Impact of sulfur-impregnated biochar amendment on microbial communities and mercury methylation in contaminated sediment

Sustainable removal of gaseous Hg0 using sulfur functionalized biochar: Adsorption experiment and life cycle assessment

Maximizing the sorption capacity of gaseous Hg0 by sulfur-functionalized biochar can lead to increased energy consumption and the production of secondary environmental pollutants such as greenhouse gases. This study evaluates the environmental impact of producing sulfurized biochar through a life cycle assessment (LCA), weighing these impacts against the benefits of enhanced Hg removal efficiencies. The biochar's Hg0 adsorption capacity, which ranges between 3 and 22 μg-Hg0/g-biochar, is influenced by several factors: it increases with higher sulfur loading (0-15 %), higher O2 levels (0-21 %), and longer pyrolysis times (1-5 h). However, it also decreases with increased pyrolysis temperature (100-500 °C). XPS and FT-IR analysis confirm that the sulfur in the biochar primarily exists as elemental sulfur, but each sulfurization condition also resulted in the formation of sulfate, organic sulfur, and sulfone. LCA results indicate that using biochar as a sorbent for Hg0 is carbon-negative when the biochar is disposed of in landfills. Sensitivity analysis showed that increasing mercury adsorption capacity through excessive investment in energy and resources does not necessarily reduce the overall environmental impact. Consequently, when selecting an adsorbent for mercury removal, it is crucial to consider both sorption capacity and environmental impact.

Novel enhancement strategy for Hg adsorption in wastewater: Nonthermal plasma-mediated advanced modification of zero-valent iron-carbon galvanic cells with thiol functionalization

Mercury (Hg) pollution poses a critical threat to human health and the environment, necessitating urgent control measures. This study introduces a novel modification method for the common zero-valent iron-carbon (ZVI-AC) galvanic cells using a two-step process, nonthermal (NTP) irradiation followed by targeted functionalization, aiming to enhance Hg adsorption potential by adjusting the physicochemical properties of the cells. The NTP irradiated functionalized adsorbent demonstrated superior Hg adsorption performance across various concentrations and pH variations. Multichannel adsorption mechanisms were confirmed by fitting a total of 22 different adsorption isotherm models, indicating the coexistence of monolayer and multilayer adsorption processes. The NTP irradiation modifies the ZVI and AC, inducing nitrogen and oxygen doping on carbon-based surfaces and oxidizing ZVI to Fe(II)-Fe(III) species. The deepened oxidation of Fe in NTP-Fe-C, coupled with Hg2+ reduction to elemental Hg by raw Fe, contributed to Hg removal. NTP irradiation facilitated electron transfer between Fe and Hg, promoting oxidation of Fe and reduction of Hg2+ cations. The emergence of diverse Hg species further supported the multichannel adsorption/removal mechanism achieved by NTP-irradiated cells. This method offers a promising solution to Hg pollution and expands the application of the traditional iron-carbon galvanic cells in treating hazardous heavy metal wastes.

Plasma-engineered sugarcane bagasse: a novel strategy for efficient mercury removal from aqueous solutions

Metal ion adsorption using agro-industrial residues has shown promising results in remediating contaminated waters. However, adsorbent effectiveness relies on their properties, often necessitating processing for modification. Considering this, plasma treatment is effective in modifying material surfaces physically and chemically. This study investigated the modification of sugarcane bagasse (SB) using plasma treatment and evaluated its efficacy as a novel adsorbent for mercury removal from aqueous solutions. SB underwent low-temperature plasma treatment with sulfur hexafluoride (SF6) as the working gas, varying treatment times (2, 30, and 60 min), and fixed powers (80, 190, and 300 W) at 16 Pa pressure. Characterization via SEM/EDS, FTIR, XPS, and pHpzc revealed significant structural changes like increased in porosity and alteration in proportion atomic. Additionally, the successful incorporation of fluorine was confirmed in all treatment conditions, while sulfur was detected in only some samples. Amongst the tested conditions, the SB treated with 300 W for 60 min demonstrated the highest mercury removal efficiency, achieving an impressive 83.67% removal rate compared to untreated SB, which yielded only 57.95%. The adsorption mechanism exhibited both physical and chemical behavior, with chemisorption being the dominant process. The Freundlich model provided the best fit to the experimental data, with an R2 value of 0.97. In conclusion, plasma treatment can be a promising alternative for improving the physical and chemical characteristics of SB adsorbents, thereby improving their efficiency in removing mercury from aqueous solutions.

