Performance of Activated Carbon Supported Catalyst during Wet Oxidation of Pulping Effluent

Bholu Ram Yadav and Anurag Garg*

Centre for Environmental Science and Engineering, Indian Institute of Technology Bombay,
Powai, Mumbai, 400076, INDIA
*Corresponding author: a.garg@iitb.ac.in

Abstract

In a pulp and paper mill, the pulping process generates a high strength colored liquid stream primarily containing not easily biodegradable compounds. The major contaminants include lignin, hemicellulose and phenols. The intense color of the effluent is primarily due to the presence of lignin. Wet oxidation (WO) is one of the potential pre-treatment alternatives for the high waste streams which not only reduce the intense color from the effluent but also make the wastewater amenable to the conventional biological process. The reaction follows free radical mechanism and occurs in three steps: initiation, propagation and termination. In the chain reaction, the formation of hydroxyl radical (HO·) is desired which has higher oxidation potential compared to molecular oxygen. The generation of such radicals can be accelerated with the addition of a suitable catalyst. The present study was aimed to investigate the efficacy of activated carbon supported non-noble based metals as catalysts for the oxidative degradation of the diluted pulping liquor obtained from a wood based kraft pulp and paper mill. The characteristics of the diluted pulping liquor were as follows: pH = 11.23, chemical oxygen demand (COD) = 16000 mg/l, total organic carbon (TOC) = 4500 mg/l, and 5-day biochemical oxygen demand (BOD5) = 2600 mg/l.
The catalytic wet oxidation (CWO) of the wastewater (adjusted pH = 8.5) was carried out at
moderate operating conditions (120 – 170°C temperature and 0.5 MPa of oxygen partial pressures) in a stainless steel high pressure reactor (capacity = 0.7 L). For CWO runs, two heterogeneous catalysts, namely, Cu/Ce/AC and Cu/Mn/AC (mass ratio = 3:3:4) were used which were synthesized in the laboratory. After 3 h reaction, the maximum TOC reduction of
65% was found in the presence of Cu/Ce/AC catalyst at a reaction temperature of 170°C compared to only 53% and 44% reductions were achieved at 150oC and 120oC temperature, respectively. The other catalyst (i.e., Cu/Mn/AC) was found to be less efficient and its presence could reduce the TOC to 58% of the initial value at 170oC temperature. In comparison, the non- catalytic oxidation reaction could cause only 42% reduction in TOC. The color reduction with
Cu/Ce/AC and Cu/Mn/AC catalysts was 86% and 74% after 3 h compared to only 44% after non-catalytic WO process at the same temperature. After CWO with Cu/Ce/AC and Cu/Mn/AC at 170°C temperature, the COD reductions from the wastewater were 67% and 63%, respectively while only 44% COD reduction was obtained in the absence of catalyst. The degradation of the organics was also confirmed from pH change with time. The pH of treated wastewater (in the presence of Cu/Ce/AC catalyst) was 6.04 from an initial pH of 8.5. The pH drop is an indication of the formation of low molecular weight acidic species.
For non-catalytic WO and CWO (with Cu/Ce/AC catalyst), the TOC reduction data with time was fit in second order rate equation. The CWO rate constants at different temperatures were 2-
4 times of that obtained for non-catalytic reaction. Based on the activation energies of non- catalytic WO and CWO, it can be suggested that the reaction in the absence of catalyst was more temperature sensitive than CWO (37 kJ mol-1 and 21.55 kJ mol-1, respectively). The illustration shown below presents the effect of catalytic oxidation on TOC removal from the wastewater and the kinetic rate constants (at different temperatures) with the activation energy.
In summary, it can be suggested that CWO process with activated carbon supported catalyst is a good proposition to treat the highly colored pulping effluent. However, the catalytic stability and its reusability should to be checked in future studies.

Figure: Effect of temperature on TOC reduction and kinetic constants

Keywords: Wet oxidation; Pulping liquor; Heterogeneous catalysts