Municipal solid wastes as a resource for environmental recovery: Impact of water treatment residuals and compost on the microbial and biochemical features of As and trace metal-polluted soils

Using biochar for environmental recovery and boosting the yield of valuable non-food crops: The case of hemp in a soil contaminated by potentially toxic elements (PTEs)

Hemp (Cannabis sativa L.) is known to tolerate high concentrations of soil contaminants which however can limit its biomass yield. On the other hand, organic-based amendments such as biochar can immobilize soil contaminants and assist hemp growth in soils contaminated by potentially toxic elements (PTEs), allowing for environmental recovery and income generation, e.g. due to green energy production from plant biomass. The aim of this study was therefore to evaluate the suitability of a softwood-derived biochar to enhance hemp growth and promote the assisted phytoremediation of a PTE-contaminated soil (i.e., Sb 2175 mg kg-1; Zn 3149 mg kg-1; Pb 403 mg kg-1; and Cd 12 mg kg-1). Adding 3% (w/w) biochar to soil favoured the reduction of soluble and exchangeable PTEs, decreased soil dehydrogenase activity (by ∼2.08-fold), and increased alkaline phosphomonoesterase and urease activities, basal respiration and soil microbial carbon (by ∼1.18-, 1.22-, 1.22-, and 1.66-fold, respectively). Biochar increased the abundance of selected soil culturable microorganisms, while amplicon sequencing analysis showed a positive biochar impact on α-diversity and the induction of structural changes on soil bacterial community structure. Biochar did not affect root growth of hemp but significantly increased its aboveground biomass by ∼1.67-fold for shoots, and by ∼2-fold for both seed number and weight. Biochar increased the PTEs phytostabilisation potential of hemp with respect to Cd, Pb and Zn, and also stimulated hemp phytoextracting capacity with respect to Sb. Overall, the results showed that biochar can boost hemp yield and its phytoremediation effectiveness in soils contaminated by PTEs providing valuable biomass that can generate profit in economic, environmental and sustainability terms.

Effect of Industrial Solid Waste as Fillers on the Rheology and Surface Free Energy of Asphalt Mastic

The continuous growth of industrial solid waste production has generated many environmental problems. We evaluated the potential of industrial solid waste as a substitute filler in asphalt mastic, with the aim of increasing the use of sustainable road construction materials. In this study, X-ray fluorescence spectroscopy (XRF) and scanning electron microscopy (SEM) were used to characterize the oxide composition and micromorphology of limestone (LS), red mud (RM), steel slag (SS), and ground granulated blast-furnace slag (GGBFS). Four asphalt mastics containing LS, RM, SS, and GGBFS with a filler-to-binder weight ratio of one were prepared. An evaluation of the rheology and wetting of the solid-waste-filler asphalt mastic was conducted using a frequency sweep, temperature sweep, linear amplitude sweep (LAS), multiple stress creep and recovery (MSCR), and surface free energy (SFE) methods. The results showed that SS increased the complex modulus, elastic component of the asphalt mastic and decreased the nonrecoverable creep compliance at stress levels of 0.1 and 3.2 kPa, which improved the rutting resistance of the asphalt mastic and reduced deformation under high-temperature conditions. The RM and GGBFS increased the fatigue performance of the asphalt mastic under strain loading, enhanced its fatigue life, and maintained good performance under long-term loading. The dispersive component of the SFE parameter of the solid-waste-filler asphalt mastic was larger than the polar component for the largest share of the surface energy composition. The SFE of the asphalt mastic prepared from the industrial solid-waste filler was reduced; however, the difference was insignificant compared to the limestone asphalt mastic. Solid-waste-filler asphalt mastic has performance characteristics, and its actual application can be based on different performance characteristics to select an appropriate solid-waste filler. The results of this study provide new technological solutions for solving the utilization rate of solid waste materials and sustainable road construction in the future.

Mixing Compost and Biochar Can Enhance the Chemical and Biological Recovery of Soils Contaminated by Potentially Toxic Elements

Biochar and compost are able to influence the mobility of potentially toxic elements (PTEs) in soil. As such, they can be useful in restoring the functionality of contaminated soils, albeit their effectiveness can vary substantially depending on the chemical and/or the (micro)biological endpoint that is targeted. To better explore the potential of the two amendments in the restoration of PTE-contaminated soils, biochar, compost (separately added at 3% w/w), and their mixtures (1:1, 3:1, and 1:3 biochar-to-compost ratios) were added to contaminated soil (i.e., 2362 mg kg-1 of Sb and 2801 mg kg-1 of Zn). Compost and its mixtures promoted an increase in soil fertility (e.g., total N; extractable P; and exchangeable K, Ca, and Mg), which was not found in the soil treated with biochar alone. All the tested amendments substantially reduced labile Zn in soil, while biochar alone was the most effective in reducing labile Sb in the treated soils (-11% vs. control), followed by compost (-4%) and biochar-compost mixtures (-8%). Compost (especially alone) increased soil biochemical activities (e.g., dehydrogenase, urease, and β-glucosidase), as well as soil respiration and the potential catabolic activity of soil microbial communities, while biochar alone (probably due to its high adsorptive capacity towards nutrients) mostly exhibited an inhibitory effect, which was partially mitigated in soils treated with both amendments. Overall, the biochar-compost combinations had a synergistic effect on both amendments, i.e., reducing PTE mobility and restoring soil biological functionality at the same time. This finding was supported by plant growth trials which showed increased Sb and Zn mineralomass values for rigid ryegrass (Lolium rigidum Gaud.) grown on biochar-compost mixtures, suggesting a potential use of rigid ryegrass in the compost-biochar-assisted phytoremediation of PTE-contaminated soils.

