A KINETIC-STUDY OF THE REDUCTION OF CHROMIUM(VI) TO CHROMIUM(III) BY THIOUREA

The reduction of chromium(VI) to chromium(III) by thiourea is a key as pect of the chromium(VI)-thiourea-polyacrylamide gel polymer system us ed in oil recovery processes. A study was undertaken to develop a kine tic model for the reduction reaction. Reaction mixtures were prepared and chromium(VI) concentrations were determined from the mixtures's vi sible absorbance. The reaction rate was found to depend on the concent rations of chromium(VI), thiourea, and polyacrylamide and on solution pH. A kinetic model for the reactions was developed using the experime ntal data and previously proposed reaction mechanisms. The model accur ately predicts the reaction rate in solutions that do not contain poly acrylamide and predicts a rate that is approximately correct for solut ions containing polyacrylamide. (C) 1998 Elsevier Science B.V.