Influence of humic substances on bioavailability of Cu and Zn during sewage sludge composting

A comprehensive review on agricultural waste utilization through sustainable conversion techniques, with a focus on the additives effect on the fate of phosphorus and toxic elements during composting process

The increasing trend of using agricultural wastes follows the concept of "waste to wealth" and is closely related to the themes of sustainable development goals (SDGs). Carbon-neutral technologies for waste management have not been critically reviewed yet. This paper reviews the technological trend of agricultural waste utilization, including composting, thermal conversion, and anaerobic digestion. Specifically, the effects of exogenous additives on the contents, fractionation, and fate of phosphorus (P) and potentially toxic elements (PTEs) during the composting process have been comprehensively reviewed in this article. The composting process can transform biomass-P and additive-born P into plant available forms. PTEs can be passivated during the composting process. Biochar can accelerate the passivation of PTEs in the composting process through different physiochemical interactions such as surface adsorption, precipitation, and cation exchange reactions. The addition of exogenous calcium, magnesium and phosphate in the compost can reduce the mobility of PTEs such as copper, cadmium, and zinc. Based on critical analysis, this paper recommends an eco-innovative perspective for the improvement and practical application of composting technology for the utilization of agricultural biowastes to meet the circular economy approach and achieve the SDGs.

Potential of combined reactor and static composting applications for the removal of heavy metals and antibiotic resistance genes from chicken manure

Chicken manure (CM) can pose a serious threat to environmental and human health, and need to be managed properly. The compost can effectively treat CM. However, there is limited research on the heavy metals and antibiotic resistance genes (ARGs) during compost CM. In this study, the combined application of reactor and static composting (RSC) was used to produce organic fertilizer of CM (OCM), and heavy metals, ARGs and bacterial community structure was investigated. The results show that RSC could be used to produce OCM, and OCM meet the National organic fertilizer standard (NY/T525-2021). Compared to the initial CM, DTPA-Cu, DTPA-Zn, DTPA-Pb, DTPA-Cr, DTPA-Ni and DTPA-As in OCM decreased by 40.83%, 23.73%, 34.27%, 38.62%, 16.26%, and 43.35%, respectively. RSC decreased the relative abundance of ARGs in CM by 84.06%, while the relative abundance of sul1 and ermC increased. In addition, the relative abundance and diversity of ARGs were mainly influenced by the bacterial community, with Actinobacteria, Firmicutes, and Proteobacteria becoming the dominant phyla during composting, and probably being the main carriers and dispersers of most of the ARGs. Network analyses confirmed that Gracilibacillus, Lactobacillus, Nocardiopsis, Mesorhizobium and Salinicoccus were the main potential hosts of ARGs, with the main potential hosts of sul1 and ermC being Mesorhizobium and Salinicoccus. The passivation and physicochemical properties of heavy metals contribute to the removal of ARGs, with sul1 and ermC being affected by the toal heavy metals. Application of RSC allows CM to produce mature, safe organic fertilizer after 32 d and reduces the risk of rebound from ARGs, but the issues of sul1 and ermC gene removal cannot be ignored.

The transformation of heavy metal speciation during rapid high-temperature aerobic fermentation of food waste and their potential mechanisms

In this study, the content changes of multiple trace heavy metals (HMs) in food waste using a new rapid high-temperature aerobic fermentation (RTAF) technology and their relationships with different physicochemical factors were researched. The results indicated that the content of HMs in the decomposed products met the industry standards for organic fertilizers (NY/T525-2021, China). Physicochemical factors played an important role in controlling the changes in HM content. The component evolution of dissolved organic matter was studied, and its influences on the transformation of HM speciation showed that the RTAF process converted proteins into humus-like substances. Redundancy analysis revealed that the main factors driving the speciation transformation of HMs were tyrosine-like substances or microbial-derived humus (C3), molecular weight of dissolved organic matter (SUVA254) and humification degree (E250/E365). The increase in humification degree contributed to passivating HMs. The correlation network analysis results showed that the exchangeable HMs (Exc-HMs) were related to Lactobacillus and Pediococcu. Additionally, the cytoskeleton, coenzyme transport and metabolic function of microorganisms affected the Exc-HM content. These research results can provide a scientific basis for the prevention and control of HM pollution during the treatment of food waste.

Exploring carbon conversion and balance with magnetite-amended during pig manure composting

This study was designed to explore the magnetite in maturation and humification during pig manure (PM) and wolfberry branch fillings (BF) composting. Different proportions of magnetite (T1, 0%; T2, 2.5%; T3, 5%; T4, 7.5%;) were blended with PM for 50 days of composting. The findings indicated magnetite amendment has no influence on the maturity, and the 5% ratio significantly promoted humic acid (HA) formation and fulvic acid (FA) decomposition compared to other treatments. Compared to T1, magnetite addition significantly increased CO2 and CH4 emissions by 106.39%-191.69% and 6.88-13.72 times. The further analysis suggested that magnetite improved Ruminofilibacter activity were significantly positively associated with HA, and C emissions. The further PICRUSt 2 analysis showed membrane transport may enhance environmental information processing by magnetite. Overall, these results demonstrated higher organic matter (OM) degradation and HA formation with an additional increase in microbial activity highlighted advantages of using magnetite during PM composting.

Composting pig manure with nano-zero-valent iron amendment: Insights into the carbon cycle and balance

The effectiveness of nano-zero-valent-iron (NZVI) addition during composting of pig manure (PM) was investigated. Different dosages of NZVI were mixed with PM substrate during a 50 days composting process. The results revealed that the higher share of NZVI addition, the higher OM degradation rate is. On contrary, it was observed that the higher share of NZVI addition, the lower the fulvic acid and the humin degradation rate is. Meanwhile, NZVI amendment increased the CO₂ and CH₄ emissions by 29-47 % and 53-57 %, respectively. The in-depth analysis showed that NZVI addition increased the activity of Sphaerobacter and Luteimonas, which eventually led to the degradation of hard-to-degrade OM faster. Additionally, NZVI was found to increase the filtration of microorganisms, reducing the toxicity and hygiene of compost products. No significant improvement in humic substance enhancement was observed during composting with NZVI addition but improved OM degradation.

