A Review on Enzymatic Treatment of Phenols in Wastewater
Tyrosinase is a natural enzyme and mainly catalyses the o-hydroxylation of monophenols into their corresponding o-diphenols. The synthesis of o-diphenols is a potentially valuable catalytic ability and thus tyrosinase has attracted a lot of attention with respect to industrial applications. In environmental technology, it is used for the detoxification of phenol-containing wastewaters and contaminated soils and also used in cosmetic and food industries as important catalytic enzyme. Phenols are present in effluents of a number of industries such as coal conversion, resins, plastic, petroleum refining, textiles, dyes and organic chemicals. Conventional processes for removal of phenols have drawbacks of incomplete removal of phenols. Hence, an alternative based on enzymes has been investigated. This review summarizes the current research based on removal of phenol from waste streams by enzyme polyphenoloxidase (tyrosinase).
Phenolic compounds are mainly found in wastewater of many industries such as coal conversion, resin, plastic and petroleum refineries. These compounds are toxic pollutants in industrial waste imposing risk to human health and some of them are suspected carcinogens. Phenols and cresols are highly corrosive and toxic, cause, damage to the respiratory, scarring of the skin damage to gastrointestinal tracts, kidney failure, hematological changes and nervous system depression. Therefore, some steps should be taken to treat wastewaters and also to degrade such environmental pollutants. Some commonly used conventional treatment often fails to generate final effluents at affordable cost with the required discharge quality. To overcome conventional biological treatment drawbacks, alternative technologies are sought for the treatment of phenolic wastewater such as treatment of narrow range of contaminant concentrations, generation of high sludge volumes and delays related to biological acclimatization.
Characterisation of tyrosinase activity was done using L-tyrosine as a substrate. At pH 7 the maximum catalytic activity was observed. Tyrosinase appeared to be unstable at low pH and at elevated temperature. According to the 5 min Microtox assay, most chlorinated phenol solutions and phenol treated with tyrosinase enzyme had lower toxicities when compared to their corresponding untreated solutions. However, toxic products and coloured products were formed during wastewaters treatment. Chitosan was found to be effective in inducing their precipitation, resulting in detoxified and colourless solutions. The addition of chitosan was very effective in inducing the precipitation of the coloured products generated by the transformation of phenol with tyrosinase.
It was found that the toxicities of the phenol solutions when treated with tyrosinase were substantially lower than the solutions treated with peroxidase enzymes. This represents a very strong advantage when considering this enzyme for applications in wastewater treatment. In addition, tyrosinase requires oxygen as an oxidant, which is comparatively much less expensive than the hydrogen peroxide, required by peroxidise enzymes. These might be critical points of interest while considering tyrosinase for applications in wastewater treatment.
Journal of Biotechnology & Biomaterials
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