Selenium-phosphorus modified biochar reduces mercury methylation and bioavailability in agricultural soil

Biochar is a frequently employed for solidifying and stabilizing mercury (Hg) contamination in soil. However, it often results in an elevated presence of soil methylmercury (MeHg), which introduces new environmental risks. Consequently, there is a necessity for developing a safer modified biochar for use in Hg-contaminated soil. This study employed sodium selenite (at a safe dosage for soil) and hydroxyapatite to modify straw biochar (BC) based on the interaction between selenium (Se) and phosphorus (P). This process led to the formation of Se-modified biochar (Se-BC), P-modified biochar (P-BC), and Se and P co-modified biochar (Se-P-BC). Additionally, solvent adsorption experiments and pot experiments (BC/soil mass ratio: 0.5 %) were conducted to investigate the impacts of these soil amendments on soil Hg methylation and bioavailability. Se and P co-modification substantially increased the surface area, pore volume, and Hg adsorption capacity of BC. BC treatment increased the simulated gastric acid-soluble Hg, organo-chelated Hg, and MeHg in the soil. Conversely, Se-P-BC significantly reduced these forms of Hg in the soil, indicating that Se-P-BC can transform soil Hg into less bioavailable states. Among the different biochar treatments, Se-P-BC exhibited the most pronounced reductions in soil MeHg, total Hg, and MeHg in water spinach, achieving reductions of 63 %, 71 %, and 70 %, respectively. The co-modification of Se and P displayed a synergistic reduction effect in managing soil Hg pollution, which is associated with the increase of available Se in the soil due to phosphorus addition. The significantly reduced dissolved organic carbon and the abnormally high SO42- concentration in the soil of Se-P-BC treatment also inhibited Hg methylation and bioavailability in the soil. In summary, Se-P-BC substantially increased reduction percentage in plant Hg content while mitigating the risk of secondary pollution arising from elevated soil MeHg.

Human exposure to mercury in the atmosphere and soils in Konongo: an age-old mining centre in the Ashanti Region of Ghana

The dramatic upsurge of artisanal and small-scale gold mining (ASGM) activities in Ghana has resulted in environmental degradation, water pollution and human exposure to mercury-the main hazardous element used in gold extraction. This study evaluated the degree of human exposure to mercury based on the concentrations found in the air and soil samples taken at a resolution of 1 km² across Konongo, a historic mining town in Ghana's Ashanti Region. The highest atmospheric mercury concentration was 193 ng/m³, which is much higher than the levels the European Union and Japan allowed, which are 10 ng/m³ and 40 ng/m³, respectively. The concentration in the soil was 3.6 mg Hg/kg, which is around ten times higher than the background concentration in nature. Additionally, the soil concentrations were higher above the worrisome levels of soil contamination in agricultural land (4 mg/kg) and industrial areas (16 mg/kg), respectively. Soils are extremely contaminated with mercury at sites artisanal mining activities take place. The concentrations of mercury in the air and soils were significantly higher (p < 0.5) at locations of prominent mining activities compared to areas not close to mining sites. The inhabitants of the Konongo community are therefore exposed to mercury, most likely emitted from artisanal mining activities. A non-carcinogenic risk is posed to the people by inhaling mercury vapour through the air and vapourisation from the soil. Children are exposed to a higher risk than adults as they receive higher daily doses of mercury than adults.

Enrichment cultivation of VOC-degrading bacteria using diffusion bioreactor and development of bacterial-immobilized biochar for VOC bioremediation

Volatile organic compounds (VOCs) have been globally reported at various sites. Currently, limited literature is available on VOC bioremediation using bacterial-immobilized biochar (BC-B). In this study, multiple VOC-degrading bacteria were enriched and isolated using a newly designed diffusion bioreactor. The most effective VOC-degrading bacteria were then immobilized on rice husk-derived pristine biochar (BC) to develop BC-B. Finally, the performances of BC and BC-B for VOCs (benzene, toluene, xylene, and trichloroethane) bioremediation were evaluated by establishing batch microcosm experiments (Control, C; bioconsortium, BS; pristine biochar, BC; and bacterial-immobilized biochar, BC-B). The results revealed that the newly designed diffusion bioreactor effectively simulated native VOC-contaminated conditions, easing the isolation of 38 diverse ranges of VOC-degrading bacterial strains. Members of the genus Pseudomonas were isolated in the highest (26.33%). The most effective bacterial strain was Pseudomonas sp. DKR-23, followed by Rhodococcus sp. Korf-18, which degraded multiple VOCs in the range of 52-75%. The batch microcosm experiment data showed that BC-B remediated the highest >90% of various VOCs, which was comparatively higher than that of BC, BS, and C. In addition, compared with C, the BS, BC, and BC-B microcosms abundantly reduced the half-life of various VOCs, implying a beneficial impact on the degradation behavior of VOCs. Altogether, this study suggests that a diffusion bioreactor system can be used as a cultivation device for the isolation of a wide range of VOC-degrading bacterial strains, and a compatible combination of biochar and bacteria may be an attractive and promising approach for the sustainable bioremediation of multiple VOCs.