The influence of humic acids on the weathering of pyrite: Electrochemical mechanism and environmental implications

A novelty strategy for AMD prevention by biogas slurry: Acetate acid inhibition effect on chalcopyrite biooxidation and leachate

With the widespread application of anaerobic digestion technology, biogas slurry become the main source of organic amendments in practice. Comprehensive studies into the inhibitory effects of low molecular weight (LMW) organic acids, essential components in biogas slurry, on the sulfide minerals biooxidation and its bioleaching (AMD) have been lacking. In this study, acetic acid (AA) served as a representative of LMW organic acids in biogas slurry to investigate its impact on the inhibition of chalcopyrite biooxidation by Acidithiobacillus ferrooxidans (A. ferrooxidans). It was shown that AA could slow down the chalcopyrite biooxidation and inhibit the jarosite formation on the mineral surface. Compared with the control group (0 ppm AA), the sulfate increment in the leachate of the 50 ppm, 100 ppm, and 200 ppm AA-treated groups decreased by 36.4%, 66.8%, and 69.0%, respectively. AA treatment (≥50 ppm) could reduce the oxidation of ferrous ions in the leachate by one order of magnitude. At the same time, the bacterial concentration of the leachate in the 50 ppm, 100 ppm, and 200 ppm AA-treated groups decreased by 70%, 93%, and 94%, respectively. These findings provide a scientific basis for new strategies to utilize biogas slurry for mine remediation and contribute to an enhanced comprehension of organic amendments to prevent AMD in situ in mining soil remediation.

Inhibition photooxidation of pyrite under illumination via altering photogenerated carrier migration pathways: Role of DTC-TETA surface passivation

The oxidation of pyrite is the main cause of acidic mine drainage (AMD), which is a very serious environmental problem in numerous mining areas around the world. Previous studies have shown that passivation agents create a hydrophobic film on the surface of pyrite, effectively isolating oxygen and water. However, the presence of abundant sulfide minerals in tailings ponds may exacerbate AMD when exposed to solar radiation, due to the semiconductor properties of pyrite. It remains uncertain whether the current surface passivation coating can effectively prevent the oxidation of pyrite under light conditions. This paper is the first to investigate the passivation effect as well as the mechanism of surface passivation coating on pyrite under illumination from the perspective of materials science. The results demonstrated that the triethylenetetramine-bisdithiocarbamate (DTC-TETA) passivation coating on pyrite almost completely suppressed the photooxidation of pyrite under illumination by changing the migration path of photogenerated charge carriers. The formation of NC(S)2-Fe chelating groups provides atomic-level interface channels for DTC-TETA to transfer electrons to pyrite and creates a favorable reduction environment for pyrite. Besides, DTC-TETA coating greatly improves the electron-hole pairs recombination efficiency of pyrite, which significantly inhibits the photogenerated electron reduction of oxygen to generate reactive oxygen species (ROS). Moreover, DTC-TETA coating captures the photogenerated holes, avoiding direct oxidation of pyrite by holes. Density functional theory (DFT) calculations revealed that the DTC-TETA coating increases the adsorption energy barrier for oxygen and water. The results extend the existing knowledge on passivation mechanisms on pyrite and hold significant implications for the future screening, evaluation, and practical application of surface passivating agents.

Artificial humic acid coated ferrihydrite strengthens the adsorption of phosphate and increases soil phosphate retention

Phosphorus (P) leaching loss from farmland soils is one of the main causes of water eutrophication. Thus, effective methods must be developed to maintain sustainability in agricultural soils. Herein, we design artificial humic acid (A-HA) coated ferrihydrite (Fh) particles for fixing P in soil. The experiments in water and soil are successively conducted to explore the phosphate adsorption mechanism and soil P retention performance of A-HA coated ferrihydrite particles (A-Fh). Compared with unmodified ferrihydrite (Fh), the phosphate adsorption capacity of A-Fh is increased by 15 %, the phosphate adsorption speed and selectivity are also significantly improved. The ligand exchange, electrostatic attraction and hydrogen bonding are the dominant mechanisms of phosphate adsorption by A-Fh. In soil experiments, the addition of 2 % A-Fh increases the soil P retention performance from 0.15 to 0.7 mg/kg, and A-Fh are able to convert more phosphate adsorbed by itself into soil available P to improve soil fertility. Overall, this work highlights the importance of this a highly effective amendment for improving poor soils.