Long-term effect of municipal solid waste compost on the recovery of a potentially toxic element (PTE)-contaminated soil: PTE mobility, distribution and bioaccessibility

Compost from municipal solid waste (MSWC) can represent a resource for the environmental management of soils contaminated with potentially toxic elements (PTEs), since it can reduce their mobility and improve soil fertility. However, the long-term impact of compost on soil recovery has been poorly investigated. To this end, the influence of a MSWC added at different rates (i.e. 1.5, 3.0 and 4.5% w/w) to a multi-PTE-contaminated (e.g. Sb 412 mg kg-1, Pb 2664 mg kg-1 and Zn 7510 mg kg-1) sub-acidic soil (pH 6.4) was evaluated after 6 years since its addition. The MSWC significantly enhanced soil fertility parameters (i.e. total organic carbon, Olsen P and total N) and reduced the PTE labile fractions. The distribution maps of PTEs detected through µXRF analysis revealed the presence of Zn and Pb carbonates in the amended soils, or the formation of complexes between these PTEs and the functional groups of MSWC. A higher oral, inhalation and dermal bioaccessibility of each PTE was detected in the soil fine-grained fractions (< 2 and 2-10 µm) than in coarse particles (10-20 and 20-50 µm). The MSWC amendment generally did not modify the PTE bioaccessibility, while the relative bioaccessibility of cationic PTEs was greater than that of anionic ones (e.g. Cd > Zn > Pb > Sb > As). Pb and Sb showed the highest hazard quotients (e.g. 2.2 and 10 for Sb and Pb, respectively, in children). Overall, the results indicated that the MSWC used can be an effective option for the recovery of PTE-contaminated soils, even in the long term.

Microbial genome (Illumina MiSeq) sequencing of drinking water treatment residuals to evaluate compatibility with environmental applications

The clarification of drinking water leads to the production of large quantities of water treatment residuals (WTRs). DNA was extracted from six WTR samples collected from water treatment plants within the UK to compare their bacterial communities and examine whether factors such as coagulant usage (aluminium versus iron salt), the type of water source (reservoir or river), or leachable chemical composition influence these communities. Bacterial 16S variable region 4 (V4) was amplified and sequenced using Illumina MiSeq sequencing. The most abundant phyla in WTR samples were Proteobacteria, Actinobacteria, Bacteroidetes, Acidobacteria, and Firmicutes, collectively representing 92.77-97.8% of the total bacterial sequences. Statistical analysis of microbial profiles indicated that water source played a significant role in microbial community structure, diversity, and richness, however coagulant type did not. PERMANOVA analysis showed that no single chemical variable (pH, organic matter, or extractable element concentration) influenced microbial composition significantly; however, canonical correspondence analysis of WTR microbiomes yielded a model using all these variables that could be used to explain variations in microbial community structures of WTRs (p < 0.05). No common, potentially toxic cyanobacteria, or related pathogens of concern were found. Analysis with PICRUSt showed that WTRs all had similar predicted microbial functional profiles. Overall, the results indicate that WTRs analysed in this study are unlikely to pose any threat to soil microbial community structure when applied to land as a soil conditioner or enhancer and may help to enhance the soil microbial community.

Enzyme activities and heavy metal interactions in calcareous soils under different land uses

This study was carried out to examine the interaction of enzyme activities, microbial biomass carbon, and CO2 respiration with heavy metals under different land uses in terms of quality and sustainability of the soil. There is a statistically significant positive correlation between dehydrogenase enzyme activity and Mn, Pb, Cd, and Co, while it was negative between Cr. There was a positive correlation between catalase enzyme activity and Mn and Pb and between urease and Co. The higher interaction of dehydrogenase activity with heavy metals, which is included in the endo enzyme group, has been explained as a much stronger effect of heavy metals on living microorganisms and endoenzymes than extracellular enzymes stabilized on clay minerals and organic matter. The high clay content of the soil is thought to reduce some of the negative effects of heavy metals on enzymes. The results of this study may be good indicators of enzyme activities, especially dehydrogenase, catalase, and urease, for soil health and quality, chemical degradation and restoration processes, and ecosystem functioning in soils contaminated or to be contaminated with heavy metals. It shows that the activities of these enzymes are very sensitive and can decrease rapidly in case of high concentrations of heavy metals.

Application of biochar and compost improved soil properties and enhanced plant growth in a Pb-Zn mine tailings soil

This study evaluated the effect of biochar and compost on physiochemical properties, heavy metal content, microbial biomass, enzyme activities, and plant growth in Pb-Zn mine tailings. In this study, a pot experiment was conducted to evaluate the effects of biochar, compost, and their combination on the availability of heavy metals, physicochemical features, and enzyme activities in mining soil. Compared to separate addition, the combined application of biochar and compost was more effective to improve soil pH, soil organic carbon (SOC), total nitrogen (TN), available phosphorus (AP), and potassium (AK). All amendments significantly decreased CaCl₂-extractable Pb, Zn, Cu, and Cd. Soil enzyme activities were activated by biochar and compost. Meanwhile, the addition of biochar and compost decreased heavy metal content in plant tissues and increased plant biomass. Pearson's correlation analysis showed that plant biomass was positively correlated with nutrient levels, microbial biomass, and enzyme activities, whereas it was negatively correlated with CaCl₂-extractable heavy metals. These results enhance our understanding of the ecological functions of biochar and compost on the restoration of mining soil and reveal the potential benefit of organic amendments on the improvement of mining soil quality.

