As a global pollutant process,the reduction of mercury(Hg)is especially important.One pathway is through an abiotic reduction with humic acids(HAs),which is controlled by different factors,including initial Hg and HA concentrations,pH,temperature and light.In this study,three humic acids were selected to illustrate the Hg~(2+)abiotic reduction mechanisms by HAs,and to identify the key limiting factors for reduction rates and amounts.In addition,the initial status of the HAs as a solid or in an aqueous solution were also compared,to help explain why HAs show different dominant characteristics(e.g.complexation or reduction)in the reaction process with Hg.Results indicated that HAs were able to reduce Hg abiotically.Higher initial Hg,higher HA concentrations and either high(8.1)or low(3.6)solution pH decreased the HA reduction capacity.In addition,Hg°production rates increased with increasing temperature,and the same trend was observed with light exposure.Humic acids added as an aqueous solution resulted in significantly greater Hg°production than addition as a bulk solid.Finally,the Hg reduction rate and capacity varied significantly(P<0.05)with HAs from different sources.These findings helped to explain why HAs showed different dominant characteristics(e.g.complexation or reduction)in the reaction process with Hg,and evidentially demonstrated the existence of a possible pathway of Hg~(2+)reduction,which indicated that humic substances in natural environments,especially in water bodies,could act either as a sink or a source for Hg. As a global pollutant process, the reduction of mercury (Hg) is especially important. One way is an abiotic reduction with humic acids (HAs), which is controlled by different factors, including initial Hg and HA concentrations, pH, temperature and light .In this study, three humic acids were selected to illustrate the Hg ~ (2+) abiotic reduction mechanisms by HAs, and to identify the key limiting factors for reduction rates and amounts.In addition, the initial status of the HAs as a solid or in an aqueous solution were also compared, to help explain why HAs show different dominant characteristics (eg complex or reduction) in the reaction process with Hg.Results indicated that HAs were able to reduce Hg abiotically. Hagher initial Hg, higher HA concentrations and either high (8.1) or low (3.6) solution pH decreased the HA reduction capacity. In addition, Hg ° production rates increased with increasing temperature, and the same trend was observed with light exposure. Humic acids added as an aqueous solution induced in addition greater than Hg ° production than addition as a bulk solid. Finaally, the Hg reduction rate and capacity significantly (P <0.05) with HAs from different sources. The findings helped to explain why HAs showed different dominant characteristics (eg complex or reduction) in the reaction process with Hg, and evidentially demonstrated the existence of a possible pathway of Hg ~ (2+) reduction, which indicates that humic substances in natural environments, especially in water bodies, could act either as a sink or a source for Hg.