Effects of Acid Mine Drainage on Calcareous Soil Characteristics and Lolium perenne L. Germination

Unlocking water potential in drylands: Quicklime and fly ash enhance soil microbiome structure, ecological networks and function in acid mine drainage water-irrigated agriculture

In water-stressed regions, treated acid mine drainage (AMD) water for irrigated agriculture is a potential solution to address freshwater scarcity. However, a significant knowledge gap exists on the short and long-term effects of treated AMD water on soil health. This study used high-throughput Illumina sequencing and predictive metagenomic profiling to investigate the impact of untreated AMD (AMD), quicklime- (A1Q and A2Q) and quicklime and fly ash-treated AMD water (AFQ) irrigation on soil bacterial diversity, co-occurrence networks and function. Results showed that untreated AMD water significantly increased soil acidity, electrical conductivity (EC), sulfate (SO42-), and heavy metals (HM), including reduced microbial diversity, disrupted interaction networks, and functional capacity. pH, EC, Cu, and Pb were identified as key environmental factors shaping soil microbial diversity and structure. Predominantly, Pseudomonas, Ralstonia picketti, Methylotenera KB913035, Brevundimonas vesicularis, and Methylobacteriumoryzae, known for their adaptability to acidic conditions and metal resistance, were abundant in AMD soils. However, soils irrigated with treated AMD water exhibited significantly reduced acidity (pH > 6.5), HM and SO42- levels, with an enrichment of a balanced bacterial taxa associated with diverse functions related to soil health and agricultural productivity. These taxa included Sphingomonas, Pseudoxanthomonas, Achromobacter, Microbacterium, Rhodobacter, Clostridium, Massillia, Rhizobium, Paenibacillus, and Hyphomicrobium. Moreover, treated AMD water contributed to higher connectivity and balance within soil bacterial co-occurrence networks compared to untreated AMD water. These results show that quicklime/fly ash treatments can help lessen impacts of AMD water on soil microbiome and health, suggesting its potential for irrigated agriculture in water-scarce regions.

Coupling effect of superfine zeolite and fiber on enhancing the long-term performance of stabilized/solidified Pb-contaminated clayey soils

The focus of the present study was on the applicability of superfine zeolite (SZ) and polypropylene fibers in improving the geo-environmental parameters as well as the durability of cement-based stabilized/solidified low plasticity clay containing different dosages of Pb. The leaching data revealed that while adding a low range (≤ 7.5%) of sole cement even in the severely polluted soils could fully eliminate the Pb bioavailability, the metal retention capacity might portray a marked sensitivity to the acid-washing process. A major reduction was also observed in the mechanical/leaching performance of those samples after undergoing the wetting-drying (w-d) cycle, especially at a high proportion of Pb, which could weaken the cementation bonding dramatically; hence, much more cement was needed to pass the required stabilization/solidification (S/S) standards. Besides, the micro level tests indicated that the application of SZ (with 25% cement replacement) would alleviate the Pb declining impact on the S/S reactions and modify the porous network of soil. As a result, the specimens amended by cement-SZ (CSZ) were more functional (~ 1.4 times) in immobilizing the toxic ions than the cement alone was. However, the CSZ admixture might not perfectly restrain the w-d forces/deteriorations. Such a potential drawback was found to be solvable by the insertion of fiber, in which case, an enhancement in the ductility and the metal capsulation could be also manifested. In fact, the CSZ/fiber treatment could form a well-intertwined matrix, showing high success rates in stabilizing/solidifying the contaminated soils alongside a significant decrease (~ 2-folds) in the quantity of needed amount of cement to give the S/S satisfactory operation under the harsh environmental conditions.

Remediation technologies for acid mine drainage: Recent trends and future perspectives

Acid mine drainage (AMD) is a highly acidic solution rich in heavy metals and produced by mining activities. It can severely inhibit the growth of plants, and microbial communities and disturb the surrounding ecosystem. In recent years, the use of different bioremediation technologies to treat AMD pollution has received widespread attention due to its environment-friendly and low-cost nature. Various active and passive remediation technologies have been developed for the treatment of AMD. The active treatment involves the use of different chemical compounds while passive treatments utilize natural and biological processes like constructed wetlands, anaerobic sulfate-reducing bioreactors, anoxic limestone drains, vertical flow wetlands, limestone leach beds, open limestone channels, and various organic materials. Moreover, different nanomaterials have also been successfully employed in AMD treatment. There are also reports on certain plant growth-promoting rhizobacteria (PGPR) which have the potential to enhance the growth and productivity of plants under AMD-contaminated soil conditions. PGPR applied to plants with phytoremediation potential called PGPR-assisted phytoremediation has emerged as an economical and environment-friendly approach. Nevertheless, various approaches have been tested and employed, all the approaches have certain limitations in terms of efficiency, secondary pollution of chemicals used for the remediation of AMD, and disposal of materials used as sorbents or as phytoextractants as in the case of PGPR-assisted phytoremediation. In the future, more research work is needed to enhance the efficiency of various approaches employed with special attention to alleviating secondary pollutants production and safe disposal of materials used or biomass produced during PGPR-assisted phytoremediation.

