Ecotoxicological assessment of sewage sludge-derived biochars-amended soil

Ecotoxicological assessment, in freshwater environment, of wastewater sludge coupled and uncoupled with micro-polyvinyl chloride on algae and water fleas

In the frame of bioeconomy and circular economy, wastewater sludge (WS) could be a good candidate for its use in agriculture as fertilizer, due to its high content of organic matter, N and P, but on the other hand, it is full of toxicants such as heavy metal, microplastics, detergent, antibiotics, and so on that can reach groundwater and water bodies in leachate form. In this study, we have investigated different sludge concentrations in the eluate form, combined and not with PVC on two different freshwater organisms Selenastrum capricornutum and Daphnia magna, using ecotoxicity tests. At the endpoint, we have evaluated inhibition growth rate, oxidative stress, and pigments production for S. capricornutum, while in case of D. magna, we have assessed organism immobilization and development. From our results, it emerged that at the higher WS concentration, there was not inhibition growth rate, while at oxidative stress, it was higher in algae treated with WS and PVC. Higher Chl-a production was shown for algae treated with 0.3 g/L of sludge coupled with PVC, where higher phaeopigments production were recorded for algae treated with 0.3 g/L of WS. D. magna has shown an opposite trend when compared with algae, where at the highest WS concentrations supplied was corresponding to an increased mortality explaned as the highest immobility percentage. PRACTITIONER POINTS: Wastewater sludge is used in agriculture as fertilizer. PVC microplastic presence and associate ecotoxicity was tested. PVC presence increased oxidative stress in S. capricornutum. D. magna was significantly affected by sludge concentrations supplied.

Ecotoxicological assessment of waste-derived organic fertilizers and long-term monitoring of fertilized soils using a multi-matrix and multi-species approach

The present study investigates the ecotoxicity of 7 biofertilizers, including biowaste-derived organic matrices. Real-field tests were conducted to assess the impacts of soil fertilization with sewage sludge digestate from high-solid thermophilic anaerobic digestion (HSTAD) compared to those obtained on non-amended and urea-fertilized soils. The physical-chemical and ecotoxic impact of HSTAD digestate on soil was monitored for 12 months, at 5 time points and 2 soil depths, on a maize field divided in 3 portions (non-treated, fertilized with urea, amended with digestate). The chemical and physical characteristics of the soil were previously analyzed for 3 years to provide a long-term outlook of the impacts of biofertilizer application. Seven bioindicators were utilized for direct (on whole soil) and indirect (on soil elutriates) ecotoxicological tests on fertilizers and amended soils, including plant seeds (Lepidium sativum, Sorghum saccharatum, and Sinapsis alba), the aquatic organism Daphnia magna, the alga Raphidocelis subcapitata, the luminescent bacterium Aliivibrio fischeri, and the Nematode Caenorhabditis elegans. No serious negative effects on soil fertilized with HSTAD digestate were evidenced. Conversely, bioassays rather showed positive effects, encouraging the utilization of HSTAD digestate in agriculture, considering the proper concentrations of use. The obtained data were interpolated and a test battery integrated index was generated, confirming the absence of ecotoxicological risk for the soils amended with the applied fertilizers. The long-term evolution of the physical-chemical soil characteristics (including the concentrations of potential contaminants) was similar for both HSTAD digestate and urea application as well as for non-fertilized soil, indicating no negative effects due to digestate application on land. On the contrary, digestate application improved the content of stabilized organic matter and nutrients in soil. This study proposes a more correct approach to ecotoxicity assessment of fertilized soils for biofertilizer evaluation and demonstrates the long-term safe application of HSTAD digestate on agricultural soil.