Stabilising fluoride in contaminated soils with monocalcium phosphate and municipal solid waste compost: microbial, biochemical and plant growth impact

This study evaluated the influence of a municipal solid waste compost (MSWC) and monocalcium phosphate (MCP), alone or combined, on the mobility, toxicity, bioavailability and health risk of fluoride (1000 mg F^-·kg^-1) in an artificially polluted soil (pH 7.85). The addition of MCP (0.2% w/w) and MSWC (1% w/w) (alone and combined) to the contaminated soil reduced water-soluble (e.g. by more than 50% in MCP and MCP + MSWC-treated soils) and exchangeable F^- fractions and increased the residual one. The addition of MSWC and MSWC + MCP to the contaminated soil significantly increased microbial biomass C (SMB-C; 1.3-3.6-fold) while all treatments increased the abundance of culturable heterotrophic bacteria (up to twofold in MSWC + MCP). Overall, dehydrogenase, β-glucosidase, urease and phosphomonoesterase activities were enhanced in treated soils and positively correlated with SMB-C, but not with labile F^-. All treatments increased carrot yield (up to 3.4-fold in MSWC + MCP), while bean growth was significantly enhanced only by MCP and MCP + MSWC (~ twofold). The opposite trend applied for F^- uptake which was especially reduced in the edible part of carrot after soil amendment. A limited influence of MCP and MSWC on hazard quotient (HQ), due to bean and carrot consumption, was also recorded (i.e. HQ generally > 1). Results suggest that MCP and MSWC can be used in the recovery of soil chemical, microbial and biochemical status of F-rich agricultural soils. They also indicate that the bean and carrot cultivars employed in this study are likely unsuitable in such soils due to high F^- uptake in edible parts.

Calorific Value of Festuca rubra Biomass in the Phytostabilization of Soil Contaminated with Nickel, Cobalt and Cadmium Which Disrupt the Microbiological and Biochemical Properties of Soil

The choice of optimal plant species for phytoremediation and organic fertilization plays an important role in stabilizing the functions of soils contaminated with heavy metals. The influence of nickel, cobalt and cadmium on the biomass yield and calorific value of Festuca rubra, heavy metal concentrations in soil and plants and the microbiological, biochemical and physicochemical proprieties of soil were analyzed in a pot experiment. The tolerance index (TI) describing Festuca rubra’s ability to tolerate heavy metals, as well as the translocation (TF), accumulation (AF) and bioaccumulation (BF) factors of heavy metals in Festuca rubra were calculated. The experiment was conducted in two series: In soil fertilized and not fertilized with compost. Nickel and cobalt significantly inhibited the growth and development of Festuca rubra. The experiment demonstrated that this plant species can be grown on soil contaminated with heavy metals. Festuca rubra contained on average 46.05% C, 34.59% O, 5.91% H, 3.49% N, 0.19% S and 9.76% ash. Festuca rubra has a stable calorific value which is not affected by heavy metals; therefore, biomass harvested from heavy metal-polluted soil can be used for energy generation. The calorific value of Festuca rubra ranged from 15.924 to 16.790 MJ kg−1 plant d.m., and the heat of combustion from 17.696 to 18.576 MJ kg−1. It has a stable calorific value which is not affected by heavy metals, therefore biomass harvested from heavy metal-polluted soil can be used for energy generation. Festuca rubra is particularly useful for the phytostabilization of soil contaminated with cadmium and cobalt. Compost minimizes the adverse effects of heavy metal pollution on the microbiological, biochemical and physicochemical properties of soil.

Impact of Eisenia fetida earthworms and biochar on potentially toxic element mobility and health of a contaminated soil

This study aimed to evaluate the influence of Eisenia fetida (Savigny), added to an acidic soil contaminated with potentially toxic elements (PTEs; As, Sb, Cd, Pb, Zn) and amended with a softwood-derived biochar (2 and 5% w/w), on the mobility of PTEs and soil health (i.e. nutrient availability, enzyme activity and soil basal respiration). The PTEs bioaccumulation by E. fetida and the acute ecotoxicity effects of the amended soils were also evaluated. The interaction between earthworms and biochar led to a significant increase in soil pH, organic matter, dissolved organic carbon content, cation exchange capacity, and exchangeable Ca compared to the untreated soil. Moreover, the water-soluble and readily exchangeable PTE fraction decreased (with the exception of Sb) between 1.2- and 3.0-fold in the presence of biochar and earthworms. Earthworms, biochar, and their combination, led to a reduction of phosphomonoesterase activity which in soils amended with biochar and earthworms decreased between 2.2- and 2.5-fold with respect to the untreated soil. On the other hand, biochar and earthworms also enhanced soil basal respiration and protease activity. Although the survival rate and the weight loss of E. fetida did not change significantly with the addition of 2% biochar, adding the highest biochar percentage (5%) resulted in a survival rate that was ~2-fold lower and a weight loss that was 2.5-fold higher than the other treatments. The PTE bioaccumulation factors for E. fetida, which were less than 1 for all elements (except Cd), followed the order Cd > As>Zn > Cu > Pb > Sb and were further decreased by biochar addition. Overall, these results highlight that E. fetida and biochar, especially at 2% rate, could be used for the restoration of soil functionality in PTE-polluted environments, reducing at the same time the environmental risks posed by PTEs, at least in the short time.

