Pyrite-mediated advanced oxidation processes: Applications, mechanisms, and enhancing strategies

pH-dependent mechanisms of sulfadiazine degradation by natural pyrite-driven heterogeneous Fenton-like reactions

The development of a natural pyrite/peroxymonosulfate (PMS) system for the removal of antibiotic contamination from water represented an economic and green sustainable strategy. Yet, a noteworthy knowledge gap remained considering the underlying reaction mechanism of the system, particularly in relation to its pH sensitivity. Herein, this paper investigated the impacts of critical reaction parameters and initial pH levels on the degradation of sulfadiazine (SDZ, 3 mg/L) in the pyrite/PMS system, and elucidated the pH dependence of the reaction mechanism. Results showed that under optimal conditions, SDZ could be completely degraded within 5 min at a broad pH range of 3.0-9.0, with a pseudo-first-order reaction rate of >1.0 min-1. The low or high PMS doses could lower degradation rates of SDZ through the decreased levels of active species, while the amount of pyrite was positively correlated with the removal rate of SDZ. The diminutive concentrations of anions exerted minor impacts on the decomposition of SDZ within the pyrite PMS system. Mechanistic results demonstrated that the augmentation of pH levels facilitated the transition from the non-radical to the radical pathway within the natural pyrite/PMS system, while concurrently amplifying the role of •OH in the degradation process of SDZ. This could be attributed to the change in interface electrostatic repulsion induced by pH fluctuations, as well as the mutual transformation between active species. The stable presence of the relative content of Fe(II) in the used pyrite was ensured owing to the reduced sulfur species acting as electron donors, providing the pyrite/PMS system excellent reusability. This paper sheds light on the mechanism regulation of efficient removal of organic pollutants through pyrite PMS systems, contributing to practical application.

Efficient Cr(VI) removal by pyrite/porous biochar: Critical role of potassium salt and sulphur

The excessive accumulation of hexavalent chromium (Cr(VI)) in the environment poses a risk to environment and human health. In the present study, a potassium bicarbonate-modified pyrite/porous biochar composite (PKBC) was prepared in a one-step process and applied for the efficient removal of Cr(VI) in wastewater. The results showed that PKBC can significantly remove Cr(VI) within 4 h over a wide range of pH (2-11). Meanwhile, the PKBC demonstrated remarkable resistance towards interference from complex ions. The addition of potassium bicarbonate increased the pore structure of the material and promoted the release of Fe2+. The reduction of Cr(VI) in aqueous solution was primarily attributed to the Fe(II)/Fe(III) redox cycle. The sulphur species achieved Fe(II)/Fe(III) cycle through electron transfer with iron, thus ensuring the continuous reduction capacity of PKBC. Besides, the removal rate was also maintained at more than 85% in the actual water samples treatment process. This work provides a new way to remove hexavalent chromium from wastewater and demonstrates the potential critical role of potassium bicarbonate and sulphur.

A novel iron sulfide phase with remarkable hydroxyl radical generation capability for contaminants degradation

The hydroxyl radical (·OH) stands as one of the most potent oxidizing agents, capable of engaging in non-selective and instantaneous reactions with contaminants in water. Herein, we present a novel iron sulfide phase (S-FeS) characterized by an unprecedented structure, accompanied by its remarkable hydroxyl radical generation capability and contaminant degradation efficiency surpassing that of the conventional metastable iron sulfide phase, namely, the Mackinawite (FeS). In comparison to FeS, S-FeS exhibits enhanced degradation kinetics and higher efficacy in the removal of methylene blue, ciprofloxacin, and trivalent arsenic. Utilizing density functional theory (DFT) calculations, we postulate the mechanism for the exceptional contaminant degradation performance of S-FeS to be attributed to the increased exposure of the highly reactive (110) crystal facets. This research uncovers a new metastable phase that expands the polymorphisms within the iron sulfide family and showcases its capability for driving the oxygen reduction reaction.

