Artisanal and small-scale gold mining (ASGM) is a serious growing concern in Sub-Saharan Africa. In Mauritania, recent gold discoveries in the north and northwest have led to an increase in ASGM centers, reflecting trends across the region and posing considerable risks of mercury (Hg) contamination. Notwithstanding this fact, the extent of mercury contamination in the region remains unclear due to insufficient knowledge on the mechanisms of Hg dispersion in hyper-arid regions. In light of this, the present study aimed to acquire fundamental knowledge to elucidate the dispersion mechanism of mercury through conducting soil and groundwater sampling in and around Chami town, Mauritania, where ASGM activities have intensified. We analyzed 180 soil samples and 5 groundwater samples for total mercury (total Hg) using cold vapor atomic absorption spectrometry (CVAAS) and atomic fluorescence spectrometry (AFS) methods. The total Hg levels in soil samples ranged from 0.002 to 9.3 ppm, with the highest concentrations found at ASGM sites. Groundwater samples exhibited low total Hg levels (0.25–1.25 ng/L). The total Hg content in soil and groundwater samples was below Japanese standards, yet soil samples from hotspot points exceeded other international standards. Our study emphasizes the Hg dispersion patterns around Chami town, suggesting a gradual decrease in total Hg with increasing distance from ASGM sites and a potential influence of wind dynamics. The knowledge accumulated in this study provides essential insights into the Hg dispersion mechanisms in Chami town, laying the foundation for establishing a predictive model of Hg contamination in hyper-arid regions.

The repurposing of gold (Au) mine tailings from artisanal and small-scale mining (ASGM) operations via alkali activation technology is a promising strategy for waste reduction in developing countries. Direct activation of mine tailings, however, is challenging because these materials contain relatively low aluminum (Al)-bearing minerals. In this study, palm oil fuel ash (POFA) was elucidated as a high Al-bearing waste derived-admixture for the synthesis of an ASGM tailings-based geopolymer composite. Semi-quantitative XRD analysis showed that the tailings contained quartz (SiO₂) (~58%), pyrite (FeS₂) (~20%) and calcite (CaCO₃) (~15%) with minor to trace amounts of aluminosilicates (~7%). Substantial amounts of environmentally regulated pollutants such as mercury (Hg) (40 mg/kg), lead (Pb) (8430 mg/kg) and arsenic (As) (300 mg/kg) were also found in the tailings. SEM-EDS, XRD and ATR-FTIR results showed the successful formation of a hybrid geopolymer-CASH matrix, which improved the unconfined compressive strength (UCS) of geopolymer composites from ~5 MPa to ~7 MPa. Furthermore, POFA did not significantly affect the thermal resistivity of geopolymer composites based on thermal analysis. Finally, the TCLP results showed that the Pb leaching concentrations from ASGM tailings exceeded environmental standards (~15,000 µg/L), which was suppressed after alkali activation to 300–500 µg/L. This means that POFA addition to ASGM tailings-based geopolymer composite improved not only its applicability as backfill, pavements and bricks but also its ability to immobilize toxic elements.

Microbially induced carbonate precipitation (MICP), a widespread phenomenon in nature, is gaining attention as a low-carbon alternative to ordinary Portland cement (OPC) in geotechnical engineering and the construction industry for sustainable development. In the Philippines, however, very few works have been conducted to isolate and identify indigenous, urease-producing (ureolytic) bacteria suitable for MICP. In this study, we isolated seven, ureolytic and potentially useful bacteria for MICP from marine sediments in Iligan City. DNA barcoding using 16s rDNA identified six of them as Pseudomonas stutzeri, Pseudomonas pseudoalcaligenes, Bacillus paralicheniformis, Bacillus altitudinis, Bacillus aryabhattai, and Stutzerimonas stutzeri but the seventh was not identified since it was a bacterial consortium. Bio-cementation assay experiments showed negligible precipitation in the control (without bacteria) at pH 7, 8, and 9. However, precipitates were formed in all seven bacterial isolates, especially between pH 7 and 8 (0.7–4 g). Among the six identified bacterial species, more extensive precipitation (2.3–4 g) and higher final pH were observed in S. stutzeri, and B. aryabhattai, which indicate better urease production and decomposition, higher CO₂ generation, and more favorable CaCO₃ formation. Characterization of the precipitates by scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS) and attenuated total reflectance Fourier transform spectroscopy (ATR-FTIR) confirmed the formation of three carbonate minerals: calcite, aragonite, and vaterite. Based on these results, all six identified indigenous, ureolytic bacterial species from Iligan City are suitable for MICP provided that the pH is controlled between 7 and 8. To the best of our knowledge, this is the first report of the urease-producing ability and potential for MICP of P. stutzeri, P. pseudoalcaligenes, S. stutzeri, and B. aryabhattai.