Remediation of Cd-, Pb-, Cu-, and Zn-contaminated soil using cow bone meal and oyster shell meal

To understand the environmental friendliness and high efficiency of organic materials during remediating soil polluted by heavy metals by assessing the feedback of soil ecosystems after organic materials were put into polluted soil. Incubation research was undertaken to examine the impact of amendments ranging from 0.1% to 3.0% (w/w), including single cow bone meal (BM), single oyster shell meal (OS), and a composite of 50% BM mixed with 50% OS (BO) on soil biochemical properties. The findings revealed that the implementation of BM and OS increased soil pH, the content of certain nutrients, and the activities of catalase (S-CAT), and urease (S-UE) while decreasing the availability of Cd, Pb, Cu, and Zn. Overall, the immobilization effect on Cd and Zn after a 108-day incubation was ranked as follows: BM group > OS group ≥ BO group, and the order of the immobilization effect of Pb and Cu was OS group > BO group > BM group. In addition, the dominant bacterial community flora shifted toward alleviating the re-dissolution of metal ions from the soil and promoting nutrient recycling in soil within 108 days of cultivation. RNA analyses showed that the strongest determinants for microbial communities between BM application and OS application at the genus level were soil pH, CEC, and heavy metal (Cd, Pb). These results increase our understanding of the leaching performance of Cd, Pb, Cu and Zn and the evolution trend of microorganisms when organic amendments remediate heavy metal contaminated soil.

The microbiology of rebuilding soils with water treatment residual co-amendments: Risks and benefits

Water treatment residual (WTR) is composed of sludges from the potable water treatment process, currently largely destined for landfill. This waste can be diverted to rebuild degraded soils, aligning with the UN's Sustainable Development Goals 12 (Consumption and Production) and 15 (Terrestrial Ecosystems). Biosolids are tested against stringent pathogen guidelines, yet few studies have explored the microbial risk of WTR land application, despite anthropogenic impacts on water treatment. We explored the microbial risks and benefits of amending nutrient-poor sandy soil with WTRs. Our results showed that the culturable pathogen load of wet and dry WTRs did not warrant pre-processing before land application, according to South African national quality guidelines, with fecal coliforms not exceeding 10⁴ colony forming units per gram dry weight in wet sludges sampled from four South African and Zimbabwean water treatment plants and decreasing upon drying and processing. There was no culturable pathogenic (fecal coliforms, enterococci, Salmonella, and Shigella) regrowth in soil incubations amended with dry WTR. However, the competition (microbial load and diversity) introduced by a WTR co-amendment did not limit pathogen survival in soils amended with biosolids. Application of WTR to nutrient-poor sandy soils for wheat (Triticum aestivum L.) growth improved the prokaryotic and eukaryotic culturable cell concentrations, similar to compost. However, the compost microbiome more significantly affected the bacterial beta diversity of the receiving soil than WTR when analyzed with automated ribosomal intergenic spacer analysis. Thus, although there was a low pathogen risk for WTR amendment in receiving soils and total soil microbial loads were increased, microbial diversity was more significantly enhanced by compost than WTR.

Effect of Municipal Solid Waste Compost on Antimony Mobility, Phytotoxicity and Bioavailability in Polluted Soils

The effect of a municipal solid waste compost (MSWC), added at 1 and 2% rates, on the mobility, phytotoxicity, and bioavailability of antimony (Sb) was investigated in two soils (SA: acidic soil; SB: alkaline soil), spiked with two Sb concentrations (100 and 1000 mg kg−1). The impact of MSWC on microbial activity and biochemical functioning within the Sb-polluted soils was also considered. MSWC addition reduced water-soluble Sb and favored an increase in residual Sb (e.g., by 1.45- and 1.14-fold in SA-100 and SA-1000 treated with 2% MSWC, respectively). Significant increases in dehydrogenase activity were recorded in both the amended soils, as well as a clear positive effect of MSWC on the metabolic activity and catabolic diversity of respective microbial communities. MSWC alleviated Sb phytotoxicity in triticale plants and decreased Sb uptake by roots. However, increased Sb translocation from roots to shoots was recorded in the amended soils, according to the compost rate. Overall, the results obtained indicated that MSWC, particularly at a 2% rate, can be used for the recovery of Sb-polluted soils. It also emerged that using MSWC in combination with triticale plants can be an option for the remediation of Sb-polluted soils, by means of assisted phytoextraction.

Processed animal manure improves morpho-physiological and biochemical characteristics of Brassica napus L. under nickel and salinity stress

Soil contamination with readily soluble salts and heavy metals is a major challenge concerning sustainable crop production. The use of organic wastes in agriculture not only helps in waste reduction but also acts as a soil conditioner and bio-stimulant for enhancing crop growth. In this regard, a pot experiment was conducted to investigate the effect of raw and processed animal manure (AM) on the growth, yield, and physicochemical parameters of Brassica napus L. developed under salinity and Ni stress. The experiment comprised two salinity levels (1.05 and 8 dS m^-1), two Ni levels (0 and 50 mg kg^-1), and two types of AMs (raw and processed at a rate of 2% w/w). A control treatment without AM incorporation was also included. In results, the application of AM markedly increased the growth and yield of B. napus under Ni and salinity stress; at the same time, it improved the physiological and chemical parameters of the said crop. Similarly, incorporation of processed AM significantly improved nutrient uptake and decreased Na/K ratios in the shoot and grain under the different stress conditions, as compared to the control. Likewise, Ni uptake in the grain, shoot, and root samples was also significantly reduced under the AM treatment. Also, the application of AM significantly reduced the daily intake of metal (DIM) index and the health risk index (HRI) values under the different stress conditions, as compared to the control. In conclusion, the application of processed AM constitutes an effective agricultural strategy to alleviate the adverse effects of Ni and salinity stress on growth, physiology, and yield of B. napus, thus resulting in enhanced productivity, as well as reduced risks associated with human health.