Biodegradable chelating agents for enhancing phytoremediation: Mechanisms, market feasibility, and future studies

Heavy metals in soil significantly threaten human health, and their remediation is essential. Among the various techniques used, phytoremediation is one of the safest, most innovative, and effective. In recent years, the use of biodegradable chelators to assist plants in improving their remediation efficiency has gained popularity. These biodegradable chelators aid in the transformation of metal ions or metalloids, thereby facilitating their mobilization and uptake by plants. Developed countries are increasingly adopting biodegradable chelators for phytoremediation, with a growing emphasis on green manufacturing and technological innovation in the chelating agent market. Therefore, it is crucial to gain a comprehensive understanding of the mechanisms and market prospects of biodegradable chelators for phytoremediation. This review focuses on elucidating the uptake, translocation, and detoxification mechanisms of chelators in plants. In this study, we focused on the effects of biodegradable chelators on the growth and environmental development of plants treated with phytoremediation agents. Finally, the potential risks associated with biodegradable chelator-assisted phytoremediation are presented in terms of their availability and application prospects in the market. This study provides a valuable reference for future research in this field.

Influence of natural cellulose on hydroxyl radical generation by abiotic oxidation of pyrite under acidic conditions

Natural cellulose is one of the most important substances coexisting on the surface of pyrite. Oxidation of pyrite produces hydroxyl radicals (•OH). In this study, a pyrite-cellulose binary system was constructed with natural cellulose to investigate the effect of cellulose on the mechanism of •OH generation via oxidation of pyrite, and the mechanism for abiotic oxidative •OH production by pyrite under the influence of cellulose was investigated with oxidation and quenching experiments and characterization techniques. It was demonstrated that cellulose was chemisorbed onto the pyrite surface and some of the Fe(II) on the pyrite surface was masked, thus inhibiting the reaction between pyrite and O2 and decreasing the •OH production level from 33.54 to 22.48 μM under oxic conditions. In addition, the cellulose caused SS bond breakage, resulting in defects on the pyrite surface, which oxidized H2O to produce •OH in anoxic conditions. Therefore, under anoxic conditions, cellulose promoted the production of •OH and increased the •OH content from 11.85 to 14.78 μM. In addition to •OH, pyrite oxidation also produced SO42-. The amount of SO42- produced by a single pyrite crystal was less than that produced in the pyrite-cellulose system in all cases, and the amount produced under oxic conditions was approximately 10 times greater than that produced under anoxic conditions. More sulfate production indicated more sulfur intermediates during the reaction, such as S2O32-, which may decompose to produce S0 or Sn2- adsorbed on pyrite and decrease the amount of •OH produced. During the oxidation of pyrite by H2O2, cellulose competed with pyrite to react with H2O2, which inhibited the reaction between pyrite and H2O2 and decreased •OH production. Therefore, natural cellulose influenced the abiotic oxidation of pyrite to produce •OH.

Facet-Dependent Productions of Reactive Oxygen Species from Pyrite Oxidation

Reactive oxygen species (ROS) are widespread in nature and play central roles in numerous biogeochemical processes and pollutant dynamics. Recent studies have revealed ROS productions triggered by electron transfer from naturally abundant reduced iron minerals to oxygen. Here, we report that ROS productions from pyrite oxidation exhibit a high facet dependence. Pyrites with various facet compositions displayed distinct efficiencies in producing superoxide (O2• -), hydrogen peroxide (H2O2), and hydroxyl radical (•OH). The 48 h •OH production rates varied by 3.1-fold from 11.7 ± 0.4 to 36.2 ± 0.6 nM h-1, showing a strong correlation with the ratio of the {210} facet. Such facet dependence in ROS productions primarily stems from the different surface electron-donating capacities (2.2-8.6 mmol e- g-1) and kinetics (from 1.2 × 10-4 to 5.8 × 10-4 s-1) of various faceted pyrites. Further, the Fenton-like activity also displayed 10.1-fold variations among faceted pyrites, contributing to the facet depedence of •OH productions. The facet dependence of ROS production can greatly affect ROS-driven pollutant transformations. As a paradigm, the degradation rates of carbamazepine, phenol, and bisphenol A varied by 3.5-5.3-fold from oxidation of pyrites with different facet compositions, where the kinetics were in good agreement with the pyrite {210} facet ratio. These findings highlight the crucial role of facet composition in determining ROS production and subsequent ROS-driven reactions during iron mineral oxidation.

