Utilization of solar energy for photoreduction of industrial wastewater containing hexavalent chromium with zinc oxide semiconductor catalyst Utilization of solar energy for photoreduction of industrial wastewater containing hexavalent chromium with zinc oxide semiconductor catalyst Pallavi Mitra*, Prantik Banerjee, Sampa Chakrabarti, Sekhar Bhattacharjee Department of Chemical Engineering, University of Calcutta, 92, Acharya P.C. Road, Kolkata 700 009, India Tel. +91 033 2350 8386; Fax: +91 033 2351 9755; email: pallavim8@gmail.com Received 11 June 2012; Accepted 3 December 2012 A B S T R A C T Scarcities of water and energy are going to be critical in the near future. A possible remedy is to reuse industrial wastewater after suitable treatment using a renewable energy. In this work, wastewater containing hexavalent chromium is treated with Zinc oxide (ZnO) semi- conductor photocatalyst in the presence of sunlight to reduce the more toxic hexavalent metal to its less toxic trivalent counterpart. Thirty five percent reduction was obtained after 2 h with 10 mg/L substrate, 0.4 g/L photocatalyst, and 75kLux solar radiation at 31oC tem- perature. Process parameters are initial concentration of substrate, loading of photocatalyst, pH and concentration of the electron donor. Initial rate of reduction varied only with ZnO- loading and hence it was zero order with respect to both the substrate and electron donor. An alternative rate equation based on the modified Langmuir Hinshelwood Hougen Watson (LHHW) model compares well with the mechanistic rate equation. Keywords: Hexavalent chromium; Solar energy; Photoreduction; ZnO-semiconductor 1. Introduction Among the total world population of 6,700 million, approximately 600 million people are suffering from chronic water scarcity and nearly one billion people do not get safe drinking water [1]. The most rational and reasonable remedies for this water crisis are to minimize wastage of water as well as to recycle and to reuse industrial wastewater. Treatment of industrial wastewater will demand a huge amount of energy in the coming years. Energy crisis, in turn, will worsen the problem of water crisis and vice versa since water and energy are very closely related. Energy required to produce such a large quantity of treated water will not be easily available from the conventional sources. So a serious participation of a non-conventional and renewable source of energy for the treatment of water and wastewater is required [1]. Solar energy is the most competent candidate as a source of renewable energy [1–3]. India is a tropical country and the daily average solar energy incident over India varies from 4 to 7 kWh/m2 with about 2,300–3,200 sunshine hours per year, depending upon location [4,5]. Hexavalent chromium is used in a variety of industries including leather-chrome tanning, preserva- tion of wood, production of stainless steel and textile dyes. Hence, hexavalent chromium comes into waste- water from such industries. Chromium is a priority*Corresponding author. 1944-3994/1944-3986 � 2013 Balaban Desalination Publications. All rights reserved. Desalination and Water Treatment www.deswater.com doi: 10.1080/19443994.2013.770202 51 (2013) 5451–5459 August