Heavy metal stabilization remediation in polluted soils with stabilizing materials: a review

The remediation of soil contaminated by heavy metals has long been a concern of academics. This is due to the fact that heavy metals discharged into the environment as a result of natural and anthropogenic activities may have detrimental consequences for human health, the ecological environment, the economy, and society. Metal stabilization has received considerable attention and has shown to be a promising soil remediation option among the several techniques for the remediation of heavy metal-contaminated soils. This review discusses various stabilizing materials, including inorganic materials like clay minerals, phosphorus-containing materials, calcium silicon materials, metals, and metal oxides, as well as organic materials like manure, municipal solid waste, and biochar, for the remediation of heavy metal-contaminated soils. Through diverse remediation processes such as adsorption, complexation, precipitation, and redox reactions, these additives efficiently limit the biological effectiveness of heavy metals in soils. It should also be emphasized that the effectiveness of metal stabilization is influenced by soil pH, organic matter content, amendment type and dosage, heavy metal species and contamination level, and plant variety. Furthermore, a comprehensive overview of the methods for evaluating the effectiveness of heavy metal stabilization based on soil physicochemical properties, heavy metal morphology, and bioactivity has also been provided. At the same time, it is critical to assess the stability and timeliness of the heavy metals' long-term remedial effect. Finally, the priority should be on developing novel, efficient, environmentally friendly, and economically feasible stabilizing agents, as well as establishing a systematic assessment method and criteria for analyzing their long-term effects.

Vermitoxicity of aged biochar and exploring potential damage factors

Although biochar is a promising soil amendment, its characteristics change owing to its aging in soil. Studies have shown that some aged biochar is hazardous to plants and soil microbiota. Earthworms are well-known soil ecosystem engineers; nevertheless, the toxic effects of aged biochar on them (vermitoxicity) are yet unknown, and it is necessary to explore the potential risk factors. Here, a series of soil culture experiments were conducted to systematically examine the vermitoxicity of aged biochar at various levels utilizing the earthworm Eisenia fetida and corncob biochar.. Acute toxicity bioassays were also used to evaluate several potential harm factors utilizing modified aged biochar/leaching solutions. The findings showed that both fresh and aged biochar might have adverse effects on earthworms, and that aged biochar was more toxic than fresh biochar with LC₅₀s reduced to 6.89%. Specifically, aged biochar caused earthworm death, growth inhibition with a maximum of 36.6%, and avoidance with 100% avoidance at the application rates of 2% at the individual-behavioral level. At the cellular and physiological-biochemical levels, aged biochar damaged coelomocyte lysosomal membrane stability, disrupted antioxidant enzyme activities, and improved the malondialdehyde (MDA) content in earthworms. Heat-treated and pH-modified aged biochar exhibited less acute toxicity on earthworms than aged biochar, whereas aqueous and acetone extracts showed weak vermitoxicity. As a result, earthworms may be harmed by volatile organic compounds (VOCs), an improper pH, and aqueous and acetone extracts. Additionally, the range of neural red retention times (NRRTs) was reviewed as ∼20-70 min mostly. This study, as far as we know, is the first to evaluate the vermitoxicity of aged biochar and its potential damage factors. The results may enhance our understanding of ecological toxicity of biochar, particularly over the long term, and lead to the development of application standards for biochar amendments to the soil.

Biochar derived from hydrolysis of sewage sludge influences soil properties and heavy metals distributed in the soil

Sewage sludge contains a large number of nutrients and dangerous substances, when sludge was processed into sludge hydrochar that was added to the soil, which not only solve the problem of sludge disposal, but also amend the soil and fix pollutants in the soil. However, it was lack of report on the effect of the sludge hydrochar on soil compositions and soil microorganism community structures until now. In the present study, the hydrothermal carbonization method is used to prepare hydrochar from sewage sludge at temperatures of 180 ℃ and 240 ℃ at durations of 6 h and 15 h in this paper. The effects of the prepared sludge hydrochar on soil-derived dissolved organic matter (DOM), the content of total dissolved nitrogen (TDN) and NO₃^--N in soil, and the community structure of soil bacteria and fungi were evaluated. Furthermore, the change rules in heavy metal speciation in soils treated with sludge hydrochar were investigated. With the increase in the preparation temperature and dosage of sludge hydrochar, the main components of DOM changed from soluble microbial byproducts to fulvic acid-like and humic acid-like fractions through UV and fluorescence characterization. The sludge hydrochar prepared at low temperature could significantly increase the contents of TDN and NO₃^--N in the soil. Affected by sludge hydrochar, the dominant phylum of the bacterial community changed from Proteobacteria to Actinobacteria, and the dominant phylum in the fungal community did not change, but its relative abundance increased. Finally, the sludge hydrochar obtained when the carbonization time was 15 h was more beneficial to reduce the total amount and available content of heavy metals in the soil. The study provides a basis for sludge hydrochar application for the soil amendment.

