Mitigation of cadmium and arsenic in rice grain by zero valent iron through modifying the soil chemical property and microbial community structure

Human health is seriously threatened by cadmium (Cd) and arsenic (As) poisoning in paddy soils. Due to different geochemical behavior, Cd and As are challenging to repair together. We aims to inquire into the influences of zero valent iron (ZVI) and its mixture on acidic Cd-As contaminated paddy soil and reveal the relevant mechanisms. We carried out the pot experiments of ZVI and its mixture on the Cd and As concentrations of rice and iron-plaque (Fe-plaque) on root surface. While also investigated soil chemical properties and microbial community structure. Compared with unmodified soil (CK), the soil-available Cd contents and Cd concentrations in rice grains treated with ZVI, ZVI + biochar (ZVI_BC), and ZVI + biochar + calcium carbonate (ZVI_BC_Ca) were lower (P < 0.05), and the reduction gradually increased. The mechanism may be as follows: 1) Raised the relative abundance of Firmicutes and Hydrogenispora; 2) Lossed the relative abundance of Chloroflexi, Patescibacteria, Myxococota, Anaerolineaceae, (g_norank_f__norank_o__SBR1031); 3) Gained the soil cation exchange capacity; 4) Promoted the formation of Fe-plaque on the rice roots surfaces. Only ZVI reduced (P < 0.05) the soil-available As contents. The mechanism may be as follows: 1) Decreased the relative abundance of (Clostridium_sensu_stricto_5); 2) Increased the relative abundance of Anaeromyxoactor; 3) Reduced the soil organic matter content. ZVI can simultaneously repair Cd-As contaminated soil, ultimately diminishing the concentrations of Cd and As in rice. ZVI_BC_Ca can better amend Cd contaminated soil, but not suitable for As contaminated soil. This provides reference for the toxic metal/metalloid elements remediation in paddy soils.