Interactions of ferrate(VI) and aquatic humic substances in water treatment

Aquatic humic substances, encompassing humic acid (HA) and fulvic acid (FA), can influence the treatment of ferrate(VI), an emerging water treatment agent, by scavenging Fe(VI) to accelerate its decomposition and hinder the elimination of target micro-pollutants. Meanwhile, HA and FA degrade the water quality through the transformation to disinfection byproducts over disinfection, contribution to water color, and enhanced mobility of toxic metals. However, the interplay with ferrate(VI) and humic substances is not well understood. This study aims to elucidate the interactions of ferrate(VI) with HA and FA for harnessing ferrate(VI) in water treatment. Laboratory investigations revealed distinctive biphasic kinetic profiles of ferrate(VI) decomposition in the presence of HA or FA, involving a 2^nd order kinetic reaction followed by a 1^st-order kinetic reaction. Both self-decay and reactions with the humic substances governed the ferrate(VI) decomposition in the initial phase. With increasing dissolved organic carbon (DOC), the contribution of self-decomposition to ferrate(VI) decay declined, while humic substance-induced ferrate(VI) consumption increased. To assess relative contributions of the two factors, DOC_50% was first introduced to represent the level at which the two factors equally contribute to the ferrate(VI) loss. Notably, DOC_50% (11.90 mg/L for HA and 13.10 mg/L for FA) exceeded typical DOC in raw water, implying that self-decay predominantly governs ferrate(VI) consumption. Meanwhile, ferrate(VI) could degrade and remove HA and FA across different molecular weight (MW) ranges, exhibiting treatment capabilities that are either better or, at least, equivalent to ozone. The ferrate(VI) treatment attacked high MW, hydrophobic organic molecules, accompanied by the production of low MW, more hydrophilic compounds. Particularly, FA was more effectively removed due to its smaller molecular sizes, higher solubility, and lower carbon contents. This study provides valuable insights into the effective utilization of ferrate(VI) in water treatment in presence of humic substances.