Biochar alleviating heavy metals phytotoxicity in sludge-amended soil varies with plant adaptability

Recycling sewage sludge (SS) to soil potentially causes soil heavy metal (HM) pollution and plant phytotoxicity. Biochar plays an important role in alleviating HM phytotoxicity, and responses vary with the feedstocks and usage of biochar. However, the effect of plant adaptability on biochar-mediated alleviation is poorly understood. Here, SS-derived biochar (SB) and rice straw-derived biochar (RB) applied at rates of 1.5% and 3% (W/W, SB1.5, SB3, RB1.5, and RB3) were used to improve the properties of soil amended with SS at 50% (W/W). Alleviation of phytotoxicity by biochar was further analyzed with SS-sensitive plant Monstera deliciosa and SS-resistant plant Ruellia simplex. Results revealed that both SB and RB significantly decreased the soil's bulk density and increased water retention. They also changed soil organic matter content and HMs fractionation. The addition of SB or RB alleviated the SS phytotoxicity, and they significantly promoted the growth and the root morphology and physiological index of M. deliciosa. But for R. simplex, these significant changes only synchronously occurred in SB3 treatment. The alleviation in M. deliciosa was more prominent and more closely connected with soil property changes than in R. simplex. Also, more soil property predictors were observed to play an important role in M. deliciosa growth than in R. simplex growth. These results indicated that biochar alleviating HMs phytotoxicity in SS-amended soil is associated with the changes of soil property. Moreover, the alleviation varies more prominently with plant adaptability than with biochar feedstocks and usage.

Effect of carrier gas change during sewage sludge or sewage sludge and willow pyrolysis on ecotoxicity of biochar-amended soil

Different pyrolysis conditions determine the properties of the biochar. The properties of biochar may affect directly or indirectly their influence on living organisms. The aim of this study was to determine the toxicity of biochar obtained under different conditions (temperature: 500 or 700 °C, carrier gas: N₂ or CO₂, feedstock: sewage sludge or sewage sludge/biomass mixture) after adding to the soil in long-term pot experiment (180 days). Biochars were added to the podzolic loamy sand at a 2% (w/w) dose. Samples were collected at the beginning of the experiment and after 30, 90 and 180 days. The bacteria Aliivibrio fischeri (luminescence inhibition - Microtox), the plant Lepidium sativum (root growth and germination inhibition test - Phytotoxkit F), and the invertebrate Folsomia candida (mortality and reproduction inhibition test - Collembolan test) were used as the test organisms. In the long-term perspective for most tests, changing the carrier gas from N₂ to CO₂ resulted in reduced toxicity of the biochar. A particularly beneficial effect of changing the gas to CO₂ was observed for the solid-phase test with L. sativum. The CO₂ during pyrolysis had the least beneficial effect on toxicity towards A. fischeri.

