Pristine/magnesium-loaded biochar and ZVI affect rice grain arsenic speciation and cadmium accumulation through different pathways in an alkaline paddy soil

•SC, Mg/SC and ZVI decreased concentrations of Cd and iAs in rice grain concurrently.•Mg/SC and ZVI simultaneously reduced Cd and iAs concentrations in soil porewater.•ZVI raised DMA concentrations in porewater and rice grain, but SC and Mg/SC didn't.•SC, Mg/SC and ZVI affected Cd/As accumulation through divergent mechanisms.