Sulfate-reduction behavior in waste-leachate transition zones of landfill sites

Pathways and contributions of sulfate reducing-bacteria to arsenic cycling in landfills

Sulfate-reducing bacteria (SRB) are generally found in sanitary landfills and play a role in sulfur (S) and metal/metalloid geochemical cycling. In this study, we investigated the influence of SRB on arsenic (As) metabolic pathways in refuse-derived cultures. The results indicated that SRB promote As(III) methylation and are beneficial for controlling As levels. Heterotrophic and autotrophic SRB showed significant differences during As cycling. In heterotrophic SRB cultures, the As methylation rate increased with As(III) concentration in the medium and reached a peak (85.1%) in cultures containing 25 mg L-1 As(III). Moreover, 4.0-12.6% of SO42- was reduced to S2-, which then reacted with As(III) to form realgar (AsS). In contrast, autotrophic SRB oxidized As(III) to less toxic As(V) under anaerobic conditions. Heterotrophic arsM-harboring SRB, such as Desulfosporosinus, Desulfocurvibacter, and Desulfotomaculum, express As-related genes and are considered key genera for As methylation in landfills. Thiobacillus are the main autotrophic SRB in landfills and can derive energy by oxidizing sulfur compounds and metal(loid)s. These results suggest that different types of SRB drive As methylation, redox reaction, and mineral formation in landfills. These study findings have implications for the management of As pollutants in landfills and other contaminated environments.

Sulfate reduction behavior in response to landfill dynamic pressure changes

During the long-term stabilization process of landfills, the pressure field undergoes constant changes. This study constructed dynamic pressure changes scenarios of high-pressure differentials (0.6 MPa) and low-pressure differentials (0.2 MPa) in the landfill pressure field at 25 °C and 50 °C, and investigated the sulfate reduction behavior in response to landfill dynamic pressure changes. The results showed that the pressurization or depressurization of high-pressure differentials caused more significant differences in sulfate reduction behavior than that of low-pressure differentials. The lowest hydrogen sulfide (H2S) release peak concentration under pressurization was only 29.67% of that under initial pressure condition; under depressurization, the highest peak concentration of H2S was up to 21,828 mg m-3, posing a serious risk of H2S pollution. Microbial community and correlation analysis showed that pressure had a negative impact on the sulfate-reducing bacteria (SRB) community, and the SRB community adjusted its structure to adapt to pressure changes. Specific SRBs were further enriched with pressure changes. Differential H2S release behavior under pressure changes in the 25 °C pressure environments were mediated by Desulfofarcimen (ASV343) and Desulfosporosinus (ASV1336), while Candidatus Desulforudis (ASV24) and Desulfohalotomaculum (ASV94) played a key role at 50 °C. This study is helpful in the formulation of control strategies for the source of odor pollution in landfills.

Microplastic, a possible trigger of landfill sulfate reduction process

The environmental impact of microplastics (MPs) formed from landfill has not been gained enough attention. This research investigated the characteristics of the MPs occurrence in landfills through field sampling. It shows that the MPs abundance in the landfill surface soil and non-landfill areas can reach 3573 items·g-1 and 3041 items·g-1, respectively. The vertical abundance of MPs increases significantly with depth, ranging from 387 to 11,599 items·g-1 with small size (≤10 μm, 65.61 %) and flake or wedge shape (38.48 %). The leachate movement in a longitudinal direction enables MPs to accumulate more easily in the landfill bottom layer with high moisture abundance. The abundance of MPs are significantly correlated with SO42- and S2- content, the two typical metabolic substrate and product of sulfate reduction process. In such heterogeneous environment, this significant correlation is not a random phenomenon in terms of the MPs have known substantial impact on biogeochemical processes. Microplastic is a possible trigger of landfill odor emission related with sulfate reduction. This research could serve as a reference for MPs and odor pollution management in landfills.