Effects of Co-Digestion Sludge Application on Soil Productivity

Anaerobic digestion and agricultural use of sewage sludge are effective methods to treat and dispose of sewage sludge, respectively. Then, the anaerobic digested sewage sludge is applied in agricultural land and the improvement of soil properties can be expected. In this study, with the purpose of evaluating the potential of co-digestion sludge for agricultural use, plot experiments with two vegetable species (radish and Chinese cabbage) and three application dosages were carried out in a short term of six months. Focus was on soil physical properties, soil nutrient change and plant growth responses during the whole process. Results showed that application of co-digestion sludge had little effect on soil physical properties, including the bulk density, porosity, capillary porosity and non-capillary porosity. However, after the application of co-digestion sludge, the maximum increase in content of organic matter, total nitrogen, hydrolysable nitrogen, total phosphorus and available phosphorus in soil reached 51%, 125%, 212%, 15% and 87%, respectively, which supplied the available nutrients quickly and continuously. The application of co-digestion sludge promoted the growth of radish and Chinese cabbage, which was observed through increase of the leaf, root biomass and plants height. Consequently, co-digestion sludge has a good application prospect for improving soil productivity as fertilizer.

Immobilization of Cd and Pb in soil facilitated by magnetic biochar: metal speciation and microbial community evolution

The preparation of magnetic biochar from sewage sludge and rice straw for heavy metal contaminated soil remediation has greater application prospects, but its remediation mechanism was rarely considered by combining soil physicochemical properties with microbial community. In this study, the effects of magnetic sewage sludge biochar (SSB) and rice straw biochar (RSB) on Cd and Pb immobilization in paddy soil were compared and analyzed by 60-day soil incubation experiments. The results illustrated that DTPA-Cd and DTPA-Pb were reduced by 51.53% (43.07%) and 53.57% (50.47%), while the percentage of residual fraction of the BCR procedure was enhanced by 31.27% (30.78%) of Cd and 27.25% (23.22%) of Pb in the SSB (RSB) treatment, respectively. Fe was detected on both SSB and RSB surfaces, but SSB had rougher and a larger specific surface area compared to RSB. The addition of SSB and RSB in paddy soil increased soil pH and TOC content, and affected the diversity and species of soil microbial community. Compared with the CK group, the relative abundance of Proteobacteria, Bacteroidota, and Lysobacter decreased, and the relative abundance of Actinobacteriota, Pontibacter, and Alkaliphilus increased with SSB and RSB treatments, all of which reflected the bioavailability of Cd and Pb reduction.

Assessing the influence of sewage sludge and derived-biochar in immobilization and transformation of heavy metals in polluted soil: Impact on intracellular free radical formation in maize

As one of the most common ways to get rid of municipal waste, landfill leachate, waste with complicated compositions and high levels of contaminants, has become a significant threat to the world's environment. Here, the impact of sewage sludge (SS) and derived-biochar (SSB) amendments on the immobilization and potential mobility of heavy metals in a contaminated soil-plant system was investigated. The sequential fractionation findings showed that using SS-2%, SSB-2%, and SSBC-1% reduced the potential mobility of heavy metals while increasing the residual fraction in polluted soils. The translocation and bioconcentration factors showed that heavy metals were slightly transferred into shoots from roots and lowered accumulation in roots from contaminated soils. Fourier transform infrared (FTIR) and X-ray photoelectron spectrum (XPS) comprehensive characterization results indicated the significant role of applied amendments for heavy metals transformation from the exchangeable-soluble fractions to the least available form by lowering their mobility to confirm the adsorption-based complexes, which results in the surface adsorption of heavy metals with functional groups. The electron paramagnetic resonance (EPR) results indicated the dominance of reactive oxygen species (ROS) in the intracellular formation of hydroxyl radicals (•OH) in maize plant roots and shoots. ROS (•OH) generation plays a critical influence in the interaction between the physiological processes of plants and heavy metals. Moreover, all the amendments increased maize growth and biomass production. Our study suggests that alone and combined application of SS and SSB have great potential to remediate heavy metals contaminated soil for environmental sustainability.