Inoculation of Bacillus spp. Modulate the soil bacterial communities and available nutrients in the rhizosphere of vetiver plant irrigated with acid mine drainage

Acid mine drainage (AMD) is one of an important pollution sources associated with mining activities and often inhibits plant growth. Plant growth promoting bacteria has received extensive attention for enhancing adaptability of plants growing in AMD polluted soils. The present study investigated the effect of plant growth promoting Bacillus spp. (strains UM5, UM10, UM13, UM15 and UM20) to improve vetiver (Chrysopogon zizanioides L.) adaptability in a soil irrigated with 50% AMD. Bacillus spp. exhibited P-solubilization, IAA and siderophore production. The Bacillus spp. strains UM10 and UM13 significantly increased shoot (4.2-2.5%) and root (3.4-1.9%) biomass in normal and AMD-impacted soil, respectively. Bacillus sp. strain UM20 significantly increased soil AP (379.93 mg/kg) while strain UM13 increased TN (1501.69 mg/kg) and WEON (114.44 mg/kg) than control. Proteobacteria, Chloroflexi, Acidobacteria and Bacteroidetes are the dominant phyla, of which Acidobacteria (12%) and Bacteroidetes (8.5%) were dominated in soil inoculated with Bacillus sp. strain UM20 while Proteobacteria (70%) in AMD soil only. However, the Chao1 and evenness indices were significantly increased in soil inoculated with Bacillus sp. strain UM13. Soil pH, AP and N fractions were positively correlated with the inoculation of bacterial strains UM13 and UM20. Plant growth promoting Bacillus spp. strains UM13 and UM20 were the main contributors to the variations in the rhizosphere bacterial community structure, improving soil available P, TN, WEON, NO₃^--N thus could be a best option to promote C. zizanioides adaptability in AMD-impacted soils.

Effect of microbial nutrients supply on coal bio-desulfurization

Research on coal desulfurization is very important for economic, social, and environmentally sustainable development. In this study, three batches of shake flask experiments were conducted for coal bio-desulfurization using Acidithiobacillus ferrooxidans to explore the relationship between microbial nutrients (iron-free M9 K medium) supply and coal bio-desulfurization efficiency. The results showed that the removal rates of pyritic sulfur and total sulfur from coal effectively increased following reintroduction of coal into the filtrate from previous batch. The removal rates of pyritic sulfur and total sulfur were 55.6% and 10.0%, 77.1% and 16.1%, and 86.5% and 28.2%, respectively, in the three batch experiments without iron-free M9 K medium addition. In contrast, the removal rates of pyritic sulfur and total sulfur reached 87.5% and 28.2%, 89.1% and 31.6%, and 92.0% and 29.1%, respectively, in the three batch experiments with 6.7% iron-free M9 K medium addition. However, addition of excessive iron-free M9 K medium was detrimental to coal bio-desulfurization because of the synthesis of jarosite (MFe₃(SO₄)₂(OH)₆, M = K⁺, NH₄⁺) and gypsum (CaSO₄·2H₂O), which further declined the pyritic sulfur bio-oxidation efficiency and total sulfur removal efficiency.

Stabilization of lead contaminated soil with traditional and alternative binders

The application of an innovative solidification/stabilization (S/S) process was investigated for the remediation of Pb contaminated soil. The performance of Pb stabilization was evaluated by comparing the use of calcium aluminate cement (CAC) and an alkali activated metakaolin binder vs the Ordinary Portland Cement (OPC). The phase composition of the stabilized products was investigated by XRD and correlated to the internal microstructure obtained by SEM-EDX imaging. Leaching tests were performed to ascertain the effectiveness of the proposed binders in the S/S of the contaminated soil, and Pb release was evaluated for each binding system. The overall results proved that multiple mechanisms are involved in Pb retention and that key parameters regulating the stabilization performance are strongly dependent on the type of applied binder system. Pb was found to be associated to C-S-H in the case of OPC, whereas ettringite played a key role in the retention of this contaminant using the CAC binder. The use of a NaOH activated metakaolin resulted in almost total retention of Pb, despite a lack of solidification, highlighting the importance of pH in the regulation of the leaching behavior.