The effects of vermicompost and shell powder addition on Cd bioavailability, enzyme activity and bacterial community in Cd-contaminated soil: A field study

Cadmium (Cd) contamination has become serious in soil and in situ stabilization technology has been widely used for heavy metal remediation. A field study was conducted to determine the effect of amendments with the doses of 3 kg/m2, including single vermicompost (A1), a 95% vermicompost mixed with 5% shell powder composite (A2) and a 95% vermicompost mixed with 5% modified shell powder composite (A3), on the Cd bioavailability, enzyme activity and bacterial community in soil, and the experiment was conducted with lettuce (Lactuca sativa L.) grown in a Cd-contaminated farmland soil. The results showed that the application of amendments increased the pH, cation exchange capacity (CEC), organic matter (OM), available nutrients, catalase (S-CAT), invertase (S-SC) and urease (S-UE) activities in soil, while significantly reduced the Cd bioavailability with the lowest Cd bioavailability being observed in the soil with A3 application. The soil bacterial richness and diversity increased after amendments application, and the bacterial community was characterized by an increase in metal-tolerant bacteria but a decrease in Proteobacteria, Acidobacteria and Gemmatimonadetes. In addition, the application of amendments significantly improved the growth of lettuce (Lactuca sativa L.) and inhibited Cd accumulation in its edible parts, especially, the Cd content in lettuce (Lactuca sativa L.) grown in soil with A3 application was below the limit of the National Food Safety Standard of China (maximum level ≤ 0.2 mg/kg). Thus, composite amendment obtained from vermicompost mixed with modified shell powder can be used as potential remediation material in Cd-contaminated soil. CAPSULE: Composite amendment obtained from vermicompost and modified shell powder had good effects on remediation of Cd-contaminated soil.

Potential of a novel modified gangue amendment to reduce cadmium uptake in lettuce (Lactuca sativa L.)

In this study, the modified gangue (GE) was prepared by calcination at lower temperatures using potassium hydroxide (KOH) as the activating agent. The field emission scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), Fourier transform infrared spectroscopy (FTIR), and X-ray fluorescence (XRF) methods were employed to analyze the physicochemical characteristics of GE before and after the modification. Besides, the GE and commercial zeolite (ZE) were compared in the remediation of Cd-contaminated soil in field experiments. The results showed that both the GE and ZE had positive effects on the stabilization of Cd, decreasing the available Cd by 21.2-33.9% and 22.1-28.2%, respectively, while no significant difference was observed between the two amendments, indicating that the modification of GE was successful. Moreover, the application of GE decreased the Cd mobilization and uptake in lettuce shoot and root by 54.9-61.5% and 9.3-13.2%, respectively, and at the same time, the bio-available Cd decreased by 20.9-34.5%. Moreover, with the addition of GE, activities of urease and alkaline phosphatase increased in soil, while the peroxidase and superoxide dismutase activities were notably reduced in plants. Therefore, GE could be used as an effective amendment for the alleviation of Cd accumulation and toxicity, and thereby improve food safety.

Effect of biochar, iron sulfate and poultry manure application on the phytotoxicity of a former tin mine

In phytomanagement approach the application of a combination of amendments is an option for remediating arsenic polluted areas and valorized biomass obtained. Various amendments can be used. Biochar has been shown to reduce metal(loid) availability, and increase soil fertility, while iron sulfate has a considerable As binding capacity, and poultry manure is a source of nutrients. A phytotoxicity test was performed by applying the three amendments (2% biochar, 0.15%, 0.30% and 0.45% iron sulfate and 0.4% poultry manure) to a former tin mine technosol, to investigate their effects on (i) soil pore water properties, (ii) metal(loid) immobilization and (iii) Phaseolus vulgaris L. growth, used as a bioindicator. Biochar addition alone did not affect soil properties or plant parameters. However, the addition of iron sulfate acidified the soil, decreased soil pore water As concentrations, and increased the ones of Fe and Pb. It also improved plant growth, and reduced As and Pb aerial and root concentrations. Finally, the addition of poultry manure had no effect on soil and plants. Based on our results, the combination of iron sulfate with biochar may be a solution for reducing soil toxicity of the Abbaretz mining technosol, improving its fertility, and thus ameliorating plant growth.Novelty statement:The work presented in this manuscript describes the effect of amendment application, i.e., biochar, chicken dung and/or iron sulfate, on soil properties, metals availability and dwarf bean growth, plant used as bioindicator.Our results showed that the combination of a low amount of iron sulfate with biochar is the strategy to reduce soil toxicity, improved its fertility and consequently authorizes plant growth.This study is one of the first describing the effects of combined amendments on a mining soil properties with focusing on metal(loid) mobility.

