Biochar colloids act as both transporters of organic pollutants and stimulants of respiratory chain electron efflux: a new understanding of microbial degradation of adsorbed pollutants†

Microorganisms are pivotal in decomposing persistent organic pollutants in the environment. However, the bioavailability of pollutants is often hindered by their strong adsorption to ubiquitous fine colloids. This study provides the first evidence that Rhodococcus biphenylivorans, a known intracellular oxidative degradation bacterium, can reductively degrade organochlorine pollutants adsorbed on biochar colloids (BCCs) extracellularly, besides the oxidative degradation of BCC-adsorbed pollutants intracellularly. Over a 30 day biodegradation period, the total biodegradation rates of adsorbed 2,4,4′-trichlorobiphenyl (PCB28, 10 mg L⁻¹) on BCCs (1 g L⁻¹) prepared at pyrolysis temperatures of 300, 500, and 700 °C were 59.4%, 34.8%, and 10.4%, respectively. The biodegradation products of adsorbed PCB28 on low-temperature BCCs were mainly chlorosubstituted-2-hydroxy-6-oxo-6-phenyl-2,4-hexadienoic acid-type products, while in the high-temperature BCC system, dichlorobiphenyl was the main product. Mechanistically, BCCs with low pyrolysis temperature could fragment during the degradation, facilitating the transport of adsorbed PCB28 into bacteria for the intracellular oxidative degradation; the interactions between bacterial cells and BCCs (especially with high pyrolysis temperature) could stimulate the respiratory chain electron efflux for the extracellular dechlorination degradation. These findings not only provide new insights into the modulatory influence of BCCs on microbial degradation of pollutants, but also enrich our knowledge on the environmental fate of colloid-adsorbed pollutants.