Regulation and mechanism of pyrite and humic acid on the toxicity of arsenate in lettuce

Pyrite and humic acid are common substances in nature, and the combined effects of pyrite and humic acid on arsenic phytotoxicity are more widespread in the actual environments than that of a single substance, but have received less attention. In this study, the interaction between pyrite and humic acid in arsenate solution was studied, and the effects of pyrite and humic acid on plant toxicity of arsenate were evaluated. The results showed that arsenate + pyrite + fulvic acid (V-PF) treatment immobilized more arsenic by forming chemical bonds such as AsS and Fe-As-O and reduced the migration of arsenic to plants. Compared to the arsenate + fulvic acid (VF), arsenate + pyrite (VP) and arsenate (V) group, the inorganic arsenic content of lettuce leaves in the V- PF group was reduced by 19.8 %, 13.4 % and 13.4 %, respectively. In addition, the V-PF group increased the absorption of Ca, Fe and Cu in plant roots, and improved the activity of superoxide dismutase (SOD) in plant leaves. Compared to the VF group, SOD and MDA in the V-PF group increased by 34.1 % in 30 days and decreased by 47.3 % in 40 days, respectively. The biomass of lettuce in V-PF group was increased by 29.3 % compared with that in VF group on day 50. The protein content of the V-PF group was 58.3 % higher than that of the VF group and 23.1 % higher than that of the VP group. Furthermore, metabolomics analysis showed that the V-PF group promoted glycolysis by up-regulating glyoxylic acid and dicarboxylic acid metabolism, thus reducing carbohydrate accumulation. Phosphocreatine metabolism was also up-regulated, which decreased the oxidative damage in lettuce induced by arsenic. This study will provide new ideas for scientifically and rationally assessing the ecological environmental risks of arsenic and regulating its toxicity.

Insights into the weathering behavior of pyrite in alkaline soil through electrochemical characterizations: Actual hazards or potentially benefits?

Pyrite is the most common metal sulfide mineral in the crust and readily weathers under natural circumstances to release H⁺ to acidify surrounding groundwater and soil, resulting in heavy metal ions in the surrounding environment (e.g., meadow and saline soils). Meadow and saline soils are two common, widely distributed alkaline soils and can affect pyrite weathering. Currently, the weathering behaviors of pyrite in saline and meadow soil solutions have not been systematically studied. Electrochemistry coupled with surface analysis methods were employed to study pyrite weathering behaviors in simulated saline and meadow soil solutions in this work. Experimental results suggest that saline soil and higher temperatures increase pyrite weathering rates due to the lower resistance and greater capacitance. Surface reactions and diffusion control the weathering kinetics, and the activation energies for the simulated meadow and saline soil solutions are 27.1 and 15.8 kJ mol^-1, respectively. In-depth investigations reveal that pyrite is initially oxidized to Fe(OH)₃ and S⁰, and Fe(OH)₃ further transforms into goethite γ-FeOOH and hematite α-Fe₂O₃, while S⁰ ultimately converts into sulfate. When these iron compounds enter alkaline soils, the alkalinity of soil changes, and iron (hydr)oxides effectively reduce the bioavailability of heavy metals and benefit alkaline soils. Meanwhile, weathering of natural pyrite ores containing toxic elements (such as Cr, As, and Cd) makes these elements bioavailable and potentially degrades the surrounding environment.