Patters of reactive nitrogen removal at the waters in the semi-constructed wetland

Protection and rectification patters of urban wetlands have been considered in strategies to balance services to society and negative consequences of excess reactive nitrogen (Nr) loading. However, the knowledge about strategies of semi-constructed wetlands on nitrogen (N) cycling pathways and removal Nr from the overlying water is limited. This study aimed to reveal considerable differences among rectification patterns of the typical semi-constructed wetland (Xixi wetland), comprising rational exploitation area (REA), rehabilitation and reconstruction area (RRA), and conservation area (CA) by analyzing the N distribution and N protentional pathways among them. Results pointed out that both NH4+ and NO3- concentration were prominently higher in REA, as opposed to CA and RRA. Sediments in RRA had relatively higher NH4+ content, indicating the efficiency of dissimilatory nitrate reduction (DNRA) in RRA. Moreover, there was a significant shift in the microbial community structure across different sites and sediments. Metagenomic analysis distinguished the N cycling pathways, with nitrification (M00804), denitrification (M00529), and DNRA (M00530) being the crucial pathways in the semi-constructed wetland. The relative abundance of N metabolic pathways (ko00910) varied among different types of sediments, being more abundant in shore and rhizosphere areas and less abundant in bottom sediments. Methylobacter and Nitrospira were the predominant nitrifiers in shore sediments, while Methylocystis was enriched in the bottom sediments and rhizosphere soils. Furthermore, Anaeromyxobacter, Anaerolinea, Dechloromonas, Nocardioides, and Methylocystis were identified as the primary denitrifiers with N reductase genes (nirK, nirS, or nosZ). Among these, Anaeromyxobacter, Dechloromonas, and Methylocystis were the primary contributors containing the nosZ gene in semi-constructed wetlands, driving the conversion of N2O to N2. This study provides important insights into rectification-dependent Nr removal from the overlying water in terms of N distribution and N metabolic functional microbial communities in the semi-constructed wetlands.

Sulfate reduction behavior in response to changing of pressure coupling with temperature inside landfill

The behavior of sulfate reduction, which was the source of hydrogen sulfide (H2S) odor, was investigated under changing pressure and temperature conditions inside landfills. The results showed that the release of H2S and methyl mercaptan (MM) was significantly inhibited at 25 °C and 50 °C under pressure, and the highest H2S and MM concentrations released were only 0.82 %-1.30 % and 1.87 %-4.32 % of atmospheric pressure, respectively. Analysis of the microbial community structure and identification of sulfate-reducing bacteria (SRB) revealed that temperature significantly altered the microbial community in the landfill environment, while pressure inhibited some bacteria and induced the growth and reproduction of specific bacteria. Key SRB (Desulfosporosinus-ASV212, Desulfitibacter-ASV1744) mediated differentiated sulfate reduction behavior in the pressure-bearing environment at 25 °C, while key SRB (Dethiobacter-ASV177, Desulfitibacter-ASV2355 and ASV316) were involved at 50 °C. This study provides a theoretical basis for the formulation of landfill gas management and control strategies.

Arsenate microbial reducing behavior regulated by the temperature fields in landfills

Attention should be paid to the As(V) reducing behavior in landfills under different temperature fields. In this study, microcosm tests were conducted using enrichment culture from a landfill. The results revealed that the reduction rate of As(V) was significantly affected by the temperature field, with the highest reduction rate observed at 50 °C, followed by 35 °C, 25 °C, and 10 °C. Different As cycling pathways were observed under various temperature fields. At room and medium temperatures, As4S4 was detected, indicating that both biomineralization and methylation processes occurred after As(V) reduction. However, only biogenic methylation was observed under high or low temperatures, indicating that the viability and adaptability of microorganisms varied depending on the temperature field and As contents. Pseudomonas was found to be the primary genus and dominant As(V) reduction bacteria (ARB) in all reactors. The study revealed that Pseudomonas accounted for a significant proportion of arsC genes, ranging from 87.29% to 97.59%, while arsCs genes were predominantly found in Bacillales and Closestridiales, with a contribution ranging from 89.17% to 96.59%. Interestingly, Bacillus and Clostridium were found to possess arsA genes in their metagenome-ssembled genome, resulting in a higher As(V) reducing rate under medium and high temperatures. These findings underscore the importance of temperature in modulating As(V) reducing behavior and As cycling, and could have implications for managing As pollution in landfill sites.

