Phytoavailability and uptake of arsenic in ryegrass affected by various amendments in soil of an abandoned gold mining site

Phytoremediation strategies for the reclamation of tailings and mining soils in an active open-pit site

This study evaluated two phytoremediation strategies at an active open-pit mining site within a dunite deposit, an ultramafic igneous rock rich in nickel. The strategies included phytostabilization for tailings, which are fine fractions resulting from rock processing, and phytoextraction for mining soil, composed of natural soil aggregates mixed with fine dunite fragments. Tailings had an alkaline pH (8.8), low organic matter content (9.4 g kg-1), and high available Ni concentrations, requiring immobilization using compost, vermicompost, biochar, and vermichar. These amendments were tested in 1-kg pots over 180 days, assessing soil properties, plant growth, and Ni accumulation in Lolium perenne L. In contrast, mining soil showing a more neutral pH (7.6), higher organic matter (106 g kg-1), and also high available Ni contents, underwent phytoextraction supported by EDTA and citric acid in a parallel experiment. Phytostabilization with compost, vermicompost, and vermichar significantly reduced available Ni (<5 mg kg-1), improved plant growth, and increased labile carbon, whereas biochar was less effective. Vermicompost proved particularly beneficial, enhancing nutrient and carbon availability, minimizing Ni leaching, and preventing excessive accumulation in plant aerial parts. Meanwhile, phytoextraction was successfully implemented without altering soil properties, with EDTA boosting Ni accumulation in aerial biomass up to 400 mg kg-1, outperforming citric acid. These findings highlight the adaptability of phytoremediation, demonstrating the successful application of both strategies in the same environment, with Lolium perenne L. functioning either as a metal accumulator or a phytostabilizing plant depending on the amendments used.

The roles of organic amendments and plant treatments in soil polychlorinated biphenyl dissipation under oxic and sequential anoxic-oxic conditions

Understanding polychlorinated biphenyl (PCB) degradation in sequential anaerobic-aerobic remediation is crucial for effective remediation strategies. In this study, microcosm and greenhouse experiments were conducted to dissect the effects of organic amendments (carbon-based) and plant treatments (ryegrass) on soil PCB dissipation under oxic and sequential anoxic-oxic conditions. We analyzed the soil bacterial community in greenhouse experiments using high-throughput sequencing to explore plant-pollutant-microbe interactions. Microcosm results showed that organic amendments alone did not facilitate aerobic PCB removal, but significantly accelerated PCB dechlorination under anoxic conditions altering the profiles of PCB congeners. In standard greenhouses, plant treatments substantially increased PCB dissipation to 50.8 ± 3.9%, while organic amendments aided phytoremediation by promoting plant growth, increasing PCB removal to 65.9 ± 3.2%. In sequential anaerobic-aerobic greenhouses, plant growth was inhibited by flooding treatment while flooding stress was markedly alleviated by organic amendments. Plant treatments alone during sequential treatments did not lead to PCB dissipation; however, dissipation was significantly promoted following organic amendments, achieving a removal of 41.2 ± 5.7%. This PCB removal was primarily due to anaerobic dechlorination during flooding (27.8 ± 0.5% removal), rather than from plant growth stimulation in subsequent planting phase. Co-occurrence network and functional prediction analyses revealed that organic amendments recruited specific bacterial clusters with distinct functions under different conditions, especially stimulating plant-microbe interactions and xenobiotics biodegradation pathways in planted systems. The findings provide valuable guidance for the design of practical remediation strategies under various remedy scenarios, such as in arable or paddy fields.

