chinese milk vetch) and the insoluble residues of two soils after being incubated un- der submerged conditions w ith the three manures on deposition, adso rption and deso rption of Cu and Cd were studied.When the initial concentration w as 10-4mol/L, the deposition of Cu w as promoted in low pH range ( <6) , w hile inhibited in hig her pH range ( >6) by the insolu- ble decomposition products.When the initial concentration w as 10-5mol/ L, the deposition w as promo ted w ith a greater deg ree.All the three org anic decomposition products enhanced the de- position of Cd w ith a smaller deg ree than that of Cu.At the same pH, larger amount of Cu and Cd was adsorbed by the red soil incubated w ith the manures than that incubated w ithout them. The pH v alues of the red soil w ere raised by the incubation w ith the three manures, then the adsorption w as promoted further.As for the fluvo-aquic soil, the pH-raising effect of the incu- bation with the manures w as the main cause for promoting the adso rptio n of Cu.It was con- cluded that the pH-raising effect and the complexation-enhancing effect w as the major cause fo r the insoluble decomposition products free o r incorporated with the soils to promote the deposi- tion or adsorption of Cu and Cd.The re-solution of the insoluble products in higher pH range may lead to a reduction of the deposition of Cu and Cd.The Cu and Cd adsorbed by the red soil incubated with the manures was desorbed more difficultly than that incubated w ithout ma- nures.