Catalytic effect of soil colloids on the reaction between Cr VI and p-methoxyphenol

Abstract

Adsorption of Cr VI and p-methoxyphenol (PMP) on soil colloids at different pH media was studied. The resulting k 1 and n of 1.89×10 2 and 0.53 ( r 2=0.99) and k 2 and b of 0.13 and 1.25×10 3 ( r 2=0.96) were obtained from Freundlich ( Q= k 1 C aq n ) and Langmuir [ Q= k 2 bC aq/(1+ k 2 C aq)] simulation equations, respectively, for Cr VI adsorption on soil colloids (pH 4.20). The adsorption of PMP on soil colloids in pH 5.72 media was simulated by five different equations and the results indicated that the Fritz-Schluender one ( r 2=1.00) was the most suitable among them. Adsorption quantity of Cr VI and PMP on colloids increased with increasing acidity in the pH range of 3.5–9.0. Study of Cr VI adsorption kinetics indicated that the adsorption equilibrium of Cr VI was reached rapidly within 2 h. In pure aqueous solution, Cr VI reduction by PMP was observed only when the media's pH was lower than 4.0. Oxidation and reduction reaction between Cr VI and p-methoxyphenol obviously occurred when soil colloids were involved in this system, even at pH⩾7.0, which strongly suggested that minerals in soil colloids acted as catalysts to speed the reaction of Cr VI and PMP. The oxidized product of PMP by Cr VI, extracted by chloroform in acid media and analyzed by gas chromatography–mass spectrometry, was identified as benzoquinone. The reaction included two steps of one electron process.