The role of clays, clay minerals and clay-based materials for nitrate removal from water systems: A review

Excessive use of nitrogenous fertilizers has led to increased nitrate concentrations in groundwater that pose a threat to human health, via nitrate-contaminated potable water, and contribute to eutrophication. The elimination of nitrate from water systems has been thoroughly examined; however, unconventional and low-cost technologies are greatly needed. Clays and clay minerals are widely-used for environmental applications, mostly due to their non-toxicity, worldwide abundance, low cost and physicochemical properties (high surface area, ion exchange capacity, high sorption and catalytic properties). Most are used for the degradation of cationic pollutants, nevertheless, they can be equally efficient at anionic decontamination, depending on the modification process they have undergone, or the materials they are combined with. This review aims to assemble the available literature research on the application of clays and clay minerals as well as the mechanisms that lead to successful removal of nitrate from water. The main characteristics of clays and clay minerals in nitrate uptake are evaluated and the known shortcomings of their application discussed, leading to suggestions for further research.