Adsorptive/releasing behaviors and mechanisms of sedimentary phosphate in multi-pond treatment system

Effects of pH and Eh on the adsorptive/releasing behaviors of sedimentary phosphate were studied.The adsorptive/releasing potential under different pH and Eh conditions was investigated and possible mechanisms involved were also proposed for different phosphate fractions.Additionally,the formation mechanism of iron-bound-phosphate with Eh steadily increasing was discussed.Results of this study showed that the binding capability of each phosphate differed greatly under different pH conditions as follows: iron-phosphate(Fe-P) and aluminum-phosphate(Al-P) exhibited highest binding capability with pH ranging from pH=7 to pH=8;organic phosphate extracted by alkaline solutions(OPalk) and calcium-phosphate(Ca-P) showed highest binding capability at pH=5 and pH=9,respectively.Mainly through affecting the fractional proportion of Fe(II) to Fe(III) and their possibly binding forms with phosphate,Eh significantly affected the adsorptive/releasing behaviors of sedimentary phosphate.With Eh increasing from-650 mV to-50 mV,Fe(II) was gradually oxidized to Fe(III) and the reduction of Fe(III) was inhibited due to humic covering on their surface.Consequently,Fe(III)-P,possibly existed in the form of P-Fe3+-O-HS-M+,apparently elevated with Eh increasing.