Phytostabilization of trace elements and 13C isotope composition of Atriplex atacamensis Phil. cultivated in mine tailings treated with organic amendments

Mining generates large quantities of mineral processing wastes that are typically stored in mine tailings (MT) ponds. Long-term exposure of the surrounding areas to the material from the tailings ponds has been reported to have adverse effects on both human health and the environment. The purpose of this study was to evaluate the ability of Atriplex atacamensis Phil. to phytostabilize metals (Cu, Fe, Mn, and Zn) and sulfur (S) when grown directly on mine tailings with and without compost (C) and humic substance (HS). The stress status of A. atacamensis Phil. was also evaluated through the ¹³C isotopic composition of bulk leaves. A 120-day greenhouse experiment was conducted and three treatments were evaluated: (i) MT without any amendments (control), (ii) MT + C (dose: 89 ton ha^-1), and (iii) MT + HS (0.72 ton ha^-1). Mine tailings material exhibited low salinity, alkaline pH, high extractable S-SO₄ concentrations, and low fertility; total Fe, Mn, and Zn concentrations were within the reference range for mine tailings, but total Cu concentrations were high at 1860 ± 236 mg kg^-1. The HS had higher pH, EC, CEC, and available concentrations of N, P, and K than compost, while S-SO₄ concentrations were similar in both amendments. ¹³C NMR analysis showed that the HS contained more alkyl, aromatic, and phenolic groups, while the compost was dominated by O-alkyl and carboxyl groups. At the end of the experiment, the MT + C treatment achieved a significant decrease in Cu, Fe, and Mn concentrations in the roots and aboveground parts of A. atacamensis Phil. and an increase in Zn values in both tissues. Both amendments increased the sulfur content in the aboveground parts, while metal concentrations under the HS treatment proved similar to control. Furthermore, the δ¹³CV-PDB values obtained in this study indicate that the organic amendments did not cause additional physiological stress to the plants compared to the MT treatment. Overall, A. atacamensis Phil. was shown to have the ability to phytostabilize metals and sulfur, making it a potential candidate species for in situ evaluation of the phytostabilization process on mine tailings.

Simultaneous reductions in antibiotic, antibiotic resistance genes and nitrogen loss during bioaugmentation tylosin fermentation dregs co-composting

Considering the main problems presented in the typical solid wastes antibiotic fermentation dregs (AFDs) composting that the residual antibiotics could result in the propagation of antibiotic resistance genes (ARGs), and the reduced value of agronomic production caused by the ammonia gas (NH₃) emissions. This study established a bio-augmented tylosin fermentation dregs (TFDs) aerobic co-composting system to investigate the effects of a novel isolated high-efficiency strain Klebsiella sp. TN-1 inoculation on tylosin degradation, reduction in ammonia emissions, and ARG abundances during this process. Results showed that the application of strain Klebsiella sp. TN-1 extended the thermophilic stage and promoted compost maturity. Moreover, bio-enhanced co-composting with strain Klebsiella sp. TN-1 led to a totally degradation of tylosin, and removed most of ARGs, metal resistance genes (MRGs) and mobile genetic elements (MGEs), and also effectively reduce ammonia emission by 49.76 %.via increasing ammoxidation rates. Principal co-ordinates analysis further suggested that the strain Klebsiella sp. TN-1 had little influence on the bacterial community composition, while the changes of other physical and chemical properties during this process were the main reasons for the evolution of bacterial community and propagation of ARGs in the TFDs co-composting. This study suggests the potential of the bio-enhanced strain Klebsiella sp. TN-1 for antibiotic biodegradation and its application for nitrogen conservation in the AFDs co-composting process, which could decrease the risk of ARGs spreading and make compost products more secure.

Effects of CaO2 based Fenton - like reaction on heavy metals and microbial community during co-composting of straw and sediment

In this study, a Fenton-like system was constructed by CaO₂ and nano-Fe₃O₄ in the co-composting system of straw and sediment. Its effect on the passivation of heavy metals and the evolution of microbial community were investigated. The results showed that the establishment of CaO₂-Fenton-like system increased the residual Cu and residual Zn by 27.62% and 16.80%, respectively. In addition, the CaO₂-Fenton-like system facilitated the formation of humic acid (HA) up to 20.84 g·kg^-1. Redundancy analysis (RDA) showed that the CaO₂-Fenton-like system accelerated bacterial community succession and promoted the passivation of Cu and Zn. Structural equation models (SEMs) indicated that Fenton reaction affected Cu and Zn passivation by affecting pH, bacterial communities, and HA. This study shows that the CaO₂-Fenton-like system could promote the application of composting in the remediation of heavy metals contamination in sediment.

Succession and change of potential pathogens in the co-composting of rural sewage sludge and food waste

Composting is an effective way to prevent and control the spread of pathogenic microorganisms which could put potential risk to humans and environment, from rural solid waste, especially sewage sludge and food waste. In the study, we aim to analyze the changes of pathogenic bacteria during the co-composting of rural sewage sludge and food waste. The results showed that only 27 pathogenic bacteria were detected after composting, compared to 50 pathogenic bacteria in the raw mixed pile. About 74% of pathogen concentrations dropped below 1000 copies/g after composting. Lactobacillus, Bacillus, Paenibacillus and Comamonas were the core pathogenic bacteria in the compost, of which concentrations were all significantly lower than that in the raw mixed pile at the end of composting. The concentration of Lactobacillus decreased to 3.03 × 10³ copies/g compared to 0 d with 1.25 × 10⁹ copies/g by the end of the composting, while that of Bacillus, Paenibacillus and Comamonas decreased to 2.77 × 10⁴ copies/g, 2.13 × 10⁴ copies/g and 3.38 × 10² copies/g, respectively, with 1.26 × 10⁷ copies/g, 4.71 × 10⁶ copies/g, 1.69 × 10⁸ copies/g on 0 d. Redundancy analysis (RDA) indicated that physicochemical factors and substances could affect the changes of pathogenic bacteria during composting, while temperature was the key influencing factor. In addition, certain potential pathogenic bacteria, such as Bacteroides-Bifidobacterium, show statistically strong and significant co-occurrence during composting, which may increase the risk of multiple infections and also influence their distribution. These findings provide a theoretical reference for biosafety prevention and control in the treatment and disposal of rural solid waste.

Immobilization of Zn and Cu in Conditions of Reduced C/N Ratio during Sewage Sludge Composting Process

In this paper we present results of research on the transformation of chemical forms of two elements (Cu, Zn) that occurred at the highest concentration in sewage sludge being processed in a composting process. The factor that had impact on the direction of the observed transformation was the amount of straw added to the mix with sewage sludge at the batch preparation stage including elimination of an additional source of organic carbon (straw). The analysis of contents of Cu and Zn chemical forms was performed applying Tessiere’s methodology. It was ascertained that reduction of supplementation has positive impact on the allocation of tested elements in organic (IV) and residual (V) fractions with a simultaneous decrease of heavy metals mobile forms share in bioavailable fractions, mostly ion exchangeable (I) and carbonate (II). Using an artificial neural network (ANN), a tool was developed to classify composts based on Austrian standards taking into account only I ÷ IV fractions treated as a labile, potentially bioavailable, part of heavy metals bound in various chemical forms in compost. The independent variables that were predictors in the ANN model were the composting time, C/N, and total content of the given element (total Cu, Zn). The sensitivity coefficients for three applied predictors varied around 1, which proves their significant impact on the final result. Correctness of the predictions of the generated network featuring an MLP 3-5-3 structure for the test set was 100%.

