This study analyzes the leaching behavior of elements from red mud (bauxite residue) at pH values ranging from 2 to 13. The leaching characteristics of metals and contaminated anions in five red mud samples produced by Bayer and combined processes were analyzed using the batch leaching technique following the US Environmental Protection Agency (USEPA) Method 1313. In addition, the geochemical model of MINTEQ 3.1 was used to identify the leaching mechanisms of metals. The results showed that Ca, Mg, and Ba follow the cationic leaching pattern. Al, As, and Cr show an amphoteric leaching pattern. The leaching of Cl⁻ is unaffected by the pH. The maximum leaching concentration of the proprietary elements occurs under extremely acidic conditions (pH = 2), except for As. The leaching concentration of F⁻ reaches 1.4–27.0 mg/L in natural pH conditions (i.e., no acid or base addition). At the same pH level, the leaching concentrations of Pb, As, Cr, and Cu are generally higher from red mud produced by the combined process than that those of red mud from the Bayer process. The leaching concentration of these elements is not strongly related to the total elemental concentration in the red mud. Geochemical modeling analysis indicates that the leaching of metal elements, including Al, Ca, Fe, Cr, Cu, Pb, Mg, Ba, and Mn, in red mud are controlled by solubility. The leaching of these elements depended on the dissolution/precipitation of their (hydr)oxides, carbonate, or sulfate solids.

With the increasingly serious problem of microplastic pollution, wastewater treatment plants (WWTPs) have attracted much attention as a key node for intercepting microplastics into the natural environment. In this study, the distribution characteristics of microplastics and their removal efficacy in wastewater treatment processes were systematically investigated in four typical wastewater treatment plants in Chengdu. The abundance, particle size, morphology and composition of microplastics in the influent and sludge were quantitatively analyzed by improving the sampling device and combining body-view microscopy and Fourier transform infrared (FTIR) spectroscopy. The results showed that the microplastic concentration in the influent water showed significant spatial heterogeneity, with microplastic concentrations ranging from 177 to 517 particles/L. The Chengdu Cooperative Plant, which is located in the central business district, had the highest percentage of fibers at 38.5%, while the PET fibers of the Hedong Plant, which is located in the textile industrial area of Jianyang City, had a percentage of 15%. In this study, the removal rate is defined as the difference in microplastic concentrations between influent and effluent divided by the influent concentration.The removal rate of > 1 mm microplastics in the primary treatment reached 85%~92%, but the removal rate of < 0.2 mm ultrafine particles was only 37%~45%, and the percentage of < 0.1 mm particles in the secondary treatment increased inversely, revealing the fragmentation effect caused by hydraulic shear. Microplastics in sludge were dominated by PP and PE, accounting for 76%, and the percentage of small particles (< 0.2 mm) in secondary sludge was significantly higher, with a maximum of 58%. The study showed that the overall removal rate of Chengdu wastewater treatment plant reached 89.3%~94.6%, but the residual small particle size and high density microplastics (e.g., PET fibers) in the effluent still constituted a potential ecological risk. It is recommended to optimize the process for industrial source characteristics and to strengthen the control of sludge microplastics. This study provides a scientific basis for microplastics control and process upgrading in mega-city wastewater treatment plants.

In this study, an improved DRASTIC model, including the DRASTIC-LTPD model and the AHP-DRASTIC-LTPD model, with the addition of four extra evaluation factors, including land use type (L), aquifer thickness (T’), aquifer potential (P) and pollution source distance (D’), was constructed and compared to assess the groundwater vulnerability around farmland area in Shuangsheng Industrial Park, Sichuan Province, China. From the vulnerability grading charts of the traditional DRASTIC model, the improved DRASTIC-LTPD model and the AHP-DRASTIC-LTPD model, it showed that the vulnerability presented a lower level in the western and eastern farmland areas, whereas a higher level was in the central industrial park area. This result was consistent with the actual situation where groundwater recharge by rivers, regional land use, and human activities were more active in the middle in these areas. Nevertheless, the area at the same level of vulnerability varied greatly from model to model. The vulnerability index V-level region ratio calculated by the AHP-DRASTIC-LTPD model was 0, indicating that the distribution of vulnerability was smoother without the appearance of extremely good or poor conditions. From the present study, it was revealed that the AHP-DRASTIC-LTPD model could effectively reflect the impact of human activities and dilution on groundwater vulnerability. The adopted AHP method was also of high accuracy to empower the evaluation index leading to a more reliable evaluation results of regional groundwater vulnerability in comparison with the other two models. Therefore, this research could be employed as a reference for the evaluation of groundwater pollution around other similar unplanned industrial parks.