Integrating Approaches for Bioremediation of Halogenated Organic Pollutants & their Environmental Applications

Abstract

In this study the various bioremediation strategies for removing halogenated organic pollutants in soil and water were investigated. It includes the use of auxenic cultures of bacteria, soil microbes, enzyme, and nanomaterials. A wide range of compounds from the conventional pollutants including dibenzofuran to the new targets, including a commercial polychlorinated biphenyls (PCBs) mixture – Aroclor 1248, a pharmaceuticals and personal care product – Triclosan (TCS), a synthetic dye - Remazol Brilliant Blue R (RBBR) and a new persistent organic pollutant micro-pollutant – hexabromocyclododecane (HBCD). To enhance the remediation efficiency, bimetallic iron based nanoparticles have been incorporated in to the bioremediation strategy for dehalogenation of pollutants followed by microbial degradation of the less toxic and biodegradable byproducts. Remediation strategies for the following persistence of pollutants are summarized below. 1. Dibenzofuran: The biodegradation potential of dibenzofuran (DF) by a newly isolated bacterium Agrobacterium sp. PH-08 showed that this strain grew well and mineralized the DF via both angular lateral dioxygenation. ESI LC-MS/MS analysis revealed that the presence of angular degrading by-products, including 2,3,2’ trihydroxybiphenyl, 2-hydroxy-6-(2-hydroxyphenyl)-6-oxo-2,4-hexadienoic acid as well as lateral-dioxygenation metabolites, such as1,2-dihydroxy-1,2-dihydrodibenzofuran, 3-hydroxy-3-(3’-oxobenzofuran-2’-yl)-propanoic acid, and 3-hydroxy-2,3-dihydrobenxofuran-2-carboxylic acid in the upper pathway. The metabolites in the lower pathway consisted of salicylic acid, gentisic acid and catechol. In addition, this strain also cometabolically degraded DF-related compounds, such as biphenyl (BP) and dibenzothiophene (DBT), when these compounds were supplied along with DF, demonstrating the substrate versatility of the strain PH-08. Besides the DF metabolites, biphenyl-2,3-diol and benzoic acid were identified as BP degradation products from BP and DF dual substrate degradation, while dibenzothiophene sulfoxide was identified from DBT and DF dual degradation. Strain PH-08 exhibited species of bacteria exhibited the evidence of meta-cleavage pathway as confirmed by the activity and gene expression of catechol-2,3-dioxygenase. This is the one of few reports describing the strain exhibiting DF degradation via both angular and lateral dioxygenation. 2. Integrated PCB remediation system: An integrated remediation system for polychlorinated biphenyl (PCBs) was developed using a combination of chemical catalysis and biodegradation. The dechlorination of Aroclor 1248 was achieved by treatment with bimetallic nanoparticles Pd/nFe under anoxic conditions. The resulting biphenyl was biodegraded rapidly by Burkholderia xenovorans LB400. Benzoic acid was detected as an intermediate during the biodegradation process. The toxicity of the residual PCBs after nano-bio treatment was evaluated in terms of toxic equivalent. The residual PCBs also had low cytotoxicity toward Escherichia coli as demonstrated by lower reactive oxygen species levels, lower glutathione peroxidase activity, and a reduced number of dead bacteria. 3. Immobilized laccase enzyme for wastewater treatment: A novel enzyme immobilization technique was developed that is suitable for treatment of wastewater. A perfect core-shell system composing copper alginate for the immobilization of laccase (Lac-beads) was synthesized. Additionally, nFe2O3 was incorporated for the bead recycling through magnetic force. Laccase immobilized beads were structurally stable in water, acetate buffer, and real wastewater. To test the Lac-beads reactivity, triclosan (TCS) and Remazol Brilliant Blue R (RBBR) were employed. The Lac-beads showed a high percentage of TCS removal after 8 hours and RBBR decolonization after 4 hours. The pollutants removal efficacy of the Lac-beads was significantly maintained in real wastewater with the bead recyclability, whereas that of the corresponding free laccase was severely deteriorated. 4. Transformation of 1,2,5,6,9,10-hexabromocyclododecane (HBCD) in soil incubated under various conditions: Total HBCD was slowly removed from the rhizosphere and natural soil containing humic acid, under aerobic and anaerobic conditions in 40 and 21 days, respectively. Under anaerobic condition, 44% and 45% of total HBCD was removed from the rhizosphere and natural soil containing humic acid, respectively after 21 days; whereas under aerobic condition, 29% and 57% of total HBCD were removed from the same soil treatments after 40 days. The distributions of HBCD isomers α-, β-, and γ-HBCD were compared. In the soils amended with glucose as a source of carbon and energy and incubated under aerobic condition, the fraction of γ-HBCD decreased due to increased microbial activity. Populations of Gram-positive bacteria decreased during aerobic treatment of HBCD, while populations of several Gram-negative bacteria (e.g., Alpha proteobacterium, Sphingomonas sp.) increased. Humic acid and glucose amendment increased HBCD removal efficiency and microbial diversity in both rhizosphere and natural soils. 5. Integrated remediation strategy in the presence of microbes and plant: The integration of bimetallic iron-based nanoparticles (NPs) and bioremediation using plants and microbes, to remove hexabromocyclododecane (HBCD) was investigated. In aqueous medium, > 90% of HBCD was removed by both Pd/nFe and Bi/nFe treatments after 9 h treatment. After 22 h, the Bromide concentrations (58 ppm in Pd/nFe treatments and 62 ppm in Bi/nFe treatments) indicated debromination of HBCD. After harvesting fully grown Nicotiana benthamiana plants, remaining HBCD was 86.9% in the control, 85% in soils amended with humic acid (HA), 59% in soils amended with NPs, and 73.2% in soils amended with both HA and NPs. Other results suggest that HA can increase the uptake of γ-HBCD by N. benthamiana plants, but reduce their uptake of α- and β- HBCD. We found significant effects of NPs and HA amended in soils on microbial activities in carbohydrates and amino acid consumption. The microbes in NP treatments preferentially metabolized carbon sources, such as tween 40, α-cyclodextrin, glycogen, or 4-hydroxy benzoic acid whose structures are complex as HBCD degradation byproducts. This study proposes an effective approach of integrated system using NPs, cultivated plants and soil microbes in remediation of emerging pollutants, such as HBCD.