Humic acid characterization and heavy metal behaviour during vermicomposting of pig manure amended with 13C-labelled rice straw

Aiming to reveal the humification process of organic waste and its contribution to the heavy metal behaviour affected by earthworm activity, it was studied about the variation of humic acid (HA) and heavy metal behaviour during vermicomposting of the mixed pig manure and ¹³C-labelled rice straw. The results showed that earthworms could well adapt to the culturing environment and feed organic matter for its growth and reproduction, the vermicomposting process increased the content of humic substances (HS), HA, and fulvic acid (FA) in substrate residues, but led to less transformation of HA into FA. The elemental, ultraviolet absorption spectroscopy, Fourier transform infrared (FTIR) and fluorescence excitation-emission matrix (EEM) analysis indicated that vermicomposting led to more aromatic structures and much higher humification degree in HA, whereas less protein, FA-like substances and plastein in HA. Vermicomposting could enhance the total Cu content and decrease Cu/Zn bioavailability in the substrate residues, and vermicomposting especially can help stabilize Cu in the substrate residues by forming more complexed HA-Cu.

Variation and factors on heavy metal speciation during co-composting of rural sewage sludge and typical rural organic solid waste

In this study, a co-composting of rural organic solid waste (rural sewage sludge, kitchen waste and corn stalks) was conducted to analyze the variation of heavy metals (As, Cu, Cr, Ni, Pb, Hg, and Zn) and their major influencing factors. During composting, significant changes were observed in the total contents of heavy metals (p < 0.01): the total concentrations of As, Cu, Hg, Pb and Zn increased by 7.5%, 54.1%, 26.3%, 15.8%, and 34.2%, whereas that of Cr and Ni decreased by 71.3% and 33.4%, respectively. Heavy metals were mainly bound to the oxidizable and residual fractions. Spearman and Redundancy analysis (RDA) indicated that substances were significantly correlated with the changes in speciation of heavy metals, among all the factors, while pH and temperature were the dominating environmental influencing parameters. Several metal-resistant bacterial genera (Pseudomonas, Paenibacillus, Bacillus, Acinetobacter, Desulfovibrio, and Ochrobactrum, etc) were observed, with significant explanatory capacity for the changes in heavy metals. Composting showed a poor effect on heavy metal passivation, except for that of As. After composting, the heavy metal contents were consistent with the application standards. The evaluation of potential ecological risk showed a high cumulative ecological risk (336.9) of heavy metals. This study provides technical support and practical information for the disposal and safe recycling for rural organic solid waste.

Binding characteristics of humic substances with Cu and Zn in response to inorganic mineral additives during swine manure composting

Composting is suitable for recycling livestock manure into valuable organic fertilizer, which can improve soil quality while mitigating potential risk of heavy metal pollution. Humic substances (HS) in compost have been demonstrated to play a key role in regulating the redistribution of heavy metal fractions. However, limited direct information have been reported on how different components of HS complexes with heavy metals to affect their bioavailability during composting. In this study, sequential extraction procedures (H₂O, KCl, Na₄P₂O₇, NaOH and HNO₃) were used to assess the characteristics that HS bound with Cu and Zn during composting of swine manure and straw added either 5% boron waste (BW) or 5% phosphate rock (PR). Organically complexed fraction extracted by Na₄P₂O₇ contained only 33-41% of the Cu but most of the Zn (81-87%). During composting, initially mobile fractions of Cu and Zn (extracted by H₂O or KCl) changed into more stable fractions (extracted by NaOH and HNO₃), and both organic matter and fulvic acids (FA) were identified as critical factors to explain this redistribution based on redundancy analysis. Over 80% of Cu and Zn were complexed with FA of HS. However, exogenous additives (phosphate rock and boron waste) enhanced Cu conversion by promoting humification (Humic acid/Fulvic acids, HA/FA) whereas they had limited influence on Zn, due to the relatively weak binding relationship between Zn and HA.

Responses of bacterial communities and antibiotic resistance genes to nano-cellulose addition during pig manure composting

Treatment with exogenous additives during composting can help to alleviate the accumulation of antibiotic resistance genes (ARGs) caused by the direct application of pig manure to farmland. In addition, nano-cellulose has an excellent capacity for adsorbing pollutants. Thus, the effects of adding 300, 600, and 900 mg/kg nano-cellulose to compost on the bacterial communities, mobile genetic elements (MGEs), and ARGs were determined in this study. After composting, treatment with nano-cellulose significantly reduced the relative abundance of ARGs, which was lowest in the compost product with 600 mg/kg added nano-cellulose. Nano-cellulose inhibited the rebound in ARGs from the cooling period to the maturity period, and weakened the selective pressure of heavy metals on microorganisms by passivating bio-Cu. The results also showed that MGEs explained most of the changes in the abundances of ARGs, and MGEs had direct effects on ARGs. The addition of 600 mg/kg nano-cellulose reduced the abundances of bacterial genera associated with ermQ, tetG, and other genes, and the number of links (16) between ARGs and MGEs was lowest in the treatment with 600 mg/kg added nano-cellulose. Therefore, adding 600 mg/kg nano-cellulose reduced the abundances of ARGs by affecting host bacteria and MGEs. The results obtained in this study demonstrate the positive effect of nano-cellulose on ARG pollution in poultry manure, where adding 600 mg/kg nano-cellulose was most effective at reducing the abundances of ARGs.

Using vermiwash to enhance performance of small-scale vermifiltration for swine farm wastewater

Pollution caused by swine wastewater is a growing concern in many countries. In the developing countries, swine wastewater is not properly collected and treated, the wastewater from swine farm pollutes the ecosystem. Especially for small swine farms, they could not afford to have wastewater treatment system. Therefore, farmers need cheap, sustainable technology for future mixed farming. Vermifiltration by earthworm has been introduced to be an answer for enhancing wastewater treatment. Vermiwash is the liquid gathered from vermicomposting that has high microbial activities and nutrients. This study was carried out on a small pilot scale to investigate swine wastewater treatment efficiency of vermifiltration system with and without vermiwash and compared with the geofiltration system. Vermiwash was incubated in vermifiltration and geofiltration systems for 1 week before the treatment. The result showed improved efficiency of vermifiltration incubated with vermiwash in swine wastewater treatment for biological oxygen demand (BOD), chemical oxygen demand (COD) and total suspended solids (TSS) removal, which was highest followed by vermifiltration without incubated vermiwash, geofilter incubated with vermiwash and geofilter, respectively. Good performance of vermifiltration incubated with vermiwash compared with the geofilter treatment was demonstrated for removal of BOD (91.29 ± 9.89%, n = 10), COD (91.42 ± 6.34%, n = 10) and TSS (86.02 ± 10.45%, n = 10). Furthermore, the burrowing activity of the test earthworm (Eisenia fetida) promoted the aeration condition in vermifilter which led to more dissolved oxygen (DO) in effluent (61.28 ± 20.05%, n = 10). Moreover, the amount of copper (Cu) in effluent was decreased compared with influent by up to 88% in all treatment. After 10 weeks of the experiment, the vermicompost that was incubated with vermiwash and produced from earthworm on the top layer was analyzed and showed that nutrients (nitrogen, phosphorus) and soil organic carbon were increased with vermifilter treatment (47.65, 81.61 and 31.79%, respectively) compared with geofilter treatment. In addition, bioavailability of Cu in soil in form of exchangeable Cu was decreased by increasing the bound to organic matter fraction. Transformation of Cu during vermifiltration happened and alleviated the mobility and availability of Cu. Copper in exchangeable form can change into non-toxic form. Therefore, vermifiltration process incubated with vermiwash could reduce the dispersion of copper in swine waste. In conclusion, vermiwash could enhance performance of vermifiltration for swine farm wastewater treatment. The available fraction of copper in vermicompost produced from vermifiltration decreased. Therefore, the farmer could produce vermicompost as the biofertilizer for agricultural production. Using vermifiltration for wastewater treatment in small swine farm could be the eco-solution for nutrient recovery, water resource recycles and minimize pollution.

