Adsorption-desorption characteristics of mercury in paddy soils of China

Mercury (Hg) has received considerable attention because of its association with various human health problems. Adsorption-desorption behavior of Hg at contaminated levels in two paddy soils was investigated. The two representative soils for rice production in China, locally referred to as a yellowish red soil (YRS) and silty loam soil (SLS) and classified as Gleyi-Stagnic Anthrosols in FAO/UNESCO nomenclature, were respectively collected from Jiaxin County and Xiasba District of Hangzhou City, Zhejiang Province. The YRS adsorbed more He2+ than the SLS. The characteristics of Hg adsorption could be described by the simple Langmuir adsorption equation (r(2) = 0.999 and 0.999, P < 0.01, respectively, for the SLS and YRS). The maximum adsorption values (X-m) that were obtained from the simple Langmuir model were 111 and 213 mg He2+ kg(-1) soil, respectively, for the SLS and YRS. Adsorption of He2+ decreased soil pH by 0.75 unit for the SLS soil and 0.91 unit for the YRS soil at the highest loading. The distribution coefficient (kd) of Hg in the soil decreased exponentially with increasing He2+ loading. After five successive desorptions with 0.01 mol L-1 KCl solution (pH 5.4), 0 to 24.4% of the total adsorbed He2+ in the SLS soil was desorbed and the corresponding value of the YRS soil was 0 to 14.4%, indicating that the SLS soil had a lower affinity for He2+ than the YRS soil at the same Hg2+ loading. Different mechanisms are likely involved in He2+ adsorption-desorption at different levels of He2+ loading and between the two soils.