Performance and mechanisms of PropS-SH/Ce(dbp)3 coatings in the inhibition of pyrite oxidationtion for acid mine drainage control

Inhibition of tailings oxidation could availably control the generation of acid mine wastewater from its source. Organosilanes serving as a high-efficiency inhibitor of the oxidation of pyrite, bring some problems including safety hazards caused by large amounts of organic solvents, difficult high-temperature curing, poor long-term properties, and so on. In our work, the PropS-SH/Ce (dbp)₃ (PS/Ce (dbp)₃) passivator with excellent passivation performance and self-healing properties was prepared by choosing 3-mercaptopropyltrimethoxysilane (PropS-SH) and dibutyl phosphate (Ce (dbp)₃) as the main passivating agent and the repair agent, respectively. We reduced the ratio of ethanol to water by adjusting the pH of the organosilane condensation and also achieved room-temperature curing by extending the curing time. Electrochemical and chemical leaching experiments results showed that the most appropriate addition of Ce (dbp)₃ was 0.2 wt% for enhancing the passivation performance of the passivated coating. In a 6-month chemical leaching experiment, the PS/Ce (dbp)₃-0.2 passivation coating cured at room temperature showed a better passivation effect and maintained 90.55% and 78.54% of total Fe and SO₄^2- passivation efficiencies. The passivation and self-healing mechanisms were investigated by FT-IR, XPS, ²⁹Si NMR, and other characterization methods, which were as follows: silane formed a cross-linked mesh structure by Si-O-Si bonding, in which Ce (dbp)₃ was physically filled. And the Si-OH on the surface of the passivation film formed Fe-O-Si bonds with the hydroxyl groups on the surface of the pyrite, thus attaching to the surface of the pyrite and isolating the oxidation medium. When the passivation coating was locally damaged, the oxidation reaction caused a change in pH, which accelerated the dissolution of Ce (dbp)₃ in the passivation layer. Ce³⁺ underwent a valence change and formed a CeO₂ precipitate, while dbp^- could form a complex with Fe²⁺ on the pyrite surface, both of which worked together to repair the broken passivation coating and prevent the oxidation reaction.

Electrochemical properties of humic acid and its novel applications: A tip of the iceberg

The widely existed humic acid (HA) with abundant redox-active groups has been considered to play an important role in biogeochemistry in sediments and soils. Recent studies reported that HA showed great performance in terms of electron transfer capacity (up to HA_EDC = 94 mmol e^-/mol C, HA_EAC = 42 mmol e^-/mol C). Since HA is widely available, inexpensive and environmentally friendly, the electrochemistry of HA has been explored to apply in many fields, such as environmental remediation, detection sensor and energy storage. Whereas, these prospective applications of HA and their electrochemical principles were lack of a comprehensive summary. In this review, the electrochemical properties and the prospective electrochemical applications of HA were summarized. Simultaneously, the existing problems like shortages of traditional electrochemical characterization of HA, and future research directions about HA electrochemistry were prospected. This review provides a deeper understanding of HA electrochemistry, and also inspires ideas for environmental remediation, detection sensor and energy storage by exploring the potential application values of HA.

Suppression of pyrite oxidation by co-depositing bio-inspired PropS-SH-tannic acid coatings for the source control acid mine drainage

In previous works, both tannic acid (TA) and organosilane-based passivators have been proven to possess good inhibition effects on pyrite oxidation, which could effectively prevent acid mine drainage (AMD) generation at the source. However, the hydrophilicity of TA passivation film and the complex coating process of organosilane-based passivators (high temperature conditions were required during the process carried out) may limit their further practical use. Therefore, to achieve the purpose of better coating treatment of pyrite under mild conditions, TA and γ-mercaptopropyltrimethoxysilane (PropS-SH) were introduced to synergistically passivate pyrite in this work. Electrochemistry tests and chemical leaching experiments both confirmed that PropS-SH-TA coated pyrite had better oxidation resistance than raw pyrite and single PropS-SH or TA coated pyrite. Additionally, the analyses of scanning electron microscopy (SEM) measurements and static water contact angle tests demonstrated that a scaly coating was formed on PropS-SH-TA coated pyrite surface, which may be the reason for the significant improvement of its surface hydrophobicity. Finally, the study on the film-forming mechanism of PropS-SH-TA composite passivator displayed that the benzoquinone derivatives formed by TA could copolymerize with PropS-SH through Michael addition or Schiff base reaction, which constructed a dense hydrophobic film on pyrite surface. The newly formed composite film could provide a better oxidation barrier for pyrite based on TA passivation film.