Functionalization of ultrasound enhanced sewage sludge-derived biochar: Physicochemical improvement and its effects on soil enzyme activities and heavy metals availability

Poor physicochemical characteristics and high heavy metals content are main limitations of applying sludge-based biochars in remediation studies. The present study attempts to combine two practical approaches of ultrasound pre-treatment with low-time and low-frequency and chemical functionalization using citric acid. The aims of this study are enhancement physicochemical characteristics and environmental applicability of sludge-derived biochar. The characteristics of obtained ultrasound-treated functionalized biochar (UFB), sludge-derived biochar (SDB) and sewage sludge (SS) were evaluated. Then, the effects of these additives on soil heavy metals availability, soil enzyme activities and soil physicochemical characteristics were investigated during a 2-month stabilization process. The results indicated that ultrasound pre-treatment and functionalization considerably increased pore volume, surface area, and surface functional groups of the biochar, but significantly decreased total heavy metals concentration and metals ecological risk index (Er). The results of soil amending showed that application of UFB decreased Pb, Zn and Cd availability in soil by 85.3, 82.9 and 30.6%, respectively. In all cases, except for Cd, the Pb and Zn availability decreased by UFB was two times greater than the availability decreased by SDB and SS. Compared to SDB, the UFB potentially enhanced the positive effect of additive on soil enzyme activities. The obtained results revealed that the feasible, uncomplicated physical and chemical techniques can be used as a valuable approach for enhancing the environmental applicability of sludge-derived biochar and management of the excessively produced sewage sludge in the world.

Evaluation of Cynara cardunculus L. and municipal solid waste compost for aided phytoremediation of multi potentially toxic element-contaminated soils

The suitability for aided phytoremediation of Cynara cardunculus L. var. altilis and municipal solid waste compost (MSWC) applied at 2% and 4 % rates was evaluated in a multi potentially toxic element (PTE)-contaminated mining soil (Pb ~ 15,383 mg kg^-1, Zn ~ 4076 mg kg^-1, As ~ 49 mg kg^-1, Cd ~ 67 mg kg^-1, Cu ~ 181 mg kg^-1, and Sb ~ 109 mg kg^-1). The growth of C. cardunculus significantly increased with compost amendment and followed the order: MSWC-4% > MSWC-2% > Control. PTE concentrations in the roots of plants grown on amended soils decreased compared with control plants (i.e., less than ~ 82, 94, and 88% for Pb, Zn, and Cd respectively). PTE translocation from roots to shoots depended on both PTE and amendment rate but values were generally low (i.e., < 1). However, PTE mineralomasses were always higher for plants grown on MSWC-amended soils because of their higher biomass production, which favored an overall PTE bioaccumulation in roots and shoots. After plant growth, labile As and Sb increased in amended soils, while labile Pb, Zn, Cu, and Cd significantly decreased. Likewise, dehydrogenase and urease activities increased significantly in planted soils amended with MSWC. Also, the potential metabolic activity and the catabolic versatility of soil microbial communities significantly increased in planted soils amended with MSWC. Overall, our results indicate that C. cardunculus and MSWC can be effective resources for the aided phytoremediation of multi PTE-contaminated soils.

The effect of particle size of bamboo biochar on the phytoremediation of Salix psammophila C. to multi-metal polluted soil

Biochar shows great potential in soil remediation. The benefits of biochar on soil depend onits intrinsic properties and soil characteristics. However, the influence of particle sizes of biochar on soil remediation is not clear. In a pot experiment, we evaluated the effects of bamboo biochar (BBC) particle sizes (P1 < 0.15 mm, 0.15 mm < P2 < 0.25 mm, 0.25 mm < P3 < 0.50 mm) on phytoremediation efficiency of Salix psammophila C. cultivated in multi-metal polluted soil. We added the BBC at 3% (w/w) in tested soil. Next, the BBC was thoroughly mixed with soil and weighting to the pot, and S. psammophila cuttings were planted and grown for six months in the amended soil under model growth condition.Results revealed the addition of different sizes of BBC particles affected soil quality, plant growth, and HMs accumulation in plants. All sizes of BBC treatments improved Cd and Zn accumulation, whereas plants in P2 treatment showed the greatest accumulation, increased by 52.41 and 25.55% compared with the control (1,503 and 19,928 μg·plant^-1). Overall, the results indicated BBC enhanced the phytoremediation efficiency of S. psammophila. Plants cultivated in P2 treatment showed the most significant effect on remediating contaminated soil.

Effect of Biowastes on Soil Remediation, Plant Productivity and Soil Organic Carbon Sequestration: A Review

High anthropogenic activities are constantly causing increased soil degradation and thus soil health and safety are becoming an important issue. The soil quality is deteriorating at an alarming rate in the neighborhood of smelters as a result of heavy metal deposition. Organic biowastes, also produced through anthropogenic activities, provide some solutions for remediation and management of degraded soils through their use as a substrate. Biowastes, due to their high content of organic compounds, have the potential to improve soil quality, plant productivity, and microbial activity contributing to higher humus production. Biowaste use also leads to the immobilization and stabilization of heavy metals, carbon sequestration, and release of macro and micronutrients. Increased carbon sequestration through biowaste use helps us in mitigating climate change and global warming. Soil amendment by biowaste increases soil activity and plant productivity caused by stimulation in shoot and root length, biomass production, grain yield, chlorophyll content, and decrease in oxidative stress. However, biowaste application to soils is a debatable issue due to their possible negative effect of high heavy metal concentration and risks of their accumulation in soils. Therefore, regulations for the use of biowastes as fertilizer or soil amendment must be improved and strictly employed to avoid environmental risks and the entry of potentially toxic elements into the food chain. In this review, we summarize the current knowledge on the effects of biowastes on soil remediation, plant productivity, and soil organic carbon sequestration.