Soil amendment-assisted phytoremediation with ryegrass offers a promising approach to mitigate environmental health concerns

This study aimed to examine the potential of soil amendment-assisted phytoremediation using ryegrass in reclaiming abandoned gold mine soil in southwestern Ghana, with a specific focus on the soil contamination hazards associated with metals and metalloids. A pot experiment lasting 60 days was carried out to assess the efficacy of soil amendments, such as compost, iron oxide, and poultry manure, in mitigating environmental hazards. Three soil contamination indices (soil contamination = CF, enrichment factor = ER, and pollution load index = PLI) were used to calculate the extent of soil contamination, enrichment, and pollution of the sites with Co, Hg, Ni, Mo, Se, Sb, and Pb. The findings show that Hg made the greatest contribution (with a maximum soil CF of 18.0) to the overall PLI, with a maximum value of 74.4. The sites were averagely and consequently enriched with toxic elements in the decreasing order: Ni (ER = 33.3) > Mo (20.5) > Sb (14.1) > Pb (11.0) > Hg (7.9) > Se (2.1). The bioaccumulation factor (BCF > 1) suggests that ryegrass has the ability to phytostabilize Co, Hg, Mo, and Ni. This means that the plant may store these elements in its roots, potentially decreasing their negative effects on the environment and human health. Ultimately, the addition of combined manure with iron oxides might have augmented the sequestration of these metals in the root. The elements may have accumulated through sorption on manure or Fe surfaces, dissolution from watering the plants in the pot, or mineralization of organic manure. Thus, ryegrass has shown potential for phytostabilisation of Co, Hg, Mo, and Ni when assisted with a combination of manure and iron oxides; and can consequently mitigate the environmental and human health impacts.

Arsenic-contaminated soil remediation with hyperthermophilic compost: Effects on arsenic bioavailability, soil fertility and bacterial community

Soil arsenic (As) contamination has posed a significant global environmental challenge seriously threatening human health. Compost has attracted broad interests as a kind of eco-friendly and versatile amendment. However, hyperthermophilic compost (HTC), which is newly-developed and more advantageous to environment, has not yet been widely utilized to remediate As-contaminated soil, and its effectiveness remains unclear. Herein, the effects of HTC amendment on soil fertility, As bioavailability, plant growth and soil bacterial community were investigated. After amended with HTC, soil nutrient content and enzyme activity were improved. Concurrently, the content of both total As and available As in soil was reduced, partially due to the formation of organo-As complex with the presence of humic acid and fulvic acid in HTC. Notably, Chinese white cabbage (Brassica campestris L. ssp. chinensis Makino) cultivated in HTC-treated soil exhibited better growth and less As uptake, but showed enhanced translocation of As from the below-ground part to the above-ground part. In particular, the lowest HTC addition ratio (HTC:Soil = 1:10, v:v) proved to be the most optimal, increasing the height, width and biomass of Chinese white cabbage from 9.92 ± 0.72 cm, 6.76 ± 0.31 cm and 4.43 ± 0.49 g, to 21.29 ± 0.48 cm, 19.3 ± 1.44 cm and 23.27 ± 2.45 g, respectively. The results of soil bacterial community analysis revealed that HTC amendment stimulated the growth and metabolism of soil microbes, augmenting the richness and diversity of bacteria related to the methylation and volatilization of As and plant growth. This work suggests that HTC can serve as an effective amendment for As-contaminated soil remediation, and a superior alternative to compound fertilizer for plant cultivation, displaying promising potential for agricultural applications.

Effects and mechanisms of microbial ecology and diversity on phytoremediation of cadmium-contaminated soil under the influence of biodegradable organic acids

In recent years, heavy metal pollution has become a global environmental problem and poses a great threat to the health of people and ecosystems. Therefore, strategies for the effective remediation of Cd from contaminated soil are urgently needed. In this study, ryegrass was utilized as a remediation plant, and its remediation potential was enhanced through the application of Citric Acid (CA) in conjunction with Bacillus megaterium (B. megaterium). The P3 treatment (CA + Bacillus megaterium) exhibited a significantly higher efficiency in promoting cadmium extraction by ryegrass, resulting in a 1.79-fold increase in shoot cadmium accumulation compared to the control group (CK) with no Bacillus megaterium or CA. Moreover, the P3 treatment led to an increased abundance of Actinobacteriota, Acidobacteriota, and Patescibacteria in the rhizosphere. The concentration of amino derivatives (such as betaine, sulfolithocholylglycine, N-alpha-acetyl-lysine, glycocholic acid, arginyl-threonine) showed significant upregulation following the P3 treatment. In summary, this study proposes a viable approach for phytoremediation of soil contaminated with cadmium by harnessing the mobilizing abilities of soil bacteria.