Effect of different moisture contents on hydrogen sulfide malodorous gas emission during composting

The sulfate reduction reaction releases malodorous gases (H₂S) during composting, with potential pollution risks to the environment. In this study, chicken manure (CM) with high sulfur content and beef cattle manure (BM) with low sulfur content were used to investigate the effect of control (CK) and low moisture content (LW) on sulfur metabolism. The results showed that compared to CK composting, the cumulative H₂S emission of CM and BM composting decreased by 27.27% and 21.08% under LW condition, respectively. Meanwhile, the abundance of core microorganisms related to sulfur components was reduced under LW condition. Furthermore, the KEGG sulfur pathway and network analysis suggested that LW composting weakened the sulfate reduction pathway, and reduced the number and abundance of functional microorganisms and genes. These results indicated that low moisture content had important effects on inhibiting the release of H₂S during composting, which provided a scientific basis to control environmental pollution.

Enhanced inhibition of HEDP on SRB-mediated corrosion with D-phenylalanine

Microbiologically influenced corrosion (MIC) caused by biofilm is a serious problem in many industries. D-amino acids could be a potential strategy to enhance traditional corrosion inhibitors due to their roles in biofilm reduction. However, the synergistic mechanism of D-amino acids and inhibitors remains unknown. In this study, D-Phenylalanine (D-Phe) and 1-hydroxyethane-1,1-diphosphonic acid (HEDP) were selected as the typical D-amino acid and corrosion inhibitor to evaluate their effect on the corrosion caused by Desulfovibrio vulgaris. The combination of HEDP and D-Phe obviously slowed down the corrosion process by 32.25%, decreased the corrosion pit depth and retarded cathodic reaction. SEM and CLSM analysis indicated that D-Phe reduced the content of extracellular protein and thus inhibited the biofilm formation. The molecular mechanism of D-Phe and HEDP on corrosion inhibition was further explored via transcriptome. The combination of HEDP and D-Phe down-regulated the gene expression of peptidoglycan, flagellum, electron transfer, ferredoxin and quorum sensing (QS) molecules, leading to less peptidoglycan synthesis, weaker electron transfer and stronger QS factor inhibition. This work provides a new strategy for improving traditional corrosion inhibitors, retarding MIC and mitigating subsequent water eutrophication.

Addition of exogenous microbial agents increases hydrogen sulfide emissions during aerobic composting of kitchen waste by improving bio-synergistic effects

The effect of microbial agents (MA) on hydrogen sulfide (H₂S) emissions in the compost is still a controversial issue. This study examined the effects and microbial mechanisms of MA on H₂S emissions during the composting of kitchen waste. The results showed that MA addition can promote sulfur conversion to elevate H₂S emissions by approximately 1.6 ∼ 2.8 times. Structural equations demonstrated that microbial community structure was the dominant driver on H₂S emissions. Agents reshaped the compost microbiome, showing more microorganisms participated in sulfur conversion, and enhanced the connection between microorganisms and functional genes. The relative abundance of keystone species associated with H₂S emissions increased after adding MA. Particularly, the sulfite and sulfate reduction processes were intensified, as evidenced by an increasing in the abundance and pathways cooperation of sat and asrA after MA addition. The outcome provides deeper insights into MA on regulating the mitigation of H₂S emissions in compost.