Enhancement of humic acid on plant growth in a Cd-contaminated matrix: performance, kinetics, and mechanism

Phytoremediation of heavy metal-contaminated sites has been widely used. Nonetheless, the destruction of chloroplasts and plant growth enzymes by heavy metals leads to a low germination rate and high mortality of plants. To address these issues, an experiment was conducted in which plants were grown with (SHC) and without humic acid (SC) in actual Cd-contaminated soil from the site of an industrial pollution source. The results showed that the average germination rates of SC and SHC samples were 94.17% and 98.33%, respectively, and the plant heights were approximately 5 and 7 cm after 42 days of planting, respectively. It was discovered that humic acid (HA) enhanced plant growth by increasing urease and invertase content of the soil. The Shannon index and Venn diagram revealed that SHC had the richer population diversity. High-throughput analysis demonstrated that HA increased the content of plant growth-promoting bacteria in the soil from 5.01 to 34.27%. The experimental results revealed that HA increased microbial activity and diversity, thereby providing a favorable environment for plants to thrive. This study develops an effective method to enhance the phytoremediation performance of heavy metal-contaminated soils.

Inhibition of humic acid on copper pollution caused by chalcopyrite biooxidation

Humic acid has the advantages of wide source, easy availability and environmental friendliness, which may be a good choice for inhibiting chalcopyrite biooxidation and alleviating copper pollution. However, there are few researches on the inhibitory effect and mechanism of humic acid on the biooxidation of chalcopyrite. In order to fill this knowledge gap, this study proposed and validated a novel method for inhibiting chalcopyrite biooxidation by means of humic acid. The results showed that the biooxidation of chalcopyrite could be effectively inhibited by humic acid, which consequently decreased the release of copper ions. Humic acid with a concentration of 120 ppm had the best inhibitory effect, which reduced the biooxidation efficiency of chalcopyrite from 40.7 ± 0.5 % to 29.3 ± 0.8 %. This in turn suggested that humic acid could effectively suppress the pollution of copper under these conditions. The analysis results of solution parameters, mineral surface morphology, mineral phases and element composition showed that humic acid inhibited the growth of Acidithiobacillus ferrooxidans, promoted the formation of jarosite and intensified the passivation of chalcopyrite, which effectively hindered the biooxidation of chalcopyrite, and would help to alleviate the pollution of copper.

Sulfur defect and Fe(III) (hydr)oxides on pyrite surface mediate tylosin adsorption in lake water: effect of solution chemistry and dissolved organic matter

Pyrite affects the adsorption of tylosin (TYL) due to their coexistence in the lake system. As well as the reactivity groups of S-S-H, S-OH, and Fe-OH, defects also have the possibilities to influence the adsorption of organic contaminants. However, the role of these active sites in antibiotic adsorption on pyrite has not been deeply studied. Besides, pH, N, P, dissolved oxygen, and dissolved organic matter (DOM) fluctuate greatly in lake at different seasons, which may change the surface characteristics of pyrite. Hence, the adsorption of TYL on natural pyrite considered solution chemistry and DOM in lake water was explored in this study. The fitting results of the kinetic and isotherm models showed that the adsorption included physical and chemical interactions. The neutral initial solution pH was conductive to TYL adsorption owing to the combined result of electrostatic and cover of Fe-oxyhydroxide. NO₃^- and NH₄⁺ had no effect on TYL adsorption, whereas H₂PO₄^- promoted adsorption by forming flocculent Fe(H₂PO₄)₃ precipitates. The dissolved oxygen increased adsorption. This is due to the co-promotion of the pyrite oxidation by oxygen and sulfur defects. The Fe(II)-DOM complex caused by pyrite surface oxidation reduced the concentration of TYL in solution by gathering. Except for the surface charge, reactivity groups on pyrite significantly influenced the adsorption of TYL. The bond fracture of S-S resulted in sulfur defects that contributed to pyrite oxidation. As a result, Fe(III)/Fe(II) on the surface of pyrite or in solution produced a complex Fe(III)/Fe(II) with anions and DOM. In addition, Fe(III)-S on sulfur defects interacted with the O-H of TYL through hydrogen bonding. Furthermore, the Fe-O-C bond is formed by the interaction of C-OH on TYL and Fe(III) (hydr)oxides on the surface of pyrite. The study provides a deep insight into the effect of pyrite surface active sites on amphoteric antibiotic adsorption. It helps to understand antibiotic migration and interactions with widespread pyrite in the real environment.

