Abstract
Synthetic and bioinspired metalloporphyrins are a class of redox-active catalysts able to emulate several enzymes such as cytochromes P450, ligninolytic peroxidases, and peroxygenases. Their ability to perform oxidation and degradation of recalcitrant compounds, including aliphatic hydrocarbons, phenolic and non-phenolic aromatic compounds, sulfides, and nitroso-compounds, has been deeply investigated. Such a broad substrate specificity has suggested their use also in the bleaching of textile plant wastewaters. In fact, industrial dyes belong to very different chemical classes, being their effective and inexpensive oxidation an important challenge from both economic and environmental perspective. Accordingly, we review here the most widespread synthetic metalloporphyrins, and the most promising formulations for large-scale applications. In particular, we focus on the most convenient approaches for immobilization to conceive economical affordable processes. Then, the molecular routes of catalysis and the reported substrate specificity on the treatment of the most diffused textile dyes are encompassed, including the use of redox mediators and the comparison with the most common biological and enzymatic alternative, in order to depict an updated picture of a very promising field for large-scale applications.
Highlights
Textile industry is one of the largest water-consuming industrial sectors [1], releasing huge amounts of colored, polluting (Chemical Oxygen Demand–COD, up to 60,000 ppm), and highly recalcitrant wastes in the environment.It has been estimated that every year more than 7 ˆ 105 tons of dyes are produced, and about 15%of the total amount used is released in the environment due to the low yields of textile processes [2,3].In addition to the textile industry, dyes are used by other sectors, such as food, cosmetic, paper, photographic, and plastic industries.The US Environmental Protection Agency (US-EPA) suggests dividing textile wastes into four principal categories: (i) dispersible; (ii) hard-to-treat; (iii) high-volume; (iv) hazardous and toxic wastes [4]
Most of the events catalyzed by cytochrome P450 (CYP450) enzymes, namely aliphatic and aromatic hydroxylation, double bond epoxidation, N-oxidation, N-dealkylation, S-oxidation, decarboxylation can be efficiently simulated by biomimetic models under proper conditions [51,61,63,66,119]
Redox-active metalloporphyrins are very promising bleaching catalysts for synthetic textile dyes when the ecofriendly and not too costly hydrogen peroxide is used as the oxidizing agent
Summary
Textile industry is one of the largest water-consuming industrial sectors [1], releasing huge amounts of colored (absorbance up to 200), polluting (Chemical Oxygen Demand–COD, up to 60,000 ppm), and highly recalcitrant wastes in the environment. All industrial sectors are increasing their water demands, and the water supplies are dwindling, resulting in higher water costs (up to almost 6 €/m3 in the European Community [1]) and stricter environmental policies and regulations [1,12] Following these inputs, the scientific community is working hard to develop novel and economically sustainable methods for the detoxification of textile wastewaters, aiming to respect the prescribed quality criteria and close the water cycle. The common biological (both aerobic and anaerobic) activated sludge treatment is not very flexible when faced with the high variety in textile wastes composition (e.g., pH, conductibility, turbidity); the same chemical composition of dyes is usually very recalcitrant and can greatly differ among various industries, and in the same plant over different periods of time (Table 1) [12,15,16]. In this review, we describe their use as efficient, biomimetic and possibly environmentally friendly alternative for the treatment of textile wastes
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