Efficient removal of naphthenic acids from real petroleum wastewater by natural pyrite activated persulfate system

The petroleum wastewater (PWW) contains a diverse range of recalcitrant organic contaminants. Of particular concern is the removal of naphthenic acids (NAs) due to the high toxicity and persistence. Persulfate (PS) based oxidation processes have shown promising in treating refractory wastewater, while the high costs of prepared catalysts limited their widespread implementation. This study aims to develop a cost-effective natural pyrite activated PS system for PWW treatment. The removal of NAs by pyrite/PS system was initially investigated. More than 90% of cyclohexanoic acid (CHA), a model NA, was removed in pyrite/PS system (2.0 g/L pyrite, 4.0 mM PS) at initial pH of 3-11. Scavenging experiments revealed that Fe(II) on pyrite surface was the reactive site for PS activation to generate reactive species, including sulfate radical (SO4·-), Fe(IV) and hydroxyl radical (·OH) for CHA degradation. Reactions of Fe(III) with S helped restore Fe(II) and enhance PS activation, resulting in the sustained catalytic activity of pyrites over five cycles. Cl-, SO42- and NO3- below 10 mM had minimal impact on CHA degradation in pyrite/PS system. However, over 1 mM of HCO3- inhibited 80% of CHA removal due to the buffer effect to maintain the high solution pH. Removing HCO3- from real PWW restored the removal of CHA and of total organic carbon (TOC) to over 90% and 71.3% in pyrite/PS system, respectively. Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) results indicated that O2‒6 species including NAs were primarily eliminated through mineralization and oxygen addition. Besides, O3-5S, NO3-5S and N3O2‒4 species were the most susceptible to oxidation in PWW, resulting in the increase of the oxidation level (i.e., O/Cwa) from 0.41 to 0.56 after treatment. This study provides valuable insights into the treatment of NAs in real PWW, and potential application of natural minerals in the treatment of industrial wastewater.

Novel strategy for treating high salinity oilfield produced water: Pyrite-activated peroxymonosulfate coupled with heterotrophic ammonia assimilation

Existing conventional biological treatment techniques face numerous limitations in effectively removing total petroleum hydrocarbons (TPHs) and ammonia (NH4+-N) from oilfield-produced water (OPW), highlighting the pressing need for innovative pre-oxidation and biological treatment processes. In this study, a pyrite-activated peroxymonosulfate (PMS)-coupled heterotrophic ammonia assimilation (HAA) system was established to achieve satisfactory system performance for OPW treatment. Pyrite sustained-release Fe2+-activated PMS was used to produce SO4•- and •OH, and 71.0 % of TPHs were effectively removed from the oil wastewater. The average TPHs and NH4+-N removal efficiencies in the test group with pre-oxidation were 96.9 and 98.3 %, compared to 46.5 and 77.1 % in the control group, respectively. The maximum fluorescence intensities of tryptophan protein and aromatic protein in the test group declined by 83.7 %. Fourier transform ion cyclotron resonance mass spectrometry revealed that pre-oxidation degraded more long-chain hydrocarbons and aromatic family compound, whereas the HAA process produced more proteins and carbohydrates. Pyrite-PMS promoted the enrichment of ammonia-assimilating bacteria, alleviating the explosive increase in extracellular polymeric substances and reducing sludge settleability. The low cost, efficiency, green chemistry principles, and synergies of this approach make it a powerful solution for practical OPW treatment to reduce environmental impacts and promote sustainable wastewater treatment.

Advancements and sustainable strategies for the treatment and management of wastewaters from metallurgical industries: an overview

Metallurgy is pivotal for societal progress, yet it yields wastewater laden with hazardous compounds. Adhering to stringent environmental mandates, the scientific and industrial sectors are actively researching resilient treatment and disposal solutions for metallurgical effluents. The primary origins of organic pollutants within the metallurgical sector include processes such as coke quenching, steel rolling, solvent extraction, and electroplating. This article provides a detailed analysis of strategies for treating steel industry waste in wastewater treatment. Recent advancements in membrane technologies, adsorption, and various other processes for removing hazardous pollutants from steel industrial wastewater are comprehensively reviewed. The literature review reveals that advanced oxidation processes (AOPs) demonstrate superior effectiveness in eliminating persistent contaminants. However, the major challenges to their industrial-scale implementation are their cost and scalability. Additionally, it was discovered that employing a series of biological reactors instead of single-step biological processes enhances command over microbial communities and operating variables, thus boosting the efficacy of the treatment mechanism (e.g., achieving a chemical oxygen demand (COD) elimination rate of over 90%). This review seeks to conduct an in-depth examination of the current state of treating metallurgical wastewater, with a particular emphasis on strategies for pollutant removal. These pollutants exhibit distinct features influenced by the technologies and workflows unique to their respective processes, including factors such as their composition, physicochemical properties, and concentrations. Therefore, it is of utmost importance for customized treatment and disposal approaches, which are the central focus of this review. In this context, we will explore these methods, highlighting their advantages and characteristics.

A comprehensive assessment on sludge