Development of a novel process for the removal of selected organic compounds from wastewaters.

peer-reviewedOrganic wastewater pollution arising from industry is a growing concern. Industries are increasingly faced with more restrictive environmental regulation with regards to pollutant emissions and an increased interest in reducing water consumption. Existing wastewater treatment can be limited by cost and efficiency. In this research, a treatment methodology for the treatment of organic wastewater was devised based on a two step aqueous adsorption and catalytic oxidation process. An organic pollutant selected from the EPER database was successfully adsorbed onto a transition metal modified zeolite. The saturated zeolite was then removed from the aqueous phase and then underwent temperature programmed oxidation. In this step the adsorbed pollutant was successfully oxidized into the relatively benign terminal products of carbon dioxide and water. Additionally the desorption of organic chemicals and the production of carbon monoxide where successfully minimized. As a result the modified zeolite was essentially regenerated at the same time as the pollutant was destroyed. Phenol was selected as the model pollutant due to its aqueous solubility and its environmental toxicity. It is released into waste water streams throughout Europe in large volumes and is particularly prevalent in adsorption research. The zeolite chosen was zeolite Beta (BEA), a large pore commercial zeolite with an ordered three dimensional structure. A range of silica to alumina ratios for zeolite beta were used including 25:1, 75:1, 150:1 and 300:1. The zeolites were modified with copper by a cation exchange method, and a range of zeolites was created with varying copper loading. It was found that cation exchange under normal conditions was largely dependent on the alumina content of the zeolite, however all samples could be ‘over exchanged’ by raising the pH to 7. This could result in almost 100% of the copper in solution being exchanged on the zeolite surface. The copper zeolites where found to be stable in solution under a pH range of 5-11, any pH lower than 5 resulted in significant leaching. The modified zeolites were examined by Temperature Programmed Reduction (TPR) which revealed that the nature of the copper species on the surface varied with available cation sites (silica to alumina ratio), copper loading and pH treatment. Platinum exchanged zeolites were also synthesised as well as bimetallic copper and platinum exchanged zeolites. The adsorption of phenol from the aqueous phase was examined using eight different zeolites H-β-25, H-β-75, H-β-150, H-β-300, 1.3Cu/β-25, 2.1Cu/β-25(p), 0.7Cu/β-150 and 2Cu/β-150(p). The adsorption largely followed pseudo second order kinetics and mostly occurred within the first 10 minutes of contact. It was also found to be mostly independent of pH. The adsorption capacity was found to increase as temperature decreased. The thermodynamic parameters reflected this and suggested a spontaneous and exothermic adsorption process. The increase of silica to alumina ratio was found to significantly increase the adsorption of phenol, from 17 mg g-1 (H-β-25) to 36 mg g-1 (H-β-150). It was also found that copper loading increased adsorption: 45 mg g-1 (2.1Cu/β-25(p)) and 66 mg g-1 (2Cu/β-150(p)). Temperature programmed oxidation was carried out on a wide range of phenol saturated copper, platinum and bimetallic zeolites with various loadings. The desorption/oxidation products were analysed through a mass selective detector. The results indicated that the copper modified zeolites reduced the temperature of oxidation considerably from the native zeolite. In addition to this, no desorbed phenol or fragments thereof were detected after modification. Carbon monoxide was also reduced to approximately 5% of the carbon dioxide mass. A comparison of CO2 peak area and the adsorption studies revealed that up to 95% of adsorbed phenol was oxidised. It was found that increasing the copper loading not only increased the intensity of the CO2 peak but also noticeably reduced the temperature of oxidation. For example, a peak maximum at 455oC was observed for 0.9Cu/β-25 and this was reduced to 378oC for 4.6Cu/β-25(p). The platinum and bimetallic platinum and copper zeolites all exhibited a two step oxidation taking place first at approximately 250oC and then at approximately 400oC. Platinum loading was found to have little effect on oxidation temperature within the range studied. However, increasing the copper loading in the bimetallic zeolites reduced the temperature proportionately. The over exchanged copper zeolite 2.1Cu/β-25(p) was examined over six repeat adsorption and oxidation cycles. The adsorption and oxidation was found to be consistent throughout, however atomic absorption spectroscopic analysis on the reused adsorbent/catalyst found that a substantial amount of leaching occurred leading to a loss of up to 74% of the exchanged copper over five cycles before the leaching ceased. It was discovered that the leaching was largely a result of the high temperature of oxidation. Reducing the oxidation temperature to 500oC reduced copper loss significantly. It was also found that by altering the pH of the adsorption solution to 7, that copper loss could be almost completely eliminated in batch adsorption studies