Design and preparation of reticular superabsorbent hydrogel material with nutrient slow-release and high shear strength for ecological remediation of abandoned mines with steep slopes

In order to solve ecological remediation issues for abandoned mines with steep slopes, a kind of hydrogels with high cohesion and water-retaining were designed by inorganic mineral skeleton combining with polymeric organic network cavities. This eco-friendly hydrogel (MFA/HA-g-p(AA-co-AM)) was prepared with acrylic acid (AA)-acrylamide (AM) as network, which was grafted with humic acids (HA) as network binding point reinforcement skeleton and polar functional group donors, KOH-modified fly ash (MFA) as internal supporter. The maximum water absorption capacities were 1960 g/g for distilled water, which followed the pseudo-second-order model. This super water absorption was attributed to the first stage of 62 % fast absorption due to the high specific surface area, pore volume and low osmotic pressure, moreover, the multiple hydrophilic functional groups and network structure swell contributed to 36 % of the second stage slow adsorption. In addition, the pore filling of water in mesoporous channels contributed the additional 2 % water retention on the third stage. The high saline-alkali resistance correlated with the electrostatic attraction with MFA and multiple interactions with oxygen-containing functional groups in organic components. MFA and HA also enhanced the shear strength and fertility retention properties. After 5 cycles of natural dehydration and reabsorption process, these excellent characteristics of reusability and water absorption capacity kept above 97 %. The application of 0.6 wt% MFA/HA-g-p(AA-co-AM) at 15° slope could improve the growth of ryegrass by approximately 45 %. This study provides an efficient and economic superabsorbent material for ecological restoration of abandoned mines with steep slopes.

Efficient stabilization of arsenic migration and conversion in soil with surfactant-modified iron-manganese oxide: Environmental effects and mechanistic insights

The use of iron-manganese oxide (FMO) as a promising amendment for remediating arsenic (As) contamination in soils has gained attention, but its application is limited owing to agglomeration issues. This study aims to address agglomeration using surfactant-modified FMO and investigate their stabilization behavior towards As and resulting environmental changes upon amendments. The results confirmed the efficacy of surfactants and demonstrated that cetyltrimethylammonium-bromide-modified FMO significantly reduced the leaching concentration of As by 92.5 % and effectively suppressed the uptake of As by 85.8 % compared with the control groups. The ratio of the residual fraction increased from 30.5-41.6 % in unamended soil to 67.9-69.2 %. The number of active sites was through the introduction of surfactants and immobilized As via complexation, ion exchange, and redox reactions. The study also revealed that amendments and the concentration of As influenced the soil physicochemical properties and enriched bacteria associated with As and Fe reduction and changed the distribution of C, N, Fe, and As metabolism genes, which promoted the stabilization of As. The interactions among cetyltrimethylammonium bromide, FMO, and microorganisms were found to have the greatest effect on As immobilization.

Influence of biochar on the arsenic phytoextraction potential of Pteris vittata in soils from an abandoned arsenic mining site