Evolution of humic substances and the forms of heavy metals during co-composting of rice straw and sediment with the aid of Fenton-like process

The Fenton-like process was established by Fe₃O₄ nanomaterials (NMs) and Phanerochaete chrysosporium or oxalate, and applied to the co-composting of rice straw and sediment to study its effect on the formation of humic substance and the bioavailability of Cd, Cu, and Pb. Results shown that the application of Fenton-like process significantly promoted the passivation of Cd and Cu, while not shown obvious enhancement for Pb. The decrease of exchangeable fraction Cd (EXC-Cd) and the humic acid (HA) content in pile B with Fe₃O₄ NMs and oxalate were highest, which were 22.35% and 20.3 g/kg, respectively. Redundancy analyses (RDA) manifested that the Fenton-like process enhanced the influence of humus substance on the bioavailability of Cd, Cu, and Pb. Excitation-emission matrix (EEM) fluorescence spectra analysis suggested that Fenton-like process could obviously enhance the generation of humic substance. This research provides a new perspective and way for composting to remediate heavy metals pollution.

Contamination of the Soil–Groundwater–Crop System: Environmental Risk and Opportunities

The increasing development of industries, resulting in a large volume of mining, smelting, and combustion wastes, and intense agricultural activities, due to demand for food and energy, have caused environmental hazards for food quality and ecosystems. This is a review on the contamination of the soil–groundwater–crop system and a potential reduction of the contamination by a gradual shift towards green economy within the European Union and on a worldwide scale. Available mineralogical and geochemical features from contaminated Neogene basins have shown a diversity in the contamination sources for soil and groundwater, and highlighted the need to define the contamination sources, hot spots, degree/extent of contamination, and provide ways to restrict the transfer of heavy metals/metalloids into the food chain, without the reduction of the agricultural and industrial production. Among harmful elements for human health and ecosystems, the contamination of groundwater (thousands of μg/L Cr(VI)) by industrial activities in many European countries is of particular attention. Although Cr(VI) can be reduced to Cr(III) and be completely attenuated in nature under appropriate pH and Eh conditions, the contamination by Cr(VI) of coastal groundwater affected by the intrusion of seawater often remains at the hundreds μg/L level. A positive trend between B and Cr(VI) may provide insights on the role of the borate [B(OH)4]− ions, a potential buffer, on the stability of Cr(VI) in coastal groundwater. Efforts are needed towards reducing toxic metal(loids) from the industrial wastewaters prior to their discharge into receptors, as well as the transformation of hazardous mining/industrial wastes to new products and applications to the optimization of agricultural management strategies.

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.

Evolution of phytotoxicity during the active phase of co-composting of chicken manure, tobacco powder and mushroom substrate

This study systematically investigated the phytotoxicity of chicken manure co-composted with tobacco powder and mushroom substrate on seed germination during active phase of composting. All compost products met the sanitation requirements specified in the Chinese national standard; however, only the mushroom substrate compost satisfied the maturity standard. From day 28, the composting entered the end of active phase and the concentrations of K+, Zn2+, Na+, Cu2+ and Fe3+ decreased gradually. Redundancy analysis indicated that the germination index, catalase and peroxidase activities was positively correlated with K+, Zn2+, Na+, Cu2+, Fe3+ and NO3--N, and negatively correlated with NH4+-N, Mg2+ and Ca2+, among which the most significant ions were Fe3+, Mg2+ and Zn2+ for all treatments. The malondialdehyde concentration of germinated seeds had adverse correlation with the above ions parameters.

Ecological risk of copper and zinc and their different bioavailability change in soil-rice system as affected by biowaste application

A large amount of organic fertilizer application could be accompanied by soil contamination caused by trace heavy metals. A field experiment was carried out in this study to examine the accumulation and availability of copper (Cu) and zinc (Zn) in soil, and their uptake by rice under continuous application of chicken manure, pig manure and sewage sludge. Results showed that after four years of chicken manure, pig manure and sewage sludge application, the soil Cu accumulation rates were 0.15-1.17 mg kg^-1 yr^-1, 1.01-4.22 mg kg^-1 yr^-1 and 0.13-1.15 mg kg^-1 yr^-1, respectively; Zn accumulation rates were 0.54-5.46 mg kg^-1 yr^-1, 1.51-9.65 mg kg^-1 yr^-1 and 1.13-10.47 mg kg^-1 yr^-1, respectively. Compared to the control, the chicken- and pig manure treatments significantly decreased the DTPA-extractable Cu, but increased the DTPA-extractable Zn in soils; thus decreased the Cu contents in rice grain by 2.2-40.6% and increased the grain Zn by 2.6-30.9%, respectively, with increasing application rates and number of years. The addition of sewage sludge significantly increased bioavailability of Zn in soil and its accumulation in rice, while had limited effect on Cu bioavailability. Results suggested that the continuous application of organic fertilizer with elevated Cu and Zn contents at high application rates can induce their accumulation in soil and affect their bioavailability differently.

Citric Acid Assisted Phytoremediation of Chromium through Sunflower Plants Irrigated with Tannery Wastewater

Heavy metals are rapidly polluting the environment as a result of growing industrialization and urbanization. The presence of high concentrations of chromium (Cr), along with other pollutants, is widespread in tannery wastewater. In Pakistan, as a result of a severe shortage of irrigation water, farmers use tannery wastewater to grow various crops with a consequent decline in plants’ yield. This experiment was performed to assess growth revival in sunflower plants irrigated with 0%, 25%, 50%, 75%, and 100% tannery wastewater, by foliar application of 0, 2.5, and 5.0 mM citric acid (CA). The wastewater treatment curtailed biomass accumulation, the growth rate, and chlorophyll contents by exacerbating the oxidative stress in sunflowers. Foliar application of CA considerably alleviated the outcomes of Cr toxicity by curbing the Cr absorption and oxidative damage, leading to improvements in plant growth, biological yield, and chlorophyll contents. It is concluded that foliar application of CA can successfully mitigate the Cr toxicity in sunflower plants irrigated with tannery wastewater.

