Enhancing composition and heavy metals combined with humic substances by adding black tourmaline during composting

Hydrochar drives reduction in bioavailability of heavy metals during composting via promoting humification and microbial community evolution

This study presented the effects of hydrochar on humification, heavy metals (HMs) bioavailability and bacterial community succession during composting. Results indicated that hydrochar addition led to elevated composting temperature, 7.3% increase in humic acid (HA), and 52.9% increase in ratio of humic acid to fulvic acid. The diethylene triamine pentacetic acid extractable Zn, Cu, Pb, and Ni were reduced by 19.2%, 36.3%, 37.8%, and 27.1%, respectively, in hydrochar-involved composting system. Furthermore, main mechanisms driving the reduced HMs bioavailability by hydrochar addition were revealed. The addition of hydrochar significantly modified the microbial community structure. Correlation analysis and microbial analysis demonstrated that relative abundance of bacterial groups connected with humification and HMs passivation were increased. Consequently, the HA formation was promoted and the HMs bioavailability were reduced through bacterial bioremediation and HA complexation. This study demonstrates the addition of hydrochar as a promising strategy to mitigate the HMs bioavailability during composting.

Effects of an assistive electric field on heavy metal passivation during manure composting

Composting is one of main technologies for treating and thus utilizing livestock manure and sludge. However, heavy metals are major concerns in compost utilization due to their potential environmental hazards and health risks. This study aimed to investigate the effects of electric field-assisted composting on the variations of heavy metals and the affecting factors. The results showed that electric field significantly reduced the contents of bioavailable heavy metals including Mn, Zn, Cu, Ni, and Cd, with their bioavailable concentrations decreasing by 61.7, 63.8, 64.9, 83.7, and 63.8 %, respectively. The heavy metals being transformed into stable states were increased, indicating that the electric field also passivated these heavy metals and reduced their biological toxicity and stabilized their forms. Spearman's correlation analysis revealed that the changes in substances, temperature, and organic matter were the dominant environmental factors affecting the forms of heavy metals. Microbial community analysis indicated an increase in the abundance of metal-resistant bacteria such as Pseudomonas and Bacillus during electric field-assisted composting, with their relative abundances being increased to 2.66 % and 15.63 % in the pile of electric field-assisted composting, respectively, compared to the values of 1.88 % and 4.36 % respectively in the conventional composting. The current study suggests that electric field-assisted composting can significantly reduce the availability of heavy metals in the compost, and thus mitigate the health risks associated with its application.

Independent and combined effects of sepiolite and palygorskite on humus spectral properties and heavy metal bioavailability during chicken manure composting

The effects of the independent and combined addition strategies of sepiolite and palygorskite on humification and heavy metals (HMs) during chicken manure composting were evaluated. Results showed that clay mineral addition showed a favorable effect on composting, prolonged the duration of the thermophilic phase (5-9 d) and improved the TN content (14%-38%) compared to CK. Independent strategy enhanced the humification degree in equal measures with the combined strategy. Carbon nuclear magnetic resonance spectroscopy (¹³C NMR) and fourier transform infrared spectroscopy (FTIR) confirmed that aromatic carbon species increased by 31%-33% during composting process. Excitation-emission matrix (EEM) fluorescence spectroscopy showed that humic acid-like compounds increased by 12%-15%. In addition, the maximum passivation rate of Cr, Mn, Cu, Zn, As, Cd, Pb and Ni were 51.35%, 35.98%, 30.39%, 32.46%, -87.02%, 36.61% and 27.62%, respectively. The independent addition of palygorskite exhibits the most potent effects for most HMs. Pearson correlation analysis indicated that pH and aromatic carbon were the key determinants of the HMs passivation. This study provided preliminary evidence and perspective of the application of clay minerals on the humification and safety of composting.

