Effects of rice straw on the speciation of cadmium (Cd) and copper (Cu) in soils

Four soils were collected from different sites of China in Lechang (LC, Guangdong province), Changsha (CS, Hunan province), Jiaxing UX, Zhejiang province) and Hangzhou (HZ, Zhejiang province), and were spiked with Cu (50 mg kg(-1)) and Cd (5 mg kg(-1)). The effects of rice straw addition (6%) on the chemical distribution of both metal ions were studied by measuring the soluble metal ion concentrations and the free metal ion concentrations (using the Donnan membrane technique) after 1-, 3- and 6-month incubation. Results show that the addition of rice straw increased soil pH by about 0.4 pH unit on average, and increased DOC significantly in soil LC, CS and JX, but not in soil HZ. With the addition of rice straw, total soluble Cu concentration increased from 0.82 mu mol l(-1) (0.26-2.4 mu mol l(-1)) to 1.22 mu mol l(-1) (0.70-3.60 mu mol l(-1)), whereas total soluble Cd concentration decreased from 20 nmol l(-1) (2-70 nmol l(-1)) to 15 nmol l(-1) (2-56 nmol l(-1)). When rice straw was added, both free Cu 2 and Cd 21 concentrations decreased, for free Cu2+ concentration from 217 nmol l(-1) (31 to 369 nmol l(-1)) to 124 nmol l(-1) (22 to 263 nmol l(-1)) and for free Cd2+ concentration from 16 nmol l(-1) (1-55 nmol l(-1)) to 12 nmol l(-1) (1-43 nmol l(-1)). With the increase of incubation time, free Cu2+ concentration tended to increase but free Cd2+ concentration decreased. Speciation model calculations show that compared to the binding capacity of soil organic matter, the capacity of rice straw is much less important. The decrease of free metal ion concentration upon rice straw addition can be attributed mainly to the increased pH. The higher DOC content in the rice straw treatment could be the reason for higher soluble Cu concentration when rice straw was added. Adsorption to DOC is much less important for Cd than for Cu. Calculation also shows that adsorption to clay minerals plays a more important role for Cd than for Cu, which may explain the stronger ageing effects on Cd distribution. (c) 2008 Elsevier B.V. All rights reserved.