Sanitary landfill site selection using GIS-based on a fuzzy multi-criteria evaluation technique: a case study of the City of Kraljevo, Serbia

Adequate disposal of municipal solid waste (MSW) is one of Serbia's most complex environmental challenges. The problem is more serious in urban areas, since large amounts of waste are disposed of in locations that do not comply with environmental, technical, and socio-economic standards. Such is the case for the city of Kraljevo, where about 116,000 inhabitants do not have a sanitary landfill facility. This research includes a multi-criteria analysis, conducted with the help of geographic information systems, to find a suitable landfill site location. After data collection, the first step was to process 15 environmental and socio-economic factors utilizing the fuzzy analytic-hierarchy process method. The second step comprised the visual analysis and selection of the ten most suitable locations from the synthetic convenience map. The third step involved the final ranking of sites by means of the fuzzy multi-objective analysis by ratio, plus the full multiplicative form method, based on four additional beneficial and non-beneficial criteria. The results show that sanitary landfill candidate site A4 is the most suitable location for constructing a sanitary landfill site due to its large area (569 ha) and relatively short distance from the urban zone (8 km). This study is the first to integrate geographic information systems and the fuzzy analytic-hierarchy process, multi-objective analysis by ratio, and the full multiplicative form algorithm for sanitary landfill selection. The results of the research can be used as a reference for safe waste disposal in the city of Kraljevo.

Arsenic methylation behavior and microbial regulation mechanisms in landfill leachate saturated zones

Arsenic (As) is a potential contaminant in landfill. As methylation has been considered as a detoxification mechanism to address this problem. In this study, microcosm incubation was used to simulate leachate saturation zone (LSZ) and other landfill zones scenarios to explore the As methylation behavior. The As methylation rate of LSZ is 11.75%, which is slightly higher than that of other zone of landfill (10.87%). However, the difference was greatly increased by the addition of moderate content of As(III), with values of 29.25% in LSZ and 4.61% in other zones. The microbial community structure varied greatly between zones and a higher abundance of arsM was observed in the LSZ, which enhanced As methylation. Based on the annotated As functional genes from the KEGG database, the microbial As methylated pathway was summarized. Higher relative abundances of gst and arsC promoted the formation of more trivalent As substrates, stimulating the methylation behavior for As detoxification in the LSZ. According to microbial arsM contribution analysis, unclassified_p__Gemmatimonadetes, unclassified_p__Actinobacteria, unclassified_o_Hydrogenophilales, and Intrasporangium were the primary As methylation bacteria in the LSZ, while unclassified_f__Chitinophagaceae and unclassified_c_Gammaproteobacteria were the primary contributors in other landfill zones. These results highlight the specific As methylation process in the LSZ, and these insights could improve the control of As contamination in landfill sites.

Unravelling microbial drivers of the sulfate-reduction process inside landfill using metagenomics

Hydrogen sulfide (H₂S) is one of the common landfill odor. This research demonstrates that the sulfate transformation behavior is significantly enhanced during the landfill process, accompanied by a shift in microbial structure. The relative abundance of dissimilatory sulfate reduction (DSR) and thiosulfate oxidation by SOX (sulfur-oxidation) complex gradually decreases through the landfill processes while the assimilatory sulfate reduction (ASR) demonstrates the opposite behavior. The major module for landfill sulfate reduction is ASR, accounting for 31.72% ± 2.84% of sulfate metabolism. Based on the functional genes for the sulfate pathway, the drivers for sulfate biotransformation in landfills were determined and further identified their contribution in the sulfate metabolism during landfill processes. Pseudomonas, Methylocaldum, Bacillus, Methylocystis and Hyphomicrobium were the top 5 contributors for ASR pathway, and only one genus Pseudomonas was found for DSR pathway. Among the 26 high-quality metagenome-assembled genomes of sulfate functional species, 24 were considered novel species for sulfuric metabolism. Overall, this study provides unique insight into the sulfate transformation process related to the H₂S odor control in landfill management.

