Risk Assessment of Heavy Metal in Farmlands and Crops Near Pb-Zn Mine Tailing Ponds in Niujiaotang, China

To accurately evaluate the pollution and risk of heavy metals in crops and farmlands near mines, we determined the contents of Cr, Ni, Cu, As, Cd, Pb, and Zn in 10 farmland soil sampling sites and six crops (pak choi, rice, spring onion, radish, Chinese cabbage, Chrysanthemum coronarium) in an area near the Niujiaotang Pb-Zn mine in Duyun City, China. Four evaluation methods were compared, including the potential ecological hazard index, Nemeiro comprehensive pollution assessment, risk assessment code, and the ratio of secondary phase to primary phase methods. The average concentration of As, Cd, Pb, and Zn exceeded the soil environmental background levels in Niujiaotang and Guizhou Province. Cd exceeded the standard substantially, and Zn pollution accumulation was the most evident. Heavy metal contamination of crops was in the order pak choi > Chinese cabbage > spring onion > paddy > radish > Chrysanthemum coronarium, whereas heavy metal concentration in crops were in the order Zn > As > Cr > Cd > Ni > Pb. The levels of all heavy metals except Cu exceeded Chinese food hygiene standards. Carcinogenic and non-carcinogenic chemicals in crops present significant risks to adults and children. Risk evaluation considering the morphological contents of heavy metals rather than their total concentration was more accurate for environmental quality assessment of agricultural soils. Samples should be collected at different times to study the spatial and temporal distribution, and further studies on the migration transformation of heavy metals between the tailings pond-soil-crop should be conducted.

Characteristics and diversity of microbial communities in lead-zinc tailings under heavy metal stress in north-west China

High-throughput 16S rRNA and 18S rRNA sequencing were performed to study the changes of soil microbial diversity and community structure under different heavy metal pollution levels in Chengxian lead-zinc mining area, Gansu Province. In this study, we characterized the main physicochemical properties, multiple heavy metal pollution, and microbial community structure of the soil in the tailings. The results show that the soil near the tailings pond was alkaline, barren and the heavy metals were seriously polluted. The microbial diversity and richness of S1 and S2 sites were significantly lower than that of CK2 site (P < 0·05), indicating that the heavy metal pollution could change the physicochemical properties and microbial community structure in soil. Among 97 identified core operating taxa of fungal communities, Ascomycota, Teguta and Basidiomycota were dominant at the phylum level, while among 1523 identified core operating taxa of bacterial communities, Actinomycota was dominant at the phylum level. In addition, the redundancy analysis and Spearman correlation analysis showed that the physicochemical properties and the heavy metal concentration had significant effects on the composition and distribution of soil microbial community. The basic characteristics of soil physicochemical properties, multiple heavy metal pollution and microbial community structure in the tailings were revealed, hoping to provide a basis for ecological rehabilitation of tailings by revealing the variance rule of microbial community diversity in the future.

[Subcellular Distribution and Chemical Forms of Heavy Metals in Three Types of Compositae Plants from Lead-Zinc Tailings Area]

Field investigation on the content of heavy metals in soils and three types of widely distributing compositae plants(Artemisia lavandulaefolia, Ageratum conyzoides L., Crassocephalum crepidioides) in lead-zinc tailings farmland of Yangshuo, Guangxi Zhuang Autonomous Region was carried out, and the differential centrifugation technique and sequential chemical extraction method were used to study the subcellular distribution and chemical forms of heavy metals in these plants. The results indicated that the soil in the tailings farmland was highly contaminated by Cd, Pb and Zn, and their concentrations were 37.7, 5.7 and 8.9 times higher than their respective values of national standard for soil environment quality(GradeⅡ). The contents of Cd, Pb and Zn in the analyzed plants exceeded the normal ranges. Ageratum conyzoides L. and Crassocephalum crepidioides showed strong capability in tolerance, accumulation and transport of Cd, and they could be used as pioneer plants for Cd-phytoremediation in study area and some related areas. In addition, Cd concentrations in the stem and leave of Crassocephalum crepidioides were 159.6 mg·kg^-1 and 219.5 mg·kg^-1, respectively, which exceeded the threshold of Cd hyperaccumulator. Thus, Crassocephalum crepidioides can be regarded as a Cd-hyperaccumulator. Majority of total Cd, Pb and Zn were found in soluble fraction and cell wall in the plants, while only a small quantity were distributed in organelles. As for chemical forms distribution, Cd, Zn and Pb predominated NaCl-, HAC-, and HCl-extractable forms both in roots and shoots of the plants. Therefore, cell wall binding, vacuolar compartmentalization and distribution mainly in lower active chemical forms were supposed to be the main mechanisms for tolerance to heavy metals in the study plants. A difference of Cd subcellular distribution and its chemical forms in the three Compositae plants was observed. Compared with the shoot of Artemisia lavandulaefolia, more Cd was located in the vacuolus and cellular soluble parts in Ageratum conyzoides L. and Crassocephalum crepidioides. Moreover, the proportion of active chemical Cd in the shoots was lower than that in the roots. These results showed that the subcellular distribution and chemical forms of Cd were related to the plant species,and also indicated Ageratum conyzoides L. and Crassocephalum crepidioides were likely to possess a higher tolerance and accumulation of Cd.