Biochar (BC) has a strong potential for activating arsenic (As) in soil; thus, the phytoremediation efficiency of As-polluted soils is enhanced with Pteris vittata L. A pot experiment was conducted to investigate the potential of BC to assist in phytoremediation with P. vittata. The effects of BC on physicochemical properties, available As, enzyme activities, and the bacterial community in the rhizosphere soil were investigated, and the biomass, physiology, and As uptake of P. vittata were analyzed. The results indicated that applying BC facilitated available As in the P. vittata rhizosphere soil, and the phytoremediation efficiency percentage increased in the As-polluted soils, such as 3.80% and 8.01% under the 2% and 5% BC treatments compared to the control, respectively. Phytoremediation with P. vittata and BC significantly improved soil organic matter content, available N, P, and K, enzyme activities, and the bacterial community. BC promoted Streptomyces (26.6-54.2%) and Sphingomonas (12.3-30.8%) abundance which regulated the growth and As uptake by P. vittata. Moreover, applying BC increased the biomass, and As uptake by P. vittata. Overall, BC strengthened the phytoremediation of As-polluted soils by improving soil pH, nutrient concentrations, enzyme activities, bacterial community structure, and soil arsenic activation, growth, and absorption by P. vittata.

Characteristics of Soil Arsenic Contamination and the Potential of Pioneer Plants for Arsenic Remediation in Gold Mine Tailings

Arsenic (As) contamination of gold mine tailings poses major threats to the natural environment and human health, necessitating adequate management measures. To investigate the soil As contamination level and the potential of pioneer plants for As remediation, the soil and plants of an abandoned gold mine tailings in the Qinling Mountains were analyzed. The level of As contamination was assessed using the single-factor pollution index and potential ecological risk index, and its bioeffectiveness was analyzed. The enrichment capability of plants was investigated using the bioaccumulation factor and translocation factor. Redundancy analysis and partial least squares regression were employed to investigate factors affecting the distribution of As in soil and plants. The results show that As in soil mainly existed in the difficult-available state, with serious contamination and extremely high ecological risk. Lythrum salicaria L. and Equisetum ramosissimum Desf. are the preferred plants for remediation of As contamination through screening pioneer plants. Soil total nitrogen (STN) and available phosphorus (SAP) are the main factors influencing the characteristics of As distribution in the soil. Soil available potassium (SAK), water content (SWC), and SAP promote the accumulation of As by plants. This study provides plant materials and new ideas for mine ecological remediation.

Immobilization effects of co-pyrolyzed neem seed mixed with poultry manure on potentially toxic elements in soil and the phytoremediation potentials of native Manihot esculenta and Jatropha curcas in ensuring sustainable land use

This study evaluated the effects of neem seed biochar, poultry manure, and their combinations at varying rates of 15 and 25% (w/w) on potentially toxic elements (PTEs) in soils. Afterward, the suitability of Manihot esculenta and Jatropha curcas in removing Cd, As, Zn, Pb, and Hg from mine spoils were appraised in a 270-day outdoor pot experiment. Using ICP-Mass Spectrometry, the elemental contents of target PTE in the shoot, root, and soil specimens were determined for each treatment. The obtained average values were further subjected to a nonparametric test of samples using IBM SPSS Statistic 29. The applied organic amendments resulted in significant differences p < 0.05 in PTE availability for plant uptake after the Independent-Samples Kruskal-Wallis Test was made. Nonetheless, applying a 25% (w/w) mixture of neem seed biochar and poultry manure was efficient in immobilizing more PTEs in soils which caused lower PTEs presence in plants. Organic amendments further significantly enhanced the fertility of the mine soils leading to about a 6- 25.00% increase in the biomass yield (p < 0.05) of both plants. No significant difference (p > 0.05) was however observed between the phytoremediation potentials of both plants after the Independent-Sample Mann-Whitney U test. Even that, Manihot esculenta was averagely more efficient in PTE uptake than Jatropha curcas. Larger portions of the bioaccumulated PTEs were stored in the roots of both plants leading to high bioconcentration factors of 1.94- 2.47 mg/kg and 1.27- 4.70 mg/kg, respectively, for Jatropha curcas and Manihot esculenta. A transfer factor < 1 was achieved for all PTEs uptake by both plants and indicated their suitability for phytostabilization. Techniques for easy cultivation of root-storing PTEs are required to enhance their large-scale use as their biomass could further be used in clean energy production.