Effects of phosphate-containing additives and zeolite on maturity and heavy metal passivation during pig manure composting

This study investigated the effects of the combination of phosphogypsum with calcium oxide (PPG + CaO), superphosphate with calcium oxide (SSP + CaO) and zeolite (Zeolite) on composting maturity and heavy metal passivation in pig manure composting. The results showed that all treatments reached the maturity requirements and the phosphorus-containing additive treatments had higher final germination indices (GIs). Compared with CK, additive treatments enhanced the compost maturity by promoting volatile fatty acids (VFAs) decomposition (26.4%-30.5%) and formation of stable humus substances. All additive amendment treatments increased humic acid-like substances by over 20%, and the PPG + CaO treatment had the highest level of humus. Composting process reduced the bioavailability of Cu (49.2%), Cd (5.0%), Cr (54.3%), and Pb (26.6%). Correlation analysis found that the heavy mental passivation rate was significantly negatively correlated with the contents of VFAs and nitrogenous substances, and positively correlated with the pH, GI, humic acid content and the ratio of humic acid to fulvic acid (HA/FA). Therefore, the PPG + CaO treatment further increased the passivation rates of Cu (65.6%), Cd (21.7%), and Pb (48.7%) and decreased the mobilization of Zn by promoting maturity and humification during composting.

Effects of different additives and aerobic composting factors on heavy metal bioavailability reduction and compost parameters: A meta-analysis

Additives are considered a promising approach to accelerate the composting process and alleviate the dissemination of pollutants to the environment. However, nearly all previous articles have focused on the impact of additive amounts on the reduction of HMs, which may not fully represent the main factor shaping HMs bioavailability status during composting. Simultaneously, previous reviews only explored the impacts, speciation, and toxicity mechanism of HMs during composting. Hence, a global-scale meta-analysis was conducted to investigate the response patterns of HMs bioavailability and compost parameters to different additives, composting duration, and composting factors (additive types, feedstock, bulking agents, and composting methods) by measuring the weighted mean values of the response ratio "[ln (RR)]" and size effect (%). The results revealed that additives significantly lessened HMs bioavailability by ≥ 40% in the final compost products than controls. The bioavailability decline rates were -40%, -60%, -57%, -55%, -42%, and -44% for Zn, Pb, Ni, Cu, Cr, and Cd. Simultaneously, additives significantly improved the total nitrogen (TN) (+16%), pH (+5%), and temperature (+5%), and decreased total organic carbon (TOC) (-17%), moisture content (MC) (-18%), and C/N ratio (-19%). Furthermore, we found that the prolongation of composting time significantly promoted the effect of additives on declining HMs bioavailability (p < 0.05). Nevertheless, increasing additive amounts revealed an insignificant impact on decreasing the HMs bioavailability (p > 0.05). Eventually, using zeolite as an additive, chicken manure as feedstock, sawdust as a bulking agent, and a reactor as composting method had the most significant reduction effect on HMs bioavailability (p < 0.05). The findings of this meta-analysis may contribute to the selection, modification, and application of additives and composting factors to manage the level of bioavailable HMs in the compost products.

Effects of heavy metals stress on chicken manures composting via the perspective of microbial community feedback

Heavy metal pollution was the main risk during livestock manures composting, in which microorganisms played a vital role. However, response strategies of microbial community to heavy metals stress (HMS) remained largely unclear. Therefore, the objective of this study was to reveal the ecological adaptation and counter-effect of bacterial community under HMS during chicken manures composting, and evaluating environmental implications of HMS on composting. The degradation of organic matters (more than 6.4%) and carbohydrate (more than 19.8%) were enhanced under intense HMS, suggesting that microorganisms could quickly adapt to the HMS to ensure smooth composting. Meanwhile, HMS increased keystone nodes and strengthened significant positive correlation relationships between genera (p < 0.05), indicating that bacteria resisted HMS through cooperating during composting. In addition, different bacterial groups performed various functions to cope with HMS. Specific bacterial groups responded to HMS, and certain groups regulated bacterial networks. Therefore, bacterial community had the extraordinary potential to deal with HMS and guarantee chicken manures composting even in the presence of high concentrations of heavy metals.