Interactions of humates and chlorides with cadmium drive soil cadmium chemistry and uptake by radish cultivars

In contrast to some salts such as chlorides (Cl) that enhance cadmium (Cd) phyto-uptake, complex soil organics like humates (HA) potentially minimise Cd uptake, but are depleted in soils low in organic matter. Organically-depleted and salt-affected areas frequently coincide in (semi)arid agroecosystems where inappropriate management practices may load topsoils with Cd. We evaluated the impact of HA (0-100 mg/kg) and NaCl (0-60 mM) in Cd-contaminated (0-5 mg/kg) soil on the chemical changes in the rhizosphere and Cd uptake by two radish (Raphanus sativus L.) cultivars. In the rhizosphere solution the significant HAxCd interaction resulted in a decrease in Cd concentration with increasing HA rates, whereas the NaClxCd interaction was brought about by an increase in Cd concentration with NaCl rising. Also, the NaClxCd interaction increased Cd concentration in radish hypocotyl with increasing NaCl addition; in contrast, the HAxCd interaction reduced Cd concentration in hypocotyl, notably at the highest Cd rate, with increasing soil humification. The addition of HA acted as a biostimulant in both radish cultivars and decreased Cd accumulation (up to 44%), whereas NaCl stress reduced the root growth and enhanced total Cd accumulation (by almost 50%). Dose-dependent severity of Cd toxicity was confirmed in both cultivars by reduced growth and progressive (up to 2 orders of magnitude) Cd accumulation (vs. uncontaminated soil). Ion speciation modelling suggested that chemistry of deprotonated humates and chlorides is crucial for complexation of the most bioavailable Cd²⁺ species, thus driving Cd mobility within the soil matrix, including uptake by plants. Detected differences between the tested cultivars (e.g. lower Cd concentration in Sparkler vs. Cherry Belle) and their impacts on rhizosphere chemistry and Cd soil-plant acquisition/root-hypocotyl-shoot (re)distribution, suggest that genetic improvements (by developing and introducing salt- and/or metal-resistant varieties) should be exploited in phytoremediation of contaminated soils or for minimising metal accumulation in sustainable food production.

Assessment of the genotoxicity of antibiotics and chromium in primary sludge and compost using Vicia faba micronucleus test

The objective of this study was to investigate chemical, biological and eco-toxicological parameters of a compost produced through the co-composting of dewatered primary sludge (DPS) and date palm waste to evaluate in which extent it can exploited as a bio-fertilizer. DPS and date palm waste were co-composted in aerobic conditions for 210 days. Physico-chemical parameters were evaluated during composting (total organic carbon, total nitrogen, pH, available forms of phosphorus). Furthermore, heavy metals (Cd, Cu, Cr, Pb, Ni, Zn) and antibiotics (fluoroquinolones, macrolides and tetracyclines) content were analyzed in the DPS. To evaluate the genotoxicity of substrates, Vicia faba micronucleus test was carried out. Single and combined toxicities of a mixture of antibiotics (ciprofloxacin, enroflxacin, nalidixic acid, roxithromycin and sulfapyridin) and chromium (Cr₂ (SO₄)₃ and K₂Cr₂O₇) were examined. Although the final compost product showed a significant decrease of the genotoxicity, almost 50% of the micronucleus frequency still remained, which could be explained by the persistence of several recalcitrant compounds such as chromium and some antibiotics. Overall, the presence of antibiotics and chromium showed that some specific combination of contaminants represent an ecological risk for soil health and ecosystems even at environmentally negligible concentrations.

Study of the morphological changes of copper and zinc during pig manure composting with addition of biochar and a microbial agent

Intensive and unregulated use of feed additives in China has led to high levels of heavy metals such as copper and zinc in fertilizers, considerable quantities of which find their way into the environment. Studies have shown that composting could significantly decrease the bioavailability of heavy metals. This study was to investigate the effects of addition of biochar and a microbial agent on the morphological changes in copper and zinc during composting. Results show that treatment T8 successfully immobilized 70.36% of copper as a result of biochar addition. Treatment T3 successfully immobilized 40.76% of zinc; transformation of zinc to a higher stable state was found to be closely related to the formation of fulvic and humic acids. Results of fluorescence spectrum analysis also corroborate that the conversion of copper and zinc to forms with higher stability was associated with the formation of fulvic and humic acid-like substances.

Changes of heavy metal fractions during co-composting of agricultural waste and river sediment with inoculation of Phanerochaete chrysosporium

The effects of Phanerochaete chrysosporium on the bioavailability of multiple heavy metals (Pb, Cd, Cu, and Zn) in river sediments were investigated by co-composting with the agricultural waste. The results showed that the Phanerochaete chrysosporium inoculation can greatly enhance the passivation on Cu, Pb and Cd during 60 days co-composting. The effects in the three metals followed the order: Cu > Cd > Pb. There were no differences for Zn whether inoculation with P. chrysosporium or not. Redundancy analysis (RDA) implied that more than 4/5 of the variation of all fractions data for all heavy metals was explained by all significant canonical axes. P. chrysosporium can change the significant parameters for each metal and enhance the explanatory power of RDA model. The inoculation can strengthen the effect of OM (organic matter) on the bioavailability of heavy metals, but weaken the contribution of pH.

Heavy metal distribution and uptake by maize in a mudflat soil amended by vermicompost derived from sewage sludge

Sewage sludge has been regarded as an economic and efficient soil amendment for mudflat soil amendment despite of the concern of heavy metal contamination. Converting sewage sludge into vermicompost by earthworms may be effective to minimize the risk of heavy metal contamination caused by direct application of sewage sludge in mudflat soil. The objective of this study was to assess the feasibility of vermicompost amendment (VA), and its influence on heavy metal contamination compared with sewage sludge amendment (SSA) in mudflat soil. The results showed that VA improved the physicochemical properties of mudflat soil by decreasing soil bulk density, salinity, and pH, increasing soil organic carbon, nitrogen, and phosphorus contents in the soil. Consequently, the maize biomass and yield were significantly elevated by VA. For heavy metals, VA increased total Cd, Cu, Ni, Pb, and Zn concentrations in mudflat soil, and the maximum increments occurred at 250 t ha^-1. Available Cd, Cu, Mn, Ni, and Zn concentrations significantly increased with increasing VA rates. VA increased the accumulation of Cd, Cu, Mn, Ni, Pb, and Zn in maize tissues, especially in root of maize. Compared with SSA under the condition of maintaining equal carbon input, VA allowed heavy metals to accumulate in a more stable binding form in the top 20-cm layer of mudflat soil. Thus, the risk of runoff and leaching of heavy metals and their bioavailability to plants reduced in mudflat soil. As a result, VA decreased the accumulation of heavy metals in maize plant compared with SSA in mudflat soil. In summary, vermicompost can be an effective and safe substitute for sewage sludge for mudflat amendment.

