To alleviate soil phosphorus deficiency, integrating straw-derived biochar with phosphate-solubilizing bacteria (PSB) has been recognized as a promising solution and is gaining growing attention. However, the mechanisms of bacterial immobilization and the influences of the physicochemical attributes of biochar remain unclear. In this study, we investigated the single-cell interactions of gram-negative Acinetobacter pittii and gram-positive Bacillus subtilis with cotton straw-derived biochars, subjected to progressively increasing pyrolysis temperatures, to understand the attributes of gradually modified biochar properties. The results revealed the correlations between adhesion forces and biochar properties (e.g., surface area and surface charge), and the strongest adhesion for both strains for the biochar pyrolyzed at 700 °C. The extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) model, structured to predict interaction energy, was subsequently compared with experimental observations made using atomic force microscopy (AFM). Discrepancies between the predicted high adhesion barriers and the observed attraction suggested that forces beyond Lifshitz-van der Waals also influenced the immobilization of PSB. Adhesion-distance spectroscopy and XDLVO theory jointly revealed four distinct phases in the immobilization process by biochar: planktonic interaction, secondary minimum entrapment, primary barrier transcendence, and initial reversible adherence, collectively facilitating biofilm formation. Notably, initial reversible adhesion positively correlated with increased protein and polysaccharide levels in extracellular polymeric substances (EPS) (R ² > 0.67), highlighting its importance in biofilm formation. Unraveling PSB–biochar interactions can improve the effectiveness of soil inoculants, thereby enhancing phosphorus availability in soil, a crucial factor for promoting plant growth and supporting environmental sustainability. Graphical Abstract

Persulfate-based advanced oxidation process has been proven to be a promising method for the toxic pesticide chlorpyrifos (CPY) degradation in wastewater treatment. However, due to the limitation for the short-lived intermediates detection, a comprehensive understanding for the degradation pathway remains unclear. To address this issue, density functional theory was used to analyze the degradation mechanism of CPY at the M06-2X/6-311++G(3df,3pd)//M06-2X/6-31+G(d,p) level, and computational toxicology methods were employed to explore the toxicity of CPY and its degradation products. Results show that hydroxyl radicals (·OH) and sulfate radicals (SO₄^•−) initiate the degradation reactions by adding to the P=S bond and abstracting the H atom on the ethyl group, rather than undergoing α-elimination of the pyridine ring in the persulfate oxidation process. Moreover, the addition products were attracted and degraded by breaking the P–O bond, while the abstraction products were degraded through dealkylation reactions. The transformation products, including 3,5,6-trichloro-2-pyridynol, O,O-diethyl phosphorothioate, chlorpyrifos oxon, and acetaldehyde, obtained through theoretical calculations have been detected in previous experimental studies. The reaction rate constants of CPY with ·OH and SO₄^•− were 6.32 × 10⁸ and 9.14 × 10⁸ M⁻¹·s⁻¹ at room temperature, respectively, which was consistent with the experimental values of 4.42 × 10⁹ and 4.5 × 10⁹ M⁻¹ s⁻¹. Toxicity evaluation results indicated that the acute and chronic toxicity to aquatic organisms gradually decreased during the degradation process. However, some products still possess toxic or highly toxic levels, which may pose risks to human health. These research findings contribute to understanding the transformation behavior and risk assessment of CPY in practical wastewater treatment.

Rhamnolipids, a type of biosurfactant, represent a potential strategy for both enhancing organismic resistance and in situ remediation of heavy metals contaminations. In-depth study of the mechanism of rhamnolipids synthesis in response to heavy metals stress, is indispensable for a wide use of biosurfactant-secreting microbes in bioremediation. In this study, we employed the wild-type and the rhlAB deficient strain (ΔrhlAB) of Pseudomonas aeruginosa, a prototypal rhamnolipids-producing soil microorganism, to investigate its responses to cadmium resistance based on its physicochemical, and physiological properties. Compared with the wild-type strain, the ΔrhlAB were more sensitive to Cd-stress at low Cd concentration (<50 mg/L), whereas there was little difference in sensitivity at higher Cd concentrations, as shown by spot titers and cell viability assays. Secreted rhamnolipids reduced intracellular Cd²⁺ accumulation to alleviate Cd²⁺ stress, whereas endogenous rhamnolipids played a limited role in alleviating Cd²⁺ stress. Synthesized rhamnolipids exhibited a higher critical micelle concentration (CMC) (674.1 mg/L) and lower emulsification index (4.7%) under high Cd-stress, while these parameters showed no obvious changes. High Cd-stress resulted in high hydrophilic wild-type bacterial surface and lower bioremediation ability. This study could advance a deeper understanding of the mechanism of cadmium resistance and provide a theoretical foundation for the application of biosurfactant and biosurfactant-secreted bacterium in contaminant bioremediation.