Addition of softwood biochar to contaminated soils decreases the mobility, leachability and bioaccesibility of potentially toxic elements

Softwood-derived biochar (5% w/w) was added to two mining soils (S1 and S2) contaminated with Cd (4.8-74 mg kg^-1), Pb (318-1899 mg kg^-1) and Zn (622-3803 mg kg^-1), to evaluate its immobilization capabilities towards such potentially toxic elements (PTEs). Biochar addition (S + B) increased soil pH, organic carbon content, extractable phosphorous and calcium. Sequential extractions showed that biochar reduced the labile pools of PTEs (e.g. -29, 55 and 79% of water-soluble and exchangeable Cd, Zn and Pb respectively in S1 + B compared to S1) and at the same time increased their most stable and less mobile fractions. Leaching experiments revealed a significant decrease of DOC, N-NO₃^-, P and PTEs in biochar-treated soils, and an increase of leached K. Kinetic equations derived from leaching data showed that PTEs in control soils were quickly mobilized, while those in biochar-treated soils needed longer time to leachate. In vitro tests showed that biochar was effective at reducing the bioaccessibility of Cd and Pb in the gastric phase of S2 and that of Zn and Pb in the intestinal phase of S1. The results obtained showed that biochar could be used as alternative amendment for the recovery of PTEs-contaminated soils.

Mitigation in availability and toxicity of multi-metal contaminated soil by combining soil washing and organic amendments stabilization

In order to investigate the decrease in total metal contents and to mitigate the availability and toxicity of metals from farmland near a lead mining area, a combination of two effective soil washing and eco-friendly stabilization technologies was applied in current research. The pre-treatment was performed with three types of agents including Ethylenediaminetetraacetic acid (EDTA), citric acid (CA), and mixture of hydroxylamine hydrochloride and citric acid (HA)) and the post-treatment stabilization was adopted using four rich-carbon organic waste amendments (cow manure compost (CMC), vermicompost (VC), urban sewage sludge (SS), and sludge-derived biochar (BIO)). Furthermore, the fate of residual metals (leachability, plant-availability, bioaccessibility, and chemical distribution), soil quality indicators (phytotoxicity and enzyme activities), and some soil physicochemical properties were examined before and after the two-steps remediation. The soil washing, especially using HA and CA agents, dramatically increased the labile metals and negatively changed the soil microbial activity. The two-month stabilization with SS, BIO, and VC resulted in a significant control of the leachability and plant-availability of residual Zn and Pb. However, the post-treatment was only slightly immobilized of Cd. The amendments affected the restoration of soil pH and organic carbon as well as the improvement of available nutrients. Compared to the other amendments that caused restrictions, the SS significantly restored the enzyme activities. With the exception of CMC, the SS, VC, and BIO, indicated higher germination rate and growth of wheat were also obtained. This study reveal the ability of the complementary role of stabilization with soil washing to reduce metal toxicity and confirm the usefulness of municipal and animal wastes in enhancing soil and environmental qualities.

Mobility and potential bioavailability of antimony in contaminated soils: Short-term impact on microbial community and soil biochemical functioning

Antimony (Sb) and its compounds are emerging priority pollutants which pose a serious threat to the environment. The aim of this study was to evaluate the short-term fate of antimonate added to different soils (S1 and S2) with respect to its mobility and impact on soil microbial communities and soil biochemical functioning. To this end, S1 (sandy clay loam, pH 8.2) and S2 (loamy coarse sand, pH 4.9) soils were spiked with 100 and 1000 mg Sb(V) kg^-1 soil and left in contact for three months. Sequential extractions carried out after this contact time indicated a higher percentage of labile antimony in the Sb-spiked S1 soils than S2 (e.g. ~13 and 4% in S1 and S2 treated with 1000 mg Sb(V) kg^-1 respectively), while the opposite was found for residual (hardly bioavailable) Sb. Also, a reduced number of culturable heterotrophic bacteria was recorded in Sb-spiked S1 soil (compared to the unpolluted S1), while an increased one was found in S2. Heterotrophic fungi followed the opposite trend. Actinomycetes and heat-resistant aerobic bacterial spores showed a variable trend depending on the soil type and Sb(V) treatment. The Biolog community level physiological profile indicated a reduced metabolic activity potential of microbial communities from the Sb-spiked S1 soils (e.g. <50% for Sb-1000 compared to the unpolluted S1), while an increase was recorded for those extracted from the Sb-spiked S2 soils (e.g. >2-fold for Sb-1000). The soil dehydrogenase activity followed the same trend. High-throughput 16S rRNA amplicon sequencing analysis revealed that Sb did not influence the bacterial α-diversity in both soils, while significantly affected the composition of the respective soil bacterial communities. Several phyla (e.g. Nitrosospira Nitrososphaeraceae, Adheribacter) were found positively correlated with the concentration of water-soluble Sb in soil. Overall, the results obtained suggest that the risk assessment in soils polluted with antimony should be a priority especially for alkaline soils where the high mobility of the anionic Sb(OH)₆- species can pose, at least in the short-term, a serious threat for soil microbial abundance, diversity and functionality, soil fertility and eventually human health.