Relation between changes in organic matter structure of poultry litter and heavy metals solubility during composting

Poultry litter (PL) is widely used as fertilizer because of its rich N, P and Ca content. When PL is applied to previously untreated soil, it is a potential contaminant. Composting is an alternative for stabilizing organic and mineral components. This study aimed to elucidate the structural changes and its influence on the solubility of heavy metals in poultry litter during the first 30 d of composting, which is the period when the most intense transformations occur. For this analysis, the transformation dynamics of the organic structures and the availability of mineral elements were studied via spectroscopic characterization, total heavy metal content determination and chemical fractionation at three composting times (0, 15, and 30 d). During composting, the material's aromaticity increased, while its aliphaticity decreased, and the hydrophobicity index increased as the polarity decreased. These results indicate that during the first 30 d, PL composting occurs via transformation of the most labile structures (carbohydrate, peptide and fatty acid fragments), thereby preserving the most stable and least functionalized structures. Composting increased the concentrations of Cu, Cr, Pb and Zn and the transformation of C_Alk-O and C_Alk-di-O associated with peptides and carbohydrates and favored solubilizing and leaching a water-soluble fraction rich in these compounds. The labile fractions of Fe and the humified organic matter fractions of Cu, Fe, Mn, Zn, and Al increased. The structural changes reduced heavy metal solubility, thus indicating that after 30 d of composting, the heavy metal contamination risk is low.

Effects of straw amendment on selenium aging in soils: Mechanism and influential factors

Soil dissolved organic matter (DOM) alters heavy metal availability, but whether straw amendment can manipulate soil selenium (Se) speciation and availability through DOM mineralization remains unclear. In this study, allochthonous maize straw and selenate were incubated together in four different soils for 1 y. The transformation and availability of DOM associated Se (DOM-Se) was investigated during aging. Results indicated that soil solution and soil particle surfaces were dominated by hexavalent hydrophilic acid-bound Se (Hy-Se). The amount of fulvic acid bound Se in soil solution (SOL-FA-Se) was higher than humic acid bound Se in soil solution (SOL-HA-Se), except in krasnozems, and mainly existed as hexavalent Se (Se(VI)). Tetravalent Se (Se(IV)) was the main valence state of FA-Se adsorbed on soil particle surfaces (EX-FA-Se) after 5 w of aging. The proportion of soil-available Se (SOL + EX-Se) decreased with increasing straw rate. However, under an application rate of 7500 kg·hm^-2, soluble Se fraction (SOL-Se) reduction was minimal in acidic soils (18.7%-34.7%), and the organic bound Se fraction (OM-Se) was maximally promoted in alkaline soils (18.2%-39.1%). FA and HON could enhance the availability of Se in the soil solution and on particle surfaces of acidic soil with high organic matter content. While Se incorporation with HA could accelerate the fixation of Se into the solid phase of soil. Three mechanisms were involved in DOM-Se aging: (1) Reduction, ligand adsorption, and inner/outer-sphere complexation associated with the functional groups of straw-derived DOM, including hydroxyls, carboxyl, methyl, and aromatic phenolic compounds; (2) interconnection of EX-FA-Se between non-residual and residual Se pools; and (3) promotion by soil electrical conductivity (EC), clay, OM, and straw application. The dual effect of DOM on Se aging was highly reliant on the characteristics of the materials and soil properties. In conclusion, straw amendment could return selenium in soil and reduce soluble Se loss.

Role of Ca-bentonite to improve the humification, enzymatic activities, nutrient transformation and end product quality during sewage sludge composting

This study was aimed to examine the response of Ca-bentonite (CB) amendment to improve the sewage sludge (SS) composting along with wheat straw (WS) as bulking agent. Five treatments (SS + WS) were mixed with or without blending of discrepant concentration of CB (2%, 4%, 6%, and 10%), respectively, and without CB added treatment applied as the control. The results showed that compared to control and 2%CB blended treatments, while the 6-10%CB -amended treatment indicated maximum enzymatic activities with the composting progress and highest organic matter degradation and loss. The amendment of 6-10%CB increased the humic acid, HA/FA ratio, DON, NH₄⁺-N, NO₃ and DOC but reduced the fulvic acids content and the maturity period by 2 weeks as compared to control. In addition, maturity parameters also confirmed that the highest seed germination was observed with the 10%CB applied compost followed by 6%CB, 4%CB and 2%CB applied treatments, respectively.

Humic acids decrease uptake and distribution of trace metals, but not the growth of radish exposed to cadmium toxicity

Naturally-occurring highly-complexed and polymerised organics such as humic acids (HA), due to their large negative charge, play a crucial role in biogeochemistry of trace metals (TM). Toxic (Cd) as well as essential (Zn, Cu, Mn) TM bind strongly to HA, but how these organo-metalic forms influence metal uptake by plants is poorly understood. A solution culture study was conducted to characterize the effects of different concentrations of HA (0-225mg/L) on the growth and element uptake/distribution in roots, shoots and hypocotyls of radish (Raphanus sativus L.) exposed to Cd (0.5mg/L) contamination. After 10-d-exposure to applied treatments, Cd induced phytotoxicity; in contrast, different concentrations of HA had no influence on biomass, but decreased concentration of most TM in examined tissues (Cu by 4.2-fold, Zn by 2.2-fold, Cd by 1.6-fold and Mn by 34%) and their total plant accumulation (Cu by 73%, Cd by 39%, Zn by 29% and Mn by 22%). HA influenced the transport/distribution of TM, decreasing accumulation in roots and increasing their translocation/deposition in shoots, with no effect on TM content in edible hypocotyls. Chemical speciation modelling of the rooting medium confirmed predominance of free metallic forms in the control (no HA) and the pronounced organo-metal complexation in the HA treatments. The results provide evidence of strong capacity of HA to decrease phytoavailability and uptake of Cd, Zn, Cu and Mn while being non-toxic even at relatively high concentration (225mg/L). Thus, HA, as naturally present soil components, control mobility and phyto-extraction of most TM as well as their phyto-accumulation.

Changes of chromium speciation and organic matter during low-temperature pyrolysis of tannery sludge

The application or disposal of char derived from tannery sludge is directly influenced by the mobility and bioavailability of Cr during pyrolysis process. This study focused on the changes of Cr speciation and organic matter in tannery sludge during low-temperature pyrolysis (100-400 °C) to evaluate the toxicity of char in terms of the leaching possibility of Cr. The results showed that (1) lower char yield and more porous structure were observed after pyrolysis. (2) Higher pyrolysis temperature increased Cr content in the char; however, Cr in this case was converted into the residual fraction which minimized its bioavailability therefore lowers its potential risk to the environment. (3) Organic matters in the acid and alkali leachates were mainly humic acid-like substance, and condensed organic matter might appear at 200 °C and then destruct. (4) Despite the comparatively high content of Cr in the char, the leaching toxicity of char was within the security range according to the national standard of China. The Cr content in the acid and alkali leachates decreased to the range of 16.5-35.3 and 0.2-6.8 mg/L, respectively. It was suggested that the potential toxicity of tannery sludge from Cr could be reduced before utilization or disposal by pyrolysis, especially under 400 °C.

