The effect of the novel composite material LiNbO₃@Fe₃O₄ on the nitrate removal, and Mn²⁺ oxidation efficiency by autotrophic denitrification strain Achromobacter sp. A14 was investigated in this study. The optimum conditions were tested by using five levels of initial Mn²⁺ concentrations (40, 60, 80, 100 and 120 mg/L), initial pH (5.0, 6.0, 7.0, 8.0 and 9.0) and temperature (20, 25, 30, 35 and 40°C). A maximal nitrate removal ratio of nearly 100% and a maximal Mn²⁺ oxidation ratio of 71.59% were simultaneously achieved at pH 7.0, 80 mg/L Mn²⁺ and 30°C by bacteria A14 with 300 mg/L LiNbO₃@Fe₃O₄ as catalytic material. Biomaterial cycle testing indicated that the denitrification efficiency of bacteria A14 with LiNbO₃@Fe₃O₄ remained steady after 10 batches. GRAPHICAL ABSTRACT

Increasing evidence indicates that microbial metabolism and mineral transformations within low-permeability clay layers, together with solute exchange between clay layers and adjacent aquifers, can jointly regulate arsenic (As) release, immobilization, and remobilization. However, how coupled C, N, S, and Fe cycling in clay layers regulates As oxidation state, solid–liquid partitioning, and transport, and how these effects extend across clay–aquifer interfaces, has not been critical narratively synthesized. This review focuses on subsurface clay layers and summarizes their hydrogeochemical properties, microbial communities, and vertical redox zonation. It examines how microbially mediated C–N–S–Fe processes influence As release, transformation, and immobilization, and further discusses the export of reactive constituents from clay layers, biogeochemical reactions at clay–aquifer interfaces, As enrichment in adjacent aquifers, and the effects of groundwater pumping on As transport. Available studies suggest that As behavior is condition-dependent and reflects interactions among microbial metabolism, electron-donor and electron-acceptor availability, mineral transformations, and interfacial transport, which together determine whether As is retained, released, or remobilized. Management implications and future research priorities are also considered. The resulting cross-scale framework links biogeochemical processes within clay layers with clay–aquifer exchange and aquifer responses, providing a basis for long-term groundwater As risk assessment, monitoring, and management.