Biochar and compost as gentle remediation options for the recovery of trace elements-contaminated soils

The use of organic-based amendments for gentle remediation options (GRO), i.e. the stabilization of trace elements (TE) in polluted soils and the reduction of their impact on soil microbial and biochemical features, has been constantly growing in last 10 years. To verify the effectiveness of biochar and compost in such context, biochar (1 and 3% w/w), compost (3% w/w) and their combination (compost 2% + biochar 2% w/w) were added to two sub-alkaline soils (FS and MS) contaminated with Sb (41-99 mg kg^-1 respectively), As (~18 mg kg^-1), and trace metals such as Ni (103-172 mg kg^-1 respectively) and Cr (165-132 mg kg^-1 respectively). Most of the treatments (especially 3% biochar) reduced labile TE pools (water-soluble and exchangeable) and increased their residual (non-extractable) fractions (e.g. +48, 56, 66, and 68% of residual Sb, As, Cr and Ni in MS-treated soil compared to the untreated control). The amendments addition had both stimulating and inhibiting effects on the activity of soil microbial communities, as shown by the Biolog community level physiological profiles. However, in both soils, 3% biochar produced the highest increase of metabolic potential as well as the use of carboxylic acids and polymers by the soil microbial communities. Likewise, soil dehydrogenase (DHG), β-glucosidase (β-GLU) and urease (URE) activities were significantly enhanced in FS and MS soils treated with 3% biochar (e.g. +77, 48, and 17% for DHG, URE and β-GLU in FS-3% biochar with respect to untreated FS). Overall, the results from this study showed that the amendments investigated (particularly 3% biochar) can be effectively used for GRO of sub-alkaline soils, being able to reduce labile TE and to increase the metabolic potential and actual biochemical activities of the respective soil microbial communities. The manifold environmental implications of such effects are discussed.

Aided phytostabilisation over two years using iron sulphate and organic amendments: Effects on soil quality and rye production

An outdoor macrocosm experiment using Fe-based and organic amendments over 2 years was set up to evaluate the effectiveness of aided-phytostabilisation. For that, a soil contaminated with As- and Cu-rich waste material (∼13000 mg As kg^-1 and ∼500 mg Cu kg^-1) was treated with combinations of iron sulphate (Fe) with lime, paper mill sludge (PS), holm-oak biochar (BC), olive mill waste compost (OMWC) or green waste compost (GWC). Rye (Secale cereale L.) was grown in the treated and non-treated soils 16 months after addition of the amendments. Arsenic and Cu dynamics in soil were assessed throughout the experiment and soil quality parameters (soil nutrients, organic matter and soil biology) were measured almost two years after addition of the amendments. All treatments resulted in a reduction of soluble and extractable Cu during the experiment and, despite the increase in soil pH (from 5 to 68) and DOC (from 10 up to 50 mg DOC L^-1) provoked by the amendments, As was not significantly mobilised in the treated soils. Treatments combining Fe sulphate with the organic materials, especially biochar and both composts, resulted in an increase in soil available nutrients and enhanced rye growth. In this semi-field scale experiment, the combination of Fe sulphate with holm-oak biochar showed the most promising results in terms of soil fertility (nutrient availability), plant As and Cu uptake and soil C sequestration. Further research should focus on monitoring long-term effects of the soil amendments on crops, following repeated applications.

Physicochemical features, metal availability and enzyme activity in heavy metal-polluted soil remediated by biochar and compost

Biochar and compost have been widely used for pollution remediation of heavy metals in soil. However, little research was conducted to explore the efficiency of biochar, compost and their combination to reduce heavy metals availability, and the effects of their additive on soil biological properties are often neglected. Therefore, this study investigated the effects of biochar, compost and their combination on availability of heavy metals, physicochemical features and enzyme activities in soil. Results showed that adding amendments to polluted soil significantly altered soil properties. Compared to the separate addition of biochar or compost, their combined application was more effective to improve soil pH, organic matter (OM), organic carbon (TOC) and available potassium (AK). All amendments significantly decreased the availability of Cd and Zn, but slightly activated As and Cu. In addition, soil enzyme activities were activated by compost and inhibited by biochar, but exhibited highly variable responses to their combinations. Pearson correlation analysis indicated that electrical conductivity (EC) and AK were the most important environmental factors affecting metal availability and soil enzyme activities including dehydrogenase, catalase, β-glucosidase, urease, acid and alkaline phosphatase, arylsulfatase except for protease and invertase. Availability of As, Cu, Cd and Zn affected dehydrogenase, catalase and urease activities. These results indicated that biochar, compost and their combination have significant effects on physicochemical features, metals availability and enzyme activities in heavy metal-polluted soil.

Mobility, bioaccessibility and toxicity of potentially toxic elements in a contaminated soil treated with municipal solid waste compost

The aim of this study was to assess the influence of a municipal solid waste compost (MSWC) on the mobility, bioaccessibility and toxicity of several potentially toxic elements (PTE), i.e. Pb (15,383 mg kg^-1), Zn (4076 mg kg^-1), Cu (181 mg kg^-1), Sb (109 mg kg^-1), Cd (67 mg kg^-1) and As (49 mg kg^-1), present in a contaminated sub-acidic soil (pH = 5.93). The addition of MSWC at 2 and 4% rates significantly decreased the labile fractions of PTE (with the exception of Cu and As) and at the same time increased the residual fractions of Zn and Sb. In-vitro tests also showed that compost amendment was able to decrease Cd and Cu gastric bioaccessibility, with respect to untreated soil (-19 and 13% of Cd and Cu in MSWC-4% respectively), while a significant increase of As intestinal bioaccessibility was recorded. This increment was attributed to the pH rise (up to 7.0) during the in-vitro intestinal phase, which likely favoured a release of the arsenic non-specifically bonded to MSWC. Soil enzyme activities, i.e. dehydrogenase and β-glucosidase, were significantly enhanced in MSWC-amended soils (i.e. up to ~6.0 and 1.4 times higher in MSWC-4% than in control soil, respectively), as well as soil basal respiration, and the potential metabolic activity and catabolic versatility of soil microbial communities (as assessed by the Biolog ecoplate community level physiological profile). Overall, the results obtained suggested that MSWC, particularly at 4% rate, could be useful to stabilise PTE in sub-acidic contaminated soils and to increase the microbial activity and functionality in these latter soils.