Comparison of biochar, zeolite and their mixture amendment for aiding organic matter transformation and nitrogen conservation during pig manure composting

The aim of this work was to compare the impact of biochar, zeolite and their mixture on nitrogen conservation and organic matter transformation during pig manure (PM) composting. Four treatments were set-up from PM mixed with wheat straw and then applied 10% biochar (B), 10% zeolite (Z) and 10% biochar+10% zeolite (B+Z) into composting mixtures (dry weight basis), while treatment without additives applied used as control. Results indicated that adding B, Z and B+Z could obviously (p<0.05) improve the organic matter degradation and decrease the nitrogen loss. And combined addition of B and Z further promoted the organic matter humification and reduced the heavy metals mobility. Meanwhile the highest mitigation of ammonia (63.40%) and nitrogen dioxide (78.13%) emissions was observed in B+Z added treatment. Comparison of organic matter transformation, nitrogen conservation and compost quality indicated that the combined use of biochar and zeolite could be more useful for PM composting.

Removal of Copper (II) by Biochar Mediated by Dissolved Organic Matter

The effects of humic acid (HA) and fulvic acid (FA) on Cu²⁺ adsorption on biochar were investigated, with mechanisms confirmed by excitation-emission matrix spectroscopy, Fourier transform infrared spectroscopy, and scanning electron microscopy. HA loading enhanced Cu²⁺ adsorption on biochar, with the maximum enhancement of 55.0% occurring at an HA loading of 100 mg-C/L. The adsorbed HA introduced many additional functional groups to biochar, thus enhancing Cu²⁺ adsorption, which decreased at HA concentrations >100 mg-C/L due to self-association of HA at high loading concentrations. In contrast, FA loading caused no enhancement on Cu²⁺ adsorption on biochar. FA was adsorbed through H-bonding with the functional groups of biochar, which set up a competition with Cu²⁺ for adsorption on biochar. The functional groups occupied by adsorbed FA were offset by the newly introduced functional groups of FA, thus there was no net increase in the amount of Cu²⁺ adsorption upon FA loading. These findings imply that, because of the enhanced adsorption of HA-loaded biochar, the amount of Cu²⁺ immobilized would increase by 28.2% for mature compost and 31.9% for fresh compost if there exist interaction between biochar and HA compared with the amounts immobilized by non-interactive HA and biochar.

Contrasting effects of composting and pyrolysis on bioavailability and speciation of Cu and Zn in pig manure

The intensive and unregulated application of feed additives to commercial pig foods has resulted in high levels of Cu and Zn in pig manure. The aim of this study was to assess the impacts of composting and pyrolysis processes on the bioavailability and chemical speciation of Cu and Zn in pig manure products by single and sequential extractions, and to compare metal bioavailability in composts and biochar-amended soils in incubation experiments. Composting and pyrolysis processes can convert exchangeable and carbonate-bound Cu and Zn to organic matter and residual fractions, and significantly reduce the potential availability of metals in composts and biochars. The DTPA-Cu and Zn concentrations in soils amended with biochar BC700 were lower than in composts and soils amended with biochar BC400. It is suggested that 700 °C is the preferred pyrolysis temperature for the conversion of pig manure contaminated with heavy metals to biochar, in order to minimize environmental pollution.

Biochar amendment for integrated composting and vermicomposting of sewage sludge - The effect of biochar on the activity of Eisenia fetida and the obtained vermicompost

Sewage sludge derived biochar (SSDB) was used as a supplementary material for municipal sewage sludge (SS) and wood chips mixtures (WC) treated by combined composting and vermicomposting. SSDB added to the mixture before composting resulted in significantly higher reproduction rate: on week 4 the number of cocoons increased by 213% when compared to the mixture with no biochar. On week 6 the average number of juveniles increased 11-fold in the mixture with biochar added before composting and 5-fold in the mixtures with biochar added after composting when compared to the mixture with no biochar. Biochar added before composting reduced bioavailability of Cd and Zn to E. fetida. The biochar-added vermicomposts showed good fertilizing properties except for elevated concentrations of Cr. The pH of all vermicomposts was in the range of 5.27-5.61. The obtained vermicomposts can be used as a growing medium for horticultural purposes or as an amendment in calcareous soils.

Vermicomposting of coal fly ash using epigeic and epi-endogeic earthworm species: nutrient dynamics and metal remediation

Study highlights the metal removal efficiency and nutrient dynamics of three potent earthworm species from coal fly ash with a comparative aspect.

Influence of N-rich material in valorization of industrial eggshell by co-composting

Industrial eggshell (ES) is an animal by-product (ABP) involving some risk if not properly managed. Composting is a possible treatment approved for its safe use. This study aims to assess the influence of using N-rich material (grass clippings (GC)) to improve co-composting of ES mixtures for reaching sanitizing temperatures imposed by the ABP regulation from the European Union. Two sets of mixtures (M1 and M2) were investigated, each containing industrial potato peel waste, GC and rice husks at 3:1.9:1 and 3:0:1 ratios by wet weight. In each set, ES composition ranged from 0% to 30% (w/w). Co-composting trials were performed in self-heating reactors for 25 days, followed by maturation in piles. Results showed that only M1 trials attained temperatures higher than 70°C for nine consecutive hours, but N-losses by stripping on average were four- to five-fold higher than M2. In the absence of N-rich material, biodegradability of mixtures was 'low' to 'moderate' and organic matter conversion was impaired. Physical, chemical and phytotoxic properties of finished composts were suitable for soil improvement, but M1 took 54 more days to achieve maturity. In conclusion, co-composting ES with N-rich materials is important to assure the fulfilment of sanitizing requirements, avoiding any additional thermal treatment.

Exploring metal detoxification and accumulation potential during vermicomposting of Tea factory coal ash: sequential extraction and fluorescence probe analysis

Metal contamination from coal ashes (CAs) is widely recognized as a significant environmental concern. To learn more about metal detoxification and accumulation potential of earthworm species, metal-rich tea factory coal ashes (TFCA) were fed to Eisenia fetida and Lampito mauritii by employing a fluorescent tag detection method. Fascinatingly, on feeding fluorescence probed Zn and Cd along with cow dung to Eisenia fetida, the detection of the gut-proteins with a molecular mass higher than 100 kDa was a distinct evidence of metal binding. Significant increases were observed in the content of humified organic C [humic acid (HAC) and fulvic acid C (FAC)] and degree of humification during vermicomposting. Concurrently, considerably large amount of toxic metals (Cr, Cd, Pb, and Zn) was transformed from exchangeable to recalcitrant (organic matter and mineral bound) fractions. Moreover, total metal concentrations were reduced with high removal efficiency upon vermicomposting.

