Elsevier

Bioresource Technology

Volume 99, Issue 7, May 2008, Pages 2293-2298
Bioresource Technology

Enhanced removal of three phenols by laccase polymerization with MF/UF membranes

https://doi.org/10.1016/j.biortech.2007.05.017Get rights and content

Abstract

Guaiacol, catechol, m-cresol are common phenolic compounds presented in various industrial effluents but difficult to be removed by conventional wastewater treatment schemes. To elucidate mechanisms of enhanced membrane removal by laccase polymerization, different MF and UF membranes were employed in a cross-flow module for phenol concentration of 5 mM. With 2.98 IU/l of laccase applied at room temperature, guaiacol, catechol and m-cresol were polymerized to products of averaged molecular weight of 9600, 8350 and 5400 Da (Dalton), respectively. Methoxy and hydroxyl-substituted phenols (guaiacol and catechol) were polymerized better than methyl-substituted phenol (m-cresol) due to more stable free-radical containing intermediate structure induced by oxygen-containing methoxy and hydroxyl functional groups. Removal efficiencies for the un-reacted phenols were dependent on the molecular sizes (length and width), but were dependent on the molecular weight for the polymerized phenolic compounds. Flux was declined initially but reached steady state after 180 min of filtration, indicating these MF/UF membranes can be used for removal of these polymerized phenols without significant fouling. In addition, pretreatments by the inactivated laccase only caused further flux reduction without additional removal of phenols.

Introduction

Guaiacol, catechol, m-cresol are common phenolic compounds presenting in various industrial wastewaters, and source of these phenolic pollutants can originate from agricultural and industrial activities, including the partial degradation of phenoxy herbicides, the use of wood preservatives, the generation of wastes by pulp and paper, the petrochemicals, the dying and other organic chemicals and the textile industries (Lante et al., 2000). The above phenols possess well-known adverse health effects (Fawell and Standfield, 2001) and are consequentially regulated as priority pollutants which are thoroughly regulated (European-Economic-Community, 1976, USEPA, 1981). Hence, removal of these compounds from industrial effluent is a critical issue. The removal could be effectively achieved by adsorption and advanced oxidation, however, the above process may inflict unfavorable treatment cost and generate hazardous by-products (Gianfreda et al., 2003). The toxicity of phenols could also seriously damage removal efficiencies of an operating biological wastewater treatment unit (Lante et al., 2000, Farre et al., 2002).

The applications of laccase and other oxidative enzymes on specialty wastewater treatment were increasingly reported (Gianfreda et al., 2003, Gianfreda and Rao, 2004). The enzyme applications were benefited by lower enzyme price, better commercial products availability, specific to target recalcitrant pollutants, enhanced pH and salinity endurance and easier treatment scheme under toxics fluctuation (Duran and Esposito, 2000, Ikehata et al., 2004, Kim and Nicell, 2006). Recently, enzymatic oxidative polymerization has been achieving much interests in the treatment of wastewaters containing toxic phenolic compounds (Tanaka et al., 2003).

Laccase is a blue copper containing polyphenol oxidase which plays an important role both in lignin biosynthesis as well as in lignin biodegradation (Thurston, 1994). Compared to other enzymes, laccase is reported to be rather unspecific and a wide range of substances are suitable for laccase for different application processes. Lante et al. (2000) reported the laccase immobilized onto a spiral-wound asymmetric polythersulphone membrane modules with an enzyme activity half-life of more than 150 h. The laccase membrane reactor was successful to remove 18 phenolic substrates. Tanaka et al. (2003) showed that 4-chloroguaiacol was oxidized by laccase and subsequently 4-chloroguaiacols were polymerized with phenoxy radicals formed from 4-chloroguaiacols. Reactions of laccase with different lignin model compounds and catechol were investigated with special emphasis on the high molecular products (Rittstieg et al., 2002, Aktas and Tanyolac, 2003a, Aktas and Tanyolac, 2003b). In wastewater treatment systems, polymerized phenolic compounds were proposed to be easier removed by sedimentation and filtration, as demonstrated by laccase immobilized in the packed polyvinyl-polypyrrolidone bed (Krastanov, 2000, Aktas and Tanyolac, 2003a, Aktas and Tanyolac, 2003b).

From the above reviews, focuses of previous studies are mostly on disappearing of target phenols in a batch reactor. Moreover, the fate of reacted phenols, the reasons for different removals of various phenols and correlations between product molecular weights and removal enhancement are seldom addressed. Impact of laccase polymerization on other pertinent operating parameters in membrane separation processes is also lacked. In this study, laccase was applied to polymerize synthetic phenolic wastewater to enhance the phenol removal with combination of membrane process. Due to the polymerization of these phenolic compounds, removals by membrane filtration processes would be benefited. Consequently, UF and MF were used to remove these phenolic polymerization compounds using different molecular weight cutoffs (MWCO) cross-flow filtration membranes. Therefore, the objectives of this research are listed as follows: (1) Determining effects of phenolic polymerizations by laccase from molecular weight determination. (2) Determining enhanced efficiencies for phenol removal for polymerized guaiacol, catechol and m-cresol by different membranes. (3) Assessing flux variation during removals for polymerized guaiacol, catechol and m-cresol by different membranes. (4) Comparisons of live (active) and inactivated laccase applications.

Section snippets

Methods

Laccase (Novozym 51003) was provided by Novozymes Company, produced by submerged fermentation of genetically modified Aspergillus spp., with molecular weight of 56,000 Da. The activity of laccase was measured on the rate of oxidation of syringaldazine. One IU (international unit) of laccase activity is defined as the amount of enzyme under standard conditions (pH 7.5, 30 °C) to oxidize 1 mmol syringaldazine per minute (Sealey and Ragauskas, 1998). Laccase of 2.98 IU/l was used throughout this

Determining effects of phenolic polymerizations by laccase using GPC

The GPC is operated by principle of size exclusion, in which target molecules are separated based on their sizes. Larger molecules will be eluted earlier, since they could not migrate into in finer micropore structure of column packing materials. In this work, polystyrene sulphonate standards with molecular weights of 10,600, 2220, 910 and 220 Da were eluted at 28, 31, 33 and 36 min, respectively, as shown in square marks of Fig. 3. The above calibration information was input into the

Conclusions

With 2.98 IU/l of laccase for phenol concentration of 5 mM applied at room temperature, the averaged molecular weight of guaiacol was polymerized from 124 to 9600 Da, catechol was from 110 to 8350 Da, and cresol was from 120 to 5400 Da. Methoxy and hydroxyl-substituted phenols (guaiacol and catechol) were polymerized better than methyl-substituted phenol (m-cresol) due to more stable free-radical containing intermediate structure induced by oxygen-containing methoxy and hydroxyl functional groups.

Acknowledgements

Laccase samples from Novozymes A/S and helpful technical discussions with Dr. Hui Xu are acknowledged by the authors.

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