Reduction of acid mine drainage by passivation of pyrite surfaces: A review

Acid mine drainage (AMD), a source of considerable environmental pollution worldwide, has prompted the development of many strategies to alleviate its effects. Unfortunately, the methods available for remedial treatment of AMD and the damage it cause are generally costly, labor-intensive, and time-consuming. Furthermore, such treatments may result in secondary pollution. Alternatively, treating the AMD problem at its source through pyrite surface passivation has become an important topic for research because it has the potential to reduce or prevent the generation of AMD and associated pollution. This review summarizes various pyrite anti-corrosion technologies, including the formation of various passivating coatings (inorganic, organic and organosilane) and carrier-microencapsulation. Several effective long-term passivators are identified, although many of them currently have important deficiencies that limit their practical application. Combining the mechanisms of existing passivation agents or new artificial materials, while considering environmental conditions, costs, and long-term passivation performance, is a feasible direction for future research.

Inhibition of organosilane/ATP@HQ self-healing passivator for pyrite oxidation

Organosilane, with functional organic groups attached to inorganic silicon atoms, exhibits excellent passivation performance for pyrite. However, a considerable number of micro-cracks will gradually appear on the surface of passivation film under long-term corrosion of oxidizing medium, resulting in a significant decrease of passivation effect. To improve the stability and long-term performance of organosilane coating, a novel passivator (PT-ATP@HQ) with self-healing function was prepared to inhibit the oxidation of pyrite. We chose 3-mercaptopropyltrimethoxysilane (Prop-SH) and tetraethoxysilane (TEOS) as the host coating (PT), and attapulgite clay (ATP) loaded with 8-hydroxyquinoline (8-HQ) was used to endow the coating with better passivation and self-healing performance. The electrochemical and chemical leaching results showed that the addition of ATP@HQ greatly improved the passivation performance of PT coating. The passivation efficiencies of total Fe and SO₄^2- reached to 88.1% and 79.2%, respectively. We also found that the protective capability of the scratched PT-ATP@HQ coating can be recovered automatically through 8-HQ release from ATP. The passivation and self-healing mechanisms were investigated by FT-IR, XPS, ²⁹Si NMR, and other characterization methods, which were as follows: firstly, the organosilanes hydrolyzed to form highly active silanol groups, then dehydration condensation reaction occurred between silanol molecules and ATP@HQ to obtain cross-linked network structure connected by Si-O-Si bonds. After that, Si-OH groups reacted with the hydroxyl groups of pyrite to form Fe-O-Si bonds, thereby an inert and dense passivation film attached to the surface of pyrite. Once the passivation film is locally damaged, 8-HQ will automatically release to repair the cracks.

Arsenopyrite weathering in acidic water: Humic acid affection and arsenic transformation