Comparative characterization of sewage sludge compost and soil: Heavy metal leaching characteristics

The leaching and accumulation of heavy metals are major concerns following the land application of sewage sludge compost (SSC). We comparatively characterized SSC, the reference soil, and the SSC amended soil to investigate their similarities and differences regarding heavy metal leaching behavior and then to evaluate the effect of SSC land application on the leaching behavior of soil. Results showed that organic matter, including both of particulate organic matter (POM) and dissolved organic matter (DOM), were critical factors influencing heavy metal leaching from both of SSC and the soil. When SSC was applied to soil at the application rate of 48t/ha, the increase of DOM content slightly enhanced heavy metal leaching from the amended soil over the applicable pH domain (6<pH<8) and at alkaline pH, and led to more DOM-bound species in the liquid phases. However, the increase of POM content with the SSC application had less influence on the leaching behavior of heavy metals. The geochemical speciation modeling revealed that heavy metal speciation in the solid phase were similar between the reference soil and the amended soil.

Obtaining edaphic biostimulants/biofertilizers from different sewage sludges. Effects on soil biological properties

We studied the influence of six edaphic biostimulants/biofertilizers (BSs) manufactured by the pH-stat method from different sewage sludge (SS): SS1 (an anaerobic mature sludge, one year old), SS2 (an aerobic young sludge, without maturation) and SS3 (an aerobic mature sludge, four months old), not previously autoclaved (A) and autoclaved (B), by analysing their effects on soil biological properties. Soil enzymatic activities were measured at 1, 3, 5, 7, 15, 30 and 60 days of the incubation period, whereas the 16S rDNA-DGGE profiles were determined at 0, 5 and 60 days. The enzymatic activities were significantly stimulated. The highest stimulation was found in the B2 treatment followed by B3, A2, A3, B1 and A1 treatments. Increasing the number of lower molecular weight proteins in the BS enhances the stimulation of soil enzymatic activities. The application of BS caused at 5 days of the incubation period temporal variations in the soil bacterial community structure.

Heavy metal and nutrient changes during vermicomposting animal manure spiked with mushroom residues

A pilot-scale trial of four months was conducted to investigate the responses of heavy metal and nutrient to composting animal manure spiked with mushroom residues with and without earthworms. Results showed that earthworm activities accelerated organic matter mineralization (e.g. reduction in C/N ratio, increase in total concentrations of N, P, K) and humification (e.g. increase in humic acid concentration, humification ratio and humification index). Despite composting increased total heavy metal (i.e. As, Pb, Cu, Zn) concentrations irrespective of earthworm, the availability of heavy metals extracted by DTPA significantly (P<0.05) decreased particularly in treatments with earthworms introduced. The shift from available to unavailable fractions of heavy metals was either due to earthworm bioaccumulation, as indicated by total heavy metal concentrations being higher in earthworm tissues, or due to the formation of stable metal-humus complexes as indicated by the promotion of humification. Our results suggest that vermicomposting process could magnify the nutrient quality but relieve the heavy metals risk of agricultural organic wastes.

Reduction of bioavailability and leachability of heavy metals during vermicomposting of water hyacinth

Vermicomposting of water hyacinth is a good alternative for the treatment of water hyacinth (Eichhornia crassipes) and subsequentially, beneficial for agriculture purposes. The bioavailability and leachability of heavy metals (Zn, Cu, Mn, Fe, Ni, Pb, Cd, and Cr) were evaluated during vermicomposting of E. crassipes employing Eisenia fetida earthworm. Five different proportions (trials 1, 2, 3, 4, and 5) of cattle manure, water hyacinth, and sawdust were prepared for the vermicomposting process. Results show that very poor biomass growth of earthworms was observed in the highest proportion of water hyacinth (trial 1). The water soluble, diethylenetriaminepentaacetic acid (DTPA) extractable, and leachable heavy metals concentration (percentage of total heavy metals) were reduced significantly in all trials except trial 1. The total concentration of some metals was low but its water soluble and DTPA extractable fractions were similar or more than other metals which were present in higher concentration. This study revealed that the toxicity of metals depends on bioavailable fraction rather than total metal concentration. Bioavailable fraction of metals may be toxic for plants and soil microorganisms. The vermicomposting of water hyacinth by E. fetida was very effective for reduction of bioavailability and leachability of selected heavy metals. Leachability test confirmed that prepared vermicompost is not hazardous for soil, plants, and human health. The feasibility of earthworms to mitigate the metal toxicity and to enhance the nutrient profile in water hyacinth vermicompost might be useful in sustainable land renovation practices at low-input basis.

Changes of heavy metal speciation during high-solid anaerobic digestion of sewage sludge

The sequential extraction procedure developed by Tessier et al. was used in sludge anaerobic digestion to determine the heavy metal speciation. Sludge samples were taken every three days to investigate humic substances, VS/TS, pH, VFA, alkalinity, ammonia, the total content of Zn, Pb, Cu, Ni, and Cr, and also their distribution into EXCH, CARB, FeMnOx, OMB and RESI fractions. Results showed that, (1) Heavy metals were concentrated during the anaerobic digestion process. The concentration of all five kinds of heavy metals increased by about 50%. (2) The distribution of these heavy metals differed. (3) High-solid anaerobic digestion much or less increased the bioavailability of Cu, Zn, Ni and Cr while decreased the bioavailability of Pb. (4) There were significant degrees of correlation between heavy metal fractions and changes of some selected parameters (for example, pH, VS/TS, and VFA content). Except for Pb, the contents of total mobile fractions for Cu, Zn, Ni, Cr could be predictable from its total content.

Transformation and availability of nutrients and heavy metals during integrated composting-vermicomposting of sewage sludges

Transformation and availability of nutrients and some heavy metals were assessed during the integrated composting-vermicomposting of both primary sewage sludge (PSS) and waste activated sewage sludge (WAS) using matured vermicompost as indigenous bulking material and employing Eisenia fetida as earthworm species. Vermicomposting resulted in significant increase in total N (TN) (PSS: 41.7-64.6%, F=11.6, P<0.05; WAS: 36.4-58.6%, F=6.4, P<0.05), water soluble N (WSN) (PSS: 37.1-50.5%, F=30.1, P<0.05; WAS: 40.1-53.0%, F=27.6, P<0.05), total P (TP) (PSS: 39.9-69.8%, F=27.1, P<0.05; WAS: 32.2-56.6%, F=21.4, P<0.05) and water soluble P (WSP) (PSS: 25.2-34.3%, F=163.9, P<0.05; WAS: 24.1-34.2%, F=144.3, P<0.05) as compared to the initial compost material depending on different experimental conditions. The study demonstrated that the vermicomposting significantly improved the availability of nutrients in sewage sludges. In addition, vermicomposting considerably reduced the availability of heavy metals except Fe and Mn, presumably by forming organic-bound complexes in spite of several fold increase in their total content. The environmental conditions (i.e., temperature and relative humidity), in general, showed significant effect on the transformation and availability of nutrients and heavy metals. There was no effect of earthworm density on the transformation and availability of heavy metals and nutrients except N and P, possibly due to prior exposure during acclimation period in sewage sludge.