Sustainable Synthesis and Enhanced Photocatalytic Activity of Titanium Oxide-Clay Nanocomposite Toward Persistent Polychlorinated Biphenyl Degradation in Real Samples

This study presents the green synthesis of TiO2-clay nanocomposite using an eco-friendly method based on carbohydrates. Glucose and soluble starch function as the bioreductant and stabilizer agent during the reaction, respectively. The UV-vis analysis ascertained the formation of TiO2-clay nanocomposite presenting a distinctive absorption peak at 360 nm and an optical band gap of 4.3 eV. TEM and XRD results indicated the anatase phase of spherical TiO2-clay with a median crystallite size of 18.36 nm. EDX and XRF techniques revealed the presence of titanium, oxygen peaks, and TiO2 compound (58 wt%) along with the elemental composition of clay. The BET values of specific surface area, average pore diameter, and pore volumes are 47.48 m2g− 1, 17.39 nm, 0.19 cm3g− 1, respectively. The benign TiO2-clay nanocomposite demonstrated superior photocatalytic efficiency toward photodegradation of poly polychlorinated biphenyl pollutant in industrial wastewater with a removal rate of 98.32%, greater than untreated TiO2 NPs.

Bioconversion of Biphenyl to A Polyhydroxyalkanoate Copolymer by Alcaligenes Denitrificans A41

A polyhydroxyalkanoate (PHA) copolymer, poly(3-hydroxybutyrate-co-3-hydroxyvalerate) [P(3HB-co-3HV)], was biosynthesized from biphenyl as the sole carbon source using Alcaligenes (currently Achromobacter) denitrificans A41. This strain is capable of degrading polychlorinated biphenyls (PCBs) and biphenyl. This proof-of-concept of the conversion of aromatic chemicals such as the environmental pollutant PCBs/biphenyl to eco-friendly products such as biodegradable polyester PHA was inspired by the uncovering of two genes encoding PHA synthases in the A. denitrificans A41 genome. When the carbon/nitrogen (C/N) ratio was set at 21, the cellular P(3HB-co-3HV) content in strain A41 reached its highest value of 10.1%. A two-step cultivation protocol improved the accumulation of P(3HB-co-3HV) by up to 26.2% of the dry cell weight, consisting of 13.0 mol% 3HV when grown on minimum salt medium without nitrogen sources. The highest cellular content (47.6%) was obtained through the two-step cultivation of strain A41 on biphenyl as the sole carbon source. The purified copolymer had ultra-high molecular weight (weight-average molecular weight of 3.5 × 106), as revealed through gel-permeation chromatography. Based on the genomic information related to both polymer synthesis and biphenyl degradation, we finally proposed a metabolic pathway for the production of P(3HB-co-3HV) associated with the degradation of biphenyl by strain A41. This is, as it were, a metabolic link between PHA synthesis and biphenyl degradation.

Biphenyl degradation by recombinant photosynthetic cyanobacterium Synechocystis sp. PCC6803 in an oligotrophic environment using unphysiological electron transfer

Cyanobacteria are potentially useful photosynthetic microorganisms for bioremediation under oligotrophic environments. Here, the biphenyl degradation pathway genes of β-proteobacterium Acidovorax sp. strain KKS102 were co-expressed in cyanobacterium Synechocystis sp. PCC6803 cells under control of the photo-inducible psbE promoter. In the KKS102 cells, biphenyl is dioxygenated by bphA1 and bphA2 gene products complex using electrons supplied from NADH via bphA4 and bphA3 gene products (BphA4 and BphA3, respectively), and converted to benzoic acid by bphB, bphC and bphD gene products. Unexpectedly, biphenyl was effectively hydroxylated in oligotrophic BG11 medium by co-expressing the bphA3, bphA1 and bphA2 genes without the bphA4 gene, suggesting that endogenous cyanobacteria-derived protein(s) can supply electrons to reduce BphA3 at the start of the biphenyl degradation pathway. Furthermore, biphenyl was converted to benzoic acid by cyanobacterial cells co-expressing bphA3, bphA1, bphA2, bphB, bphC and bphD. Structural gene-screening using recombinant Escherichia coli cells co-expressing bphA3, bphA1, bphA2, bphB and bphC suggested that petH, which encodes long- and short-type NADP-ferredoxin oxidoreductase isomers (FNRL and FNRS, respectively), and slr0600, which is annotated as an NADPH-thioredoxin reductase gene in CyanoBase, were BphA3-reducible proteins. Purified FNRL and FNRS, and the slr0600 gene product showed BphA3 reductase activity dependent on NADPH and the reduced form of glutathione, respectively, potentially shedding light on the physiological roles of the slr0600 gene product in cyanobacterial cells. Collectively, our results demonstrate the utility of Synechocystis sp. PCC6803 cells as a host for bioremediation of biphenyl compounds under oligotrophic environments without an organic carbon source.