Effects of pH-varying thermal modification on sewage sludge: A focus on releasing nitrogen- and phosphorus-containing substances.

Sewage sludge is promising for the recovery and utilisation of nutrient components, but its complex nature hinders the release of these components. The combination of pH and thermal modifications shows promise for the release of nutrient components from sludge. However, comprehensive studies on the full spectrum of pH levels and corresponding mechanisms of pH-varying thermal modification are lacking. In this study, the main nutrient components, physicochemical properties, molecular structure, and noncovalent interactions of sludge were comprehensively investigated through pH-varying thermal modification (within a pH range of 2.0 to 12.0 under the same thermal condition). The experimental results showed that the release of main organics, particularly nitrogen (N)-containing organics, was well-fitted, with a tick-like function (R2: 0.74-0.96). The thermal protons exhibited a notable accumulative mutagenic effect on the N-containing organics release, while the thermal hydroxyl ions had a more direct effect, as revealed by the changes in multivalent metals and molecular structures with the protonation-deprotonation of carboxyl groups. The driving force for the release of N-containing organics was identified as the fluctuation of electrostatic interactions at the solid-liquid interface of the sludge. However, the release of phosphorus (P)-containing substances exhibited a contrasting response to that of N-containing substances with varying pH, likely because the reaction sites of thermal protons and thermal hydroxyl ions for P-containing substances were different. Moreover, high concentrations of thermal protons and hydroxyl ions collapsed the Lifshitz-van der Waals interactions of sludge, resulting in a decrease in viscoelasticity and binding strength. These propositions were further confirmed through statistical analyses of the main indicators of the main nutrient components, physicochemical properties, and noncovalent interactions of sludge. These findings can provide a basis for optimising characteristic-specific methods to recovery nutrient components (N/P) from sludge.

Salt tolerance mechanism of a hydrocarbon-degrading strain: Salt tolerance mediated by accumulated betaine in cells.

Rhodococcus sp. HX-2 could degrade diesel oil in the presence of 1%-10 % NaCl. The compatible solute betaine accumulated in cells with increasing NaCl concentration, and this was found to be the main mechanism of resistance of HX-2 to high salt concentration. Exogenously added betaine can be transported into cells, which improved cell growth and the percentage degradation of diesel oil in the presence of high [NaCl] in solution and in soil. Scanning electron microscopy data suggested that addition of exogenous betaine facilitated salt tolerance by stimulating exopolysaccharide production. Fourier-transform infrared analysis suggested that surface hydroxyl, amide and phosphate groups may be related to tolerance of high-salt environments. Four betaine transporter-encoding genes (H0, H1, H3, H5) and the betaine producer gene betB were induced in Rhodococcus sp. HX-2 by NaCl stress. The maximal induction of H0, H1, H3 and H5 transcription depended on high salinity plus the presence of betaine. These results demonstrate that salt tolerance is mediated by accumulated betaine in Rhodococcus sp. HX-2 cells, and the potential of this strain for application in bioremediation of hydrocarbon pollution in saline environments.

Pb2+ biosorption from aqueous solutions by live and dead biosorbents of the hydrocarbon-degrading strain Rhodococcus sp. HX-2.

In this study, the Pb2+ biosorption potential of live and dead biosorbents of the hydrocarbon-degrading strain Rhodococcus sp. HX-2 was analyzed. Optimal biosorption conditions were determined via single factor optimization, which were as follows: temperature, 25°C; pH, 5.0, and biosorbent dose, 0.75 g L-1. A response surface software (Design Expert 10.0) was used to analyze optimal biosorption conditions. The biosorption data for live and dead biosorbents were suitable for the Freundlich model at a Pb2+ concentration of 200 mg L-1. At this same concentration, the maximum biosorption capacity was 88.74 mg g-1 (0.428 mmol g-1) for live biosorbents and 125.5 mg g-1 (0.606 mmol g-1) for dead biosorbents. Moreover, in comparison with the pseudo-first-order model, the pseudo-second-order model seemed better to depict the biosorption process. Dead biosorbents seemed to have lower binding strength than live biosorbents, showing a higher desorption capacity at pH 1.0. The order of influence of competitive metal ions on Pb2+ adsorption was Cu2+ > Cd2+ > Ni+. Fourier-transform infrared spectroscopy analyses revealed that several functional groups were involved in the biosorption process of dead biosorbents. Scanning electron microscopy showed that Pb2+ attached to the surface of dead biosorbents more readily than on the surface of live biosorbents, whereas transmission electron microscopy confirmed the transfer of biosorbed Pb2+ into the cells in the case of both live and dead biosorbents. It can thus be concluded that dead biosorbents are better than live biosorbents for Pb2+ biosorption, and they can accordingly be used for wastewater treatment.