Dynamic processes in conjunction with microbial response to unveil the attenuation mechanisms of tris (2-chloroethyl) phosphate (TCEP) in non-sanitary landfill soils

Although the environmental and health risks of chlorinated organophosphate esters (OPEs-Cl) have drawn much attention, its environmental behaviors have been insufficiently characterized. As a notable sink of this emerging contaminant, non-sanitary landfills, which may decompose/accumulate OPEs-Cl, is of particular concern. In the present study, the dynamic processes of the typical OPEs-Cl, tris(2-chloroethyl) phosphate (TCEP), in non-sanitary landfill soils were analyzed under anaerobic condition, and the microbial taxa involved in these processes were explored. Our results showed that TCEP could be simultaneously reduced by abiotic and biotic processes, as it was reduced by 73.9% and 65.5% over the 120-day experiment in landfill humus and subsoil, respectively. Notably, the degradation of TCEP was significantly (p < 0.05) enhanced under the stress of a high TCEP concentration (10 μg g^-1), while its ecological consequences were found insignificant regarding the microbial diversity and community structure and the typical soil redox processes, including Fe(III)/SO₄^2- reduction and methanogenesis, in both soils. The microbial diversity of subsoil was significantly lower, and acetate was an important factor in changing microbial communities in landfill soils. The microbes in the family Nocardioidaceae and genus Pseudomonas might contribute to in the degradation of TCEP in landfill humus and subsoil, respectively. The metabolism related to sulfur and sulfate respiration were significantly (p < 0.05) correlated with TCEP reduction, and Desulfosporosinus were found as a potentially functional microbial taxon in TCEP degradation in both soils. The results could advance our understanding of the environmental behavior of OPEs-Cl in landfill-like complex environments.

Microbial mediated arsenate reducing behavior in landfill leachate-saturated zone

As(V) reduction mediated by microorganisms might be an essential process in resisting As toxicity since As(V) is the major species in the landfill. LSZ has been considered as a trigger of all types of microbial activity inside a landfill site. This research investigated the microbial As(V)-reducing behavior in LSZ. The results revealed that higher As(V)-reduction efficiency in higher As(V) content-stress LSZ scenario. The corresponding microbial diversity also varied with the As(V) content. The microbial community structure was related to arrA and arsC distribution, which encode respiratory As(V) reductase and cytoplasmic As(V) reductase, respectively. The landfill As bio-reduction pathways were modeled, as well as the As functional gene distribution among different As(V) contents at different landfill stages. The C, N, and S metabolic processes generally affected the As(V)-resistance genes distribution. Thiosulfate oxidation, denitrification, and dissimilatory nitrate reduction positively affected arsC, while dissimilatory sulfate reduction and methanogenesis trended to play a negative role. This research provides new insight into As(V) bio-reduction inside a landfill site in terms of functional genes distribution and correlation with nutrient elements metabolic processes.

Maize intercropping enriches plant growth-promoting rhizobacteria and promotes both the growth and volatile oil concentration of Atractylodes lancea

In the Atractylodes lancea (A. lancea)-maize intercropping system, maize can promote the growth of A. lancea, but it is unclear whether this constitutes an aboveground or belowground process. In this study, we investigated the mechanisms of the root system interaction between A. lancea and maize using three different barrier conditions: no barrier (AI), nylon barrier (AN), and plastic barrier (AP) systems. The biomass, volatile oil concentration, physicochemical properties of the soil, and rhizosphere microorganisms of the A. lancea plant were determined. The results showed that (1) the A. lancea - maize intercropping system could promote the growth of A. lancea and its accumulation of volatile oils; (2) a comparison of the CK, AI, and AP treatments revealed that it was the above-ground effect of maize specifically that promoted the accumulation of both atractylon and atractylodin within the volatile oils of A. lancea, but inhibited the accumulation of hinesol and β-eudesmol; (3) in comparing the soil physicochemical properties of each treatment group, intercropping maize acidified the root soil of A. lancea, changed its root soil physicochemical properties, and increased the abundance of the acidic rhizosphere microbes of A. lancea at the phylum level; (4) in an analysis of rhizosphere microbial communities of A. lancea under different barrier systems, intercropping was found to promote plant growth-promoting rhizobacteria (PGPR) enrichment, including Streptomyces, Bradyrhizobium, Candidatus Solibacter, Gemmatirosa, and Pseudolabrys, and the biomass of A. lancea was significantly influenced by PGPR. In summary, we found that the rhizosphere soil of A. lancea was acidified in intercropping with maize, causing the accumulation of PGPR, which was beneficial to the growth of A. lancea.