The extraction of plastic microparticles, so-called microplastics, from sludge is a challenging task due to the complex, highly organic material often interspersed with other benign microparticles. The current procedures for microplastic extraction from sludge are time consuming and require expensive reagents for density separation as well as large volumes of oxidizing agents for organic removal, often resulting in tiny sample sizes and thus a disproportional risk of sample bias. In this work, we present an improved extraction method tested on return activated sludge (RAS). The treatment of 100 ml of RAS requires only 6% hydrogen peroxide (HO) for bleaching at 70 °C, followed by density separation with sodium nitrate/sodium thiosulfate (SNT) solution, and is completed within 24 h. Extracted particles of all sizes were chemically analyzed with confocal Raman microscopy. An extraction efficiency of 78 ± 8% for plastic particle sizes 20 µm and up was confirmed in a recovery experiment. However, glass shards with a diameter of less than 20 µm remained in the sample despite the density of glass exceeding the density of the separating SNT solution by 1.1 g/cm. This indicates that density separation may be unreliable for particle sizes in the lower micrometer range.

Groundwater contamination with fluoride (F) and uranium (U) has been reported in many parts of India. However, the sources and mobilization mechanisms of these contaminants remain poorly understood. The present study aimed to identify the processes governing the coexistence of elevated F and U in groundwater at a typical site in India’s middle Gangetic plain. Sustained groundwater sampling at 21 locations over two years indicated persistence of high F and U in a shallow aquifer (12 m depth), but absence of these pollutants in a deeper aquifer (30 m depth). For both the aquifers, Mn exhibited strong inverse correlations with F (−0.587; p < 0.01) and U (−0.581; p < 0.01). X-ray diffraction analysis of representative sediment cores indicated few differences in the mineralogy of the two aquifers, which consisted of fluorite (CaF2(s)) and calcite (CaCO3(s)), among others. Analysis of groundwater speciation and saturation state and sequential extraction on aquifer sediments suggested that elevated F in shallow groundwater occurred due to calcite precipitation-induced fluorite dissolution. The conditions in both the aquifers were oxidizing with respect to U, but reducing with respect to Mn. Elevated U was attributed to carbonate-promoted mobilization from iron–manganese (Mn) and residual sediment fractions. In the deeper groundwater, elevated Mn and lower pH levels persisted with conditions at saturation with respect to rhodochrosite (MnCO3(s)). Furthermore, medium (30 d) to long-term (300 d) batch experiments were performed to systematically evaluate the role of variable Mn on calcite precipitation under approximate in situ conditions. Precipitation of rhodochrosite outcompeted calcite precipitation and resulted in lower pH compared to pH of Mn-free systems, which (a) inhibited calcite precipitation and associated fluorite dissolution and (b) constrained pH and alkalinity in the deeper groundwater. These findings have implications for understanding F and U mobilization in comparable Mn-deficient sites and development of appropriate Mn-based amendments for in situ remediation.

Elevated uranium (U) (>WHO limit of 30 μg L −1 ) in Indian groundwaters is primarily considered geogenic, but the specific mineralogical sources and mechanisms for U mobilization are poorly understood. In this contribution, statistical and geochemical analyses of well-constrained metadata of Indian groundwater quality (n = 342 of 8543) were performed to identify key parameters and processes that influence U concentrations. For geochemical predictions, a unified speciation model was developed from a carefully compiled and updated thermodynamic database of inorganic, organic (Stockholm Humic model), and surface complexation reactions and associated constants. Critical U contamination was found at shallow depths (<100 m) within the Indo-Gangetic plain, as determined by bivariate nonparametric Kendall's Tau b and probability-based association tests. Analysis of aquifer redox states, multivariate hierarchical clusters, and principal components indicated that U contamination was predominant not just in oxic but mixed (oxic−anoxic) aquifers under high Fe, Mn, and SO 4 concentrations, presumably due to U release from dissolution of Fe/Mn oxides or Fe sulfides and silicate weathering. Most groundwaters were undersaturated with respect to relevant U-bearing solids despite being supersaturated with respect to atmospheric CO 2 (average pCO 2 of reported dissolved inorganic carbonate (DIC) data = 10 −1.57 atm). Yet, dissolved U did not appear to be mass limited, as predicted solubilities from reported sediment concentrations of U were ∼3 orders of magnitude higher. Integration of surface complexation models of U on typical aquifer adsorbents, ferrihydrite, goethite, and manganese dioxide, was necessary to explain dissolved U concentrations. Uranium contamination probabilities with increasing dissolved Ca and Mn exhibited minima at equilibrium solubilities of calcite [∼50 mg L −1 ] and rhodochrosite [∼0.14 mg L −1 ], respectively, at an average groundwater pH of ∼7.5. A potential indirect control of such U-free carbonate solids on U mobilization was suggested. For locations (n = 37) where dissolved organic carbon was also reported, organic complexes of U contributed negligibly to dominant U speciation at the groundwater pH. Overall, the unified model suggested competitive dissolution−precipitation and adsorption−desorption controls on U speciation. The model provides a quantitative framework that can be extended to understand dominant mobilization mechanisms of geogenic U in aquifers worldwide after suitable modifications to the relevant aquifer parameters.

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