The remarkable role of shikimic acid pathway in humic acid formation during biochar and montmorillonite addition composting

The final products of shikimic acid pathway, aromatic amino acids (AAA), can be used as humic acid (HA) precursors. Therefore, the aim of this study was to explore the contribution of shikimic acid pathway on the formation of HA during composting. Four composting treatments were carried out in this study, including the control, biochar addition, montmorillonite addition, biochar and montmorillonite combined addition. The results showed that the correlations between AAA and HA were enhanced during combined addition composting, and functional microorganisms involved in the shikimic acid pathway increased. In addition, random forest model suggested that 63.3% of the top 30 genera contributing to the HA formation were functional microorganisms involved in the shikimic acid pathway, which fully proved the critical role of shikimic acid pathway. Therefore, this study provided a new perspective for revealing the crucial factors that promoted the formation of HA during composting.

Elucidating the optimum added dosage of Diatomite during co-composting of pig manure and sawdust: Carbon dynamics and microbial community

Six dosages of DM (0%, 2.5%, 5.0%, 10%, 15% and 20%) were added into initial mixtures for 42 days of aerobic composting to investigate the optimum added dosage of Diatomite (DM) during co-composting of pig manure and sawdust. The results showed that adding DM was beneficial for reducing CH₄ emissions and greenhouse gas emission equivalent (GHGE) values by 15.63-24.25% and 14.33-69.08%, respectively. Meanwhile, the main contributor to the GHGE value was N₂O (58.76-75.98%), followed by CH₄ (17.22-29.16%) and NH₃ (6.38-13.36%). Moreover, the maximum values in the degradation of total organic matter and the formation rate of humic acid were 20.46% and 82.19% in 10% DM added treatment (T3), respectively. Furthermore, the increase in spectral parameters, including the specific UV absorbance at 254 nm (SUVA₂₅₄), the specific UV absorbance at 280 nm (SUVA₂₈₀) and Fourier transform - infrared parameters were facilitated by DM amendment. Additionally, the higher values of the relative abundances of Proteobacteria (50.98%) and Bacteroidetes (12.73%), and related metabolisms such as carbohydrate metabolism and amino acid metabolism, as well as the lower value of methane metabolism reported in T3, supported the difference in CH₄ and humification of the two treatments. In conclusion, DM was determined to be an eco-practical additive to improve the quality of end products and reduce potential risks, and the best treatment in this study was 10% added treatment based on dry weight.

An electric field immobilizes heavy metals through promoting combination with humic substances during composting

The aim of this study was to explore a novel method to immobilize heavy metals (HM) in composting through increasing the combination of these with humic substances. An electric-field assistant technique was applied to strengthen biomass biodegradation and assess the impact on the humification process and HM immobilization in composting. Results demonstrated that the application of an electric field enriched bacterial abundance and enhanced bacterial metabolism. Humic substance and humic acid (HA) contents in compost product were significantly increased by 19 and 69%, respectively. The HA-complexed Cu, Zn, As, Cd contents were increased by 34, 41, 29 and 135.1%, respectively, which was attributed to the promotion of HA formation since a positive correlation between HA and HA-complexed HM (R² = 0.60-0.87) was established. The evidence presented here supports the future development of electric field implementation as an intrinsic bioremediation technique for HM immobilization.

Heavy metals passivation driven by the interaction of organic fractions and functional bacteria during biochar/montmorillonite-amended composting

The aim of this study was to identify critical driving factors and pathways of mitigating heavy metals (HM) bioavailability during biochar/montmorillonite-amended composting: emphasize on the interaction effect between organic constituents and functional bacteria. Organic components, such as humus (HS), humic (HA) and fulvic acid (FA) and dissolved organic carbon (DOC), exhibited indivisible links with Cu and Zn speciation, which confirmed their vital roles on deactivating Cu and Zn. Network analysis indicated that biochar/montmorillonite obviously increased the diversity of Cu resistant/actor and Zn actor bacteria, which aided in HM passivation. Although multiple pathways were involved in regulating Cu/Zn passivation, the interaction of bacteria and organic constituents was the most critical driving factor. Given that, promoting potential HM resistant/actor bacteria utilizing and transforming low-humification organic fractions coupling with elevating high-humification constituents were the optimal pathway. This study is helpful to practical application of biochar/montmorillonite to inactivate HM for industrial composting.