Chao Zhang

No method to assess the risks of petroleum hydrocarbon pollutants C6–C9 in soils on construction land in China has been established. At one decommissioned petroleum refinery site in northwestern China, we performed an innovative tier 3 risk assessment method using carbon fraction proportions. Using HJ 25.3 guidelines, the risk-screening value for soil contamination of land by petroleum hydrocarbons was 192 mg kg⁻¹ for industrial land use. However, based on site-specific parameters, this value was 226 mg kg⁻¹, with a corresponding contaminated soil volume of 381,904 m³. A tier 3 risk assessment incorporating carbon fraction proportions and site-specific parameters yielded a risk control value of 2370 mg kg⁻¹ and reduced the soil volume requiring decontamination to 87,047 m³, potentially saving CNY 324 million (~USD 45.5 million as of November 2025) in remediation costs. Therefore, implementing a tier 3 risk assessment for C6–C9 pollutants can optimize remediation strategies and enhance the precision and scientific rigor of petroleum hydrocarbon-contaminated soil remediation.

Although the bioleaching of secondary copper sulfides has been industrialized for decades, the application of chalcopyrite bioleaching remains under development because of its low leaching rate. The effect of contact microbes on chalcopyrite leaching is still unclear due to the technical challenges in separating the contact (sessile micro-organisms) and the non-contact (planktonic micro-organisms) processes. Chalcopyrite bioleaching experiments were conducted using a novel device that stabilizes the redox potential and distinguishs between the microbial contact and non-contact effects. The contribution of the microbial “contact mechanism” in chalcopyrite leaching was quantified considering different redox potentials, compared to the “non-contact mechanism”. Based on the copper leaching kinetics and morphology of the leaching residue, it was demonstrated that the leaching rate of chalcopyrite was significantly influenced by the redox potential (850 mV > 650 mV > 750 mV), from 6.30% to 14.02% in 8 days leaching time. At each redox potential, the chalcopyrite leaching rate was 9.3%–30.6% higher with the presence of sessile microbes than without sessile microbes. Analysis of the leached chalcopyrite surface using time-of-flight secondary ion mass spectrometry (ToF-SIMS) and X-ray photoelectron spectrometry (XPS) revealed the formation of polysulfide and elemental sulfur at the surface. While the contacted sulfur oxidized the microbes, here, the Acidithiobacillus caldus preferred sessile at the chalcopyrite surface rather than Leptospirillum ferriphilum. Sulfur-oxidizing bacteria reduced the elemental sulfur content at the leach residue surface, thus playing an important role in degrading the sulfur passivation layer. In chalcopyrite bioleaching, the “contact mechanism” was primarily explained by sulfur-oxidizing bacteria promoting chalcopyrite oxidation through the removal of sulfur intermediates, while the “non-contact mechanism” was explained by ferrous-oxidizing microbes influencing the redox potential.