Fractionation and mobility risks of heavy metals and metalloids in wastewater-irrigated agricultural soils from greenhouses and fields in Gansu, China

Abstract

Wastewater irrigation reduces the pressure on freshwater usage but leads to the accumulation of heavy metal(loid)s in soils. This study investigated heavy metal(loid)s (As, Cr, Cu, Ni, Pb, and Zn) in the acid-soluble (F1), reducible (F2), oxidizable (F3), and residual (F4) fractions of soils from greenhouses and fields in Baiyin City, Gansu, China, which had been irrigated with treated industrial and municipal wastewater. Risk assessment (RAC) and modified risk assessment (mRAC) codes were used to estimate the environmental risks based on metal mobility. Results showed that more than half of each studied heavy metal(loid) (72.6%–97.4%) was present in the residual fraction (F4), which is non-bioavailable. Both the concentrations and percentages of metals in the bio-accessible fractions (F1, F2, and F3) showed no significant differences (p > 0.05) between rhizosphere and bulk soils. The greenhouse soils had higher concentrations and percentages of metals (except Pb) in bio-accessible fractions compared with field soils. Similarly, compared with irrigation using treated municipal wastewater, irrigation with treated industrial wastewater resulted in higher concentrations and percentages of all the studied metals in the bio-accessible fractions of soils. The average RAC value of each heavy metal(loid) in soils suggested medium risks from Zn, low risks from As, Cu, Ni, and Pb, and no risks from Cr. According to the mRAC values for 120 soil samples, there were only 13.3% of soils with no potential adverse effect, but 83.3% of soils with a low potential adverse effect and 3.3% of soils with a medium potential adverse effect.