Modification of arsenic and cadmium species and accumulation in rice using biochar-supported iron-(oxyhydr)oxide and layered double hydroxide: Insight from Fe plaque conversion and nano-bioassembly in the root

The impact of organic solid waste-supported nanomaterial application on the biogeochemical looping of arsenic (As) and cadmium (Cd) in rhizosphere-root system of rice remains unclear. This study investigated the biochar-supported Fe-(oxyhydr)oxide and layered double hydroxide (FLBC) to modify As and Cd species and their accumulation in rice rhizosphere system under all life-cycle. The results demonstrated that total As and Cd in porewater and rice tissues decreased by > 70 % following 1 wt% FLBC application, close to national standards in grain. Inorganic As levels significantly decreased in roots and grains under FLBC application, resulting in the preferential accumulation of organic As like dimethylarsenate. FLBC amendment substantially increased the Fe plaques on root surfaces due to a rise in Fe(II) sink influenced by Fe(III)-reducing bacteria, and then promoted the conversion of crystalline-like FeOx to an amorphous-like structure on Fe plaque through X-ray diffraction and transmission electron microscope technologies (TEM), eventually reducing the As and Cd availabilities. The ultrastructural characterization, employing TEM equipped EDS spectrometer, revealed the bioassembly of nanoscale layered double hydroxide for the first time within the vacuoles of xylem cells in the root apex under FLBC amendment. This process allowed the transformation of As and Cd species from metastable Fe–Al oxides-bound compounds to stable complexations of cysteine-bound As(V)/Cd and Cd(OH)2 within the intracellular spaces, as demonstrated by X-ray absorption near fine structure spectrum. In short, FLBC hindered the transformation and accumulation of As and Cd species in the rhizosphere-root system, providing a fresh perspective on organic solid waste-supported nanomaterials for ensuring safe crop production.