Aryl hydrocarbon reporter gene bioassay for screening polyhalogenated dibenzo-p-dioxins/furans and dioxin-like polychlorinated biphenyls in hydrochar and sewage sludge

The suitability of the AhR reporter gene bioassays to screen the presence of polychlorinated dibenzo-p-dioxins/furans (PCDD/Fs) and dioxin-like polychlorinated biphenyls (dl-PCBs) in sewage sludge (SL) and related hydrochar (HC) was here investigated. Samples of SL obtained from six WWTPs were processed by hydrothermal carbonization to obtain the resultant HCs and both tested with DR-CALUX® bioassay. Levels of PCDD/Fs and dl-PCBs were also determined analytically in the same samples by GC-MS/MS. Bioanalytical Toxicity Equivalent values (BEQ) resulted in one order of magnitude higher in HC compared to SL samples and those obtained from the dl-PCBs fraction higher than those from PCDD/Fs. BEQ and TEQ_WHO values, the latter obtained by GC-MS/MS analysis on the same matrices, were highly correlated showing also a similar trend in the six WWTPs (R_S= 0.8252, p < 0.001; Pearson's R R_P =0.8029, p < 0.01). The suitability of AhR bioassays and in particular of the DR-CALUX® to screen the presence and biological activity of legacy organohalogen compounds in both SL and HC matrices was demonstrated for the first time which support their usage for the assessment of potential risks associated with their further environmental applications.

Peat and bentonite amendments assisted soilless revegetation of oligotrophic and heavy metal contaminated nonferrous metallic tailing

Soilless revegetation is a promising method for ecological restoration of nonferrous metallic tailings because of its low-cost and eco-friendliness. However, revegetation is difficult to construct in the tailings due to the high heavy metal concentration, poor water retention capacity and low fertility. In this study, soilless revegetation was successfully carried out by using peat and bentonite amendments. The results showed that amendment addition significantly increased the F.elata seed germination percentage, plant length and fresh biomass by 14.9%-24.3%, 48.9%-90.4% and 51.9%-88.1%, respectively. Such improvements probably referred to the variation of rhizosphere tailing microecological characteristics. Amendment addition dramatically improved tailing available NPK by 39.76-102.13%, 2.69-40.81% and 2.42-20.02%, respectively, and reduced pH from alkaline to relative neutral. Besides, heavy metal bioavailability was significantly decreased that the acid soluble fraction decreased by 1.7%-11.5%, resulting in the reduction of heavy metal concentration in F.elata plant. Amendments also increased the rhizosphere tailing microbial species richness and the relative abundance of ecologically beneficial genera including Arthrobacter, Altererythrobacter and Bacillus. This study not only provided a green and efficient method for remediation of oligotrophic and high heavy metal contaminated nonferrous metallic tailing, but also demonstrated relevant mechanisms of amendment on promoting soilless revegetation.

Stabilization and passivation of multiple heavy metals in soil facilitating by pinecone-based biochar: Mechanisms and microbial community evolution

Soil contamination by multiple heavy metals and As is one of the major environmental hazards recognized worldwide. In this study, pinecone-biochar was used for stabilization and passivation of Pb, Cu, Zn, Cr, and As in contaminated soil around a smelter in Hubei province, China. The stabilization rate of heavy metals in soil can exceed 99%, and the leaching amount can meet the national standard of China (GB/T 5085.3-2007, less than 5, 100, 100, 15, and 5 mg/L, respectively.) within 90 days. The study confirmed that the addition of pinecone-biochar and the coexistence of indigenous microorganisms can effectively reduce the bioavailability of heavy metals. Among the heavy metals, As(III) can be oxidized to As(V) and then stabilized, and other heavy metals can be stabilized in a complex and chelated state characterized by X-ray photoelectron spectroscopy. After pinecone-biochar was added, the abundance of microbial community and intensity of metabolic activities became vigorous, the types and contents of dissolved organic matter increased significantly. A novel innovation is that the addition of pinecone-biochar increased the Bacillus and Acinetobacter in soil, which enhanced the function of inorganic ion transport and metabolism to promote the passivation and stabilization of heavy metals throughout the remediation process.