The optimum pH and Eh for simultaneously minimizing bioavailable cadmium and arsenic contents in soils under the organic fertilizer application

[Display omitted] •Soil pH and Eh have a significant effect on the speciation of Cd and As.•With increasing pH or decreasing Eh, bioavailable Cd content linearly decreased.•With increasing pH or decreasing Eh, bioavailable As content exponentially increased.•The optimum trade-off of the bioavailable Cd and As by a pH = 7.31 and an Eh = −130 mV. Cadmium (Cd) and arsenic (As) are usually found in contaminated soils. Their bioavailabilities are often related to pH and Eh, which indicate a generally contrasting or antagonistic effect. In this paper, the pH and Eh of soil samples were altered by adding organic fertilizer, and Tessier sequential extraction procedures were used to extract heavy metal speciation. With increasing pH and decreasing Eh, the content of the exchangeable, carbonate-bound, Fe-Mn oxide-bound, and organic-bound forms of the Cd decreased. The content of the residual form of Cd increased. The content of water-soluble Cd also increased. In terms of As, the content of the water-soluble, exchangeable, carbonate-bound, and organic-bound forms increased, and the content of the residual form decreased. Bioavailable forms contained water-soluble and exchangeable forms. With increasing pH or decreasing Eh, bioavailable Cd content linearly decreased, whereas bioavailable As content exponentially increased. The fitting curve showed that compared with 200 mV, the bioavailable Cd content decreased by 52.4%, and the bioavailable As content increased by 3.2 times at −400 mV. Finally, using the indicator of trade-off value, optimum pH = 7.31, and Eh = −130 mV, the bioavailable Cd and As contents were simultaneously maintained at a relatively low level. The novelty of this paper is studying the effects of different soil pH and Eh values changed by organic fertilizer on the speciation of Cd and As rather than the direct quantitative effects between organic fertilizer and the speciation of Cd and As, which can better explain the mechanism underlying the effect of organic fertilizer on the speciation of Cd and As.