Effect of Alkyl Phosphoric Acid on Extraction Rate of Fe(III) with D2EHPA : Role of alkyl phosphoric acids on metal extraction (3rd Report)

Extraction rate of Fe(III) with D2EHPA was investigated in the presence of alkyl phosphoric acids to elucidate the extraction mechanism and rate determining step. Butyl phosphoric acid and 2-ethyloctyl phosphoricacid were used for the investigation. The extraction rate of Fe(III) is extremely accelerated by addition of alkyl phosphoric acids. In the presence of 2-ethyloctyl phosphoric acid, the rate equation is expressed as, -d[Fe3+] aq/dt=kf*[Fe3+]aq[H2A]org[H+]aqn-kb*[Fe3+]org[RH]org-1[H2A]org-1[H+]aq2(-1≦n≦0),where kf* and kb* are forward and backward rate constants, and[HR]org and [H2A]org denote D2EHPA concentration and 2-ethyloctyl phosphoric acid concentration in the organic phase. Dependency of forward reaction on hydrogen ion concentration changes from-1 to 0, which is connected with the dissociation of mono-ester. The rate equation is interpreted by the idea that rate determining step is replacement reaction of water molecule in aquo Fe(III) ion with mono-ester molecule or ion. Value of kf/kf0, which is the rate constant in the presence of mono-esters devided by the rate constant in the absence of mono-esters, is a useful and quantitative measure to indicate the effect of mono-esters on the extraction