Arsenopyrite is a common metal sulfide mineral and weathers readily in the open environment, releases As, and pollutes the surrounding environment. Humic acid (HA) is ubiquitous in soils, sediments and waters, and contains various functional groups and complex with arsenic, iron and other metal ions that affect the weathering behavior of arsenopyrite. Because As, iron, and HA are redox-active compounds, electrochemical techniques, including polarization curves, electrochemical impedance spectroscopy (EIS), and cyclic voltammetry (CV), were used to fundamentally investigate the weathering process and mechanism of arsenopyrite over a wide range of environmental relevant conditions. Polarization curves showed higher HA concentrations (0-1000 mg•L^-1), higher temperatures (5-35°C) or acidities (pH 1.0-7.0) promoted arsenopyrite weathering; there was a linear relationship between the corrosion current density (i_corr), temperature (T) and acidity (pH): i_corr = -3691.2/T + 13.942 and i_corr = -0.2445pH + 2.2125, respectively. Arsenopyrite weathering readily occurred in the presence of HA as confirmed by its activation energy of 24.1 kJ•mol^-1, and EIS measurements confirmed that the kinetics were controlled by surface reaction as confirmed by decreased double layer resistance. CV and surface characterization (FTIR and XPS) showed that arsenopyrite initially oxidized to S⁰, As(III) and Fe²⁺, then S⁰ and Fe²⁺ were ultimately converted into SO₄^2- and Fe³⁺, while As(III) oxidized to As(V). Furthermore, the carboxyl (-COOH) and phenolic (-OH) of HA could bind with As(III)/(V) and Fe³⁺ via a ligand exchange mechanism forming As(III)/(V)-HA and As(III)/(V)-Fe-HA complexes that hinders the formation of FeAsO₄ and decreases the bioavailability of As. Findings gained from this study are valuable for the understanding of the fate and transport of As in acidic conditions, and have powerful implications for the remediation and management of As-bearing sites affected by mining activities.

Electrocatalytic ozonation process supplemented by EDTA-Fe complex for improving the mature landfill leachate treatment

The present study was to enhance catalytic ozonation process (COP) using ferric (Fe)- ethylenediaminetetraacetic acid (Fe-EDTA) integrated with an electrocoagulation (EC) process for landfill leachate pretreatment. For this purpose, the effect of operating parameters such as ozone and Fe-EDTA concentrations, current, initial pH, and reaction time were investigated. The findings revealed that the EC process and single ozonation process (SOP) could alone reduce chemical oxygen demand (COD) in landfill leachate by 23% and 39%; respectively. Moreover, integration of both processes at 100 mA current, 400 mg h^-1 ozone concentration, and 3 h reaction time could significantly improve COD reduction to 70%. As well, current efficiency and ozone consumption in the proposed system could considerably develop compared with EC process and SOP. The integrated electro-catalytic SOP using Fe-EDTA could be operated at neutral pH value, which the COD removal efficiency was obtained 79.7%. Subsequently, biochemical oxygen demand (BOD₅)/COD ratio of effluent increased to 0.64. Examining pseudo-first-order and pseudo-second-order kinetics, it was realized that constant rate in the system had augmented. These results also indicated that the modified process using Fe-EDTA was a promising landfill leachate pretreatment technique that could significantly enhance COD removal efficiency and BOD₅/COD ratio, and ultimately decrease time and sludge production.

Arsenopyrite weathering in sodium chloride solution: Arsenic geochemical evolution and environmental effects

In situ electrochemical techniques and surface analysis were used to investigate the weathering behavior of arsenopyrite in chlorine-containing brine. Cyclic voltammetry measurements showed that arsenopyrite weathering releases S°, As (III) and Fe (II) during the initial step, even contains different concentrations of H⁺ and Cl^-, and terminal transformation into SO₄^2-, As (V) and Fe (III), respectively. Cl^- ions promote the arsenopyrite weathering through diffusion control or adsorption control when Cl- ions are at low or high concentrations. When C_cl^- increased from 0.00 to 0.05 mol/L, As (III) release increases from 549.33 to 1135.86 g·m^-2·y^-1, and the promotion efficiency is 107 %; whereas from 0.20 to 0.40 mol/L, the promotion efficiency is only 15.1 %. H⁺ ions accelerate arsenopyrite weathering for O₂ + 4H⁺ + 4e^- → 2H₂O, and the relationship between corrosion current density (i_corr) and pH is i_corr = -26.54 pH + 199.75. Raman spectra confirm that corrosion produces S° and As (V) and EDX shows the passivation layers are mainly composed of elements Fe, As, S and O, while the adsorption layer are mainly composed of elements Fe, As, S and Cl. The experimental results are of great significance for arsenopyrite geological environment assess and removal of arsenic ions.