Organic amendments for in situ immobilization of heavy metals in soil: A review

There is a growing need for soil remediation due to the increase in heavy metals (HMs) migrating into the soil environment, especially those from man-made sources dominated by industry and agriculture. In situ immobilization technology, because of its lower life cycle environmental footprint, can achieve "green and sustainable remediation" of soil heavy-metal pollution. Among the various in situ immobilization remediation agents, organic amendments (OAs) stand out as they can act as soil conditioners while acting as HMs immobilization agents, and therefore have excellent application prospects. In this paper, the types and remediation effects of OAs for HMs in situ immobilization in soil are summarized. OAs have an important effect on the soil environment and other active substances in soil while interacting with HMs in soil. Based on these factors, the principle and mechanism of HMs in situ immobilization in soil using OAs are summarized. Given the complex differential characteristics of soil itself, it is impossible to determine whether it can remain stable after heavy-metal remediation; therefore, there is still a gap in knowledge regarding the compatibility and long-term effectiveness of OAs with soil. In the future, it is necessary to develop a reasonable HMs contamination remediation program for in situ immobilization and long-term monitoring through interdisciplinary integration techniques. These findings are expected to provide a reference for the development of advanced OAs and their applications in engineering.

Biogeochemical behavior and pollution control of arsenic in mining areas: A review

Arsenic (As) is one of the most toxic metalloids that possess many forms. As is constantly migrating from abandoned mining area to the surrounding environment in both oxidation and reducing conditions, threatening human health and ecological safety. The biogeochemical reaction of As included oxidation, reduction, methylation, and demethylation, which is closely associated with microbial metabolisms. The study of the geochemical behavior of arsenic in mining areas and the microbial remediation of arsenic pollution have great potential and are hot spots for the prevention and remediation of arsenic pollution. In this study, we review the distribution and migration of arsenic in the mining area, focus on the geochemical cycle of arsenic under the action of microorganisms, and summarize the factors influencing the biogeochemical cycle of arsenic, and strategies for arsenic pollution in mining areas are also discussed. Finally, the problems of the risk control strategies and the future development direction are prospected.

Luffa cylindrica Intercropping with Semen cassiae-A Production Practice of Improving Land Use in Soil Contaminated with Arsenic

In recent years, research on the safe utilization and green remediation of contaminated soil by intercropping has become common. In this study, the growth of an intercropping system of Luffa cylindrica-Semen cassiae in soil contaminated with medium amounts of arsenic (As) was studied using field (91.60 mg kg^-1) and pot (83.34 mg kg^-1) experiments. The field experiments showed that intercropping significantly increased the yield per plant of L. cylindrica by 27.36%, while the yield per plant of S. cassiae decreased by 21.66%; however, this difference was not significant. Intercropping reduced the concentration of As in all organs of L. cylindrica but increased the concentration of As in all parts of S. cassiae. The accumulation of As per plant of L. cylindrica was reduced by 20.72%, while that in a single plant of S. cassiae was increased by 201.93%. In addition, the concentration of As in the fruit of these two crops in these two planting modes was low enough to meet the National Food Safety Standard of China (GB2762-2017). In addition, the land equivalent ratio and As metal removal equivalent ratio of the intercropping mode was 1.03 and 2.34, indicating that the intercropping mode had advantages in land use and As removal. In the pot experiment, the biomass and As concentration of L. cylindrica and S. cassiae were roughly consistent with those in the field experiment. During the sampling period, intercropping reduced the concentration of As in the rhizosphere soil solution of L. cylindrica by 3.1-23.77%, while it increased the concentration of As in the rhizosphere soil solution of S. cassiae by 13.30-59.40%. The changes in pH and redox potential were also closely related to the content of water-soluble As in the rhizosphere environment, which affects the absorption of As by plants. In general, the L. cylindrica-S. cassiae intercropping system is a planting mode that can effectively treat soil that is moderately contaminated with As and remove it from the soil to an extent.