Temporal-spatial distribution and variability of cadmium contamination in soils in Shenyang Zhangshi irrigation area,China

Heavy metal contamination in soils has been of wide concern in China in the last several decades. The heavy metal contamination was caused by sewage irrigation, mining and inappropriate utilization of various agrochemicals and pesticides and so on. The Shenyang Zhangshi irrigation area （SZIA） in China is a representative area of heavy metal contamination of soils resulting from sewage irrigation for about 30 years duration. This study investigated the spatial distribution and temporal variation of soil cadmium contamination in the SZIA. The soil samples were collected from the SZIA in 1990 and 2004; Cd of soils was analyzed and then the spatial distribution and temporal variation of Cd in soils was modelled using kriging methods. The kriging map showed that long-term sewage irrigation had caused serious Cd contamination in topsoil and subsoil. In 2004, the Cd mean concentrations were 1.698 and 0.741 mg/kg, and the maxima 10.150 and 7.567 mg/kg in topsoils （0-20 cm） and subsoils （20-40 cm） respectively. These values are markedly more than the Cd levels in the second grade soil standard in China. In 1990, the Cd means were 1.023 and 0.331 mg/kg, and the maxima 9.400 and 3.156 mg/kg, in topsoils and subsoils respectively. The soil area in 1990 with Cd more than 1.5 mg/kg was 2701 and 206.4 hnl2 in topsoils and subsoils respectively; and in 2004, it was 7592 and 1583 hm^2, respectively. Compared with that in 1990, the mean and maximum concentration of Cd, as well as the soil area with Cd more than 1.5 mg/kg had all increased in 2004, both in topsoils and subsoils.

Study on principles and mechanisms of new biochar passivation of cadmium in soil

At present,many researchers have studied the modification of biochar and explored the adsorption of cadmium by modified biochar.The adsorption capacity of cadmium for general modified biochar is about 30-150 mg/g.The new biochar(BM)we studied greatly improves the adsorption effect of cadmium in water,and the maximum adsorption capacity of cadmium can reach 400 mg/g,so we tried to apply it to the passivated cadmium in polluted farmland soil.This paper discusses the mechanism of BM passivation of heavy metal cadmium in soil and its protective effect on crop growth.The study found that:(1)the content of cadmium in wheat was 3.98 mg/kg in the soil with BM800.The addition of BM800 reduced the cadmium content of wheat by 75.43%compared with the blank control group;(2)BM contains special iron-containing functional group(-Fe-OOC-,-Fe-R-COOH,-Fe-R-OH,etc.)and aromatic structure C=Cπ,and these structures can react with cadmium to form stable complexes(C=Cπ-Cd,(FeO)2Cd,(Fe-RO)2Cd,etc.);(3)there are special mineral crystal structures XiFeYjOk in BM.XiFeYjOk can react with cadmium to form Cd-XiFejOk.Cd-XiFejOk can stably exist in biochar,and can provide more adsorption sites,which strengthens the adsorption and passivation of cadmium in soil by BM.