Chromium removal from flocculation effluent of liquid-phase oil-based drill-cuttings using powdered activated carbon

The practice of injecting the flocculation effluent of liquid-phase oil-based drill-cuttings (LPOBDCs) into the ground is a popular current disposal-option for this type of drilling waste in Nigeria. Unfortunately, this practice leads to environmental degradation because this flocculation effluent contains high concentrations of chromium (Cr6+), which is a heavy-metal pollutant. Consequently, it is recommended that the flocculation effluent be treated in a batch adsorption process, using powdered activated-carbon (PAC) in order to improve the quality of the flocculation effluent before its sub-surface injection. A representative sample of the untreated LP-OBDC was collected from a mud pit close to an oil-well in the Niger Delta region of Nigeria. Initial characterization of the untreated LP-OBDC showed that the concentration of Cr6+ was 5.26 g/m(3). Flocculation of the LP-OBDCs was carried out using aluminum sulphate and sodium chloride as coagulant and flocculant, respectively. After flocculation, the Cr6+ was reduced to 5.01 g/m(3) (i.e. a 4.75% reduction). The flocculation effluent was then subjected to an activated-carbon batch-adsorption process. At the end of the process, the Cr6+ content was further reduced to 2.77 g/m(3) (i.e. a 44.7% reduction), which shows a significant improvement on the quality of the flocculation effluent. The adsorption mechanism of Cr6+ onto the PAC fitted the 2 second-order kinetic model (with R-2 = 1.00 approximately) with equilibrium being attained within 60 minutes of contact time. Performance characteristics of the PAC show that the maximum adsorption capacity of the adsorbent at equilibrium is 1.60 mg/g (i.e. a 64.0% adsorption) at the initial chromium-concentration, CO, of 2.50 g/m(3). Also, adsorption-capacity data, obtained using the regressed Freudlich's isotherm (Q(e) = 0.768 C-e(1/1.32)), were quite close to the pertinent experimental data (with R-2 = 0.981). The adsorption intensity, n, is 1.32, indicating a strongly favourable adsorption, which shows that a large amount of chromium is adsorbed at low concentrations of adsorbate in the flocculation effluent. (c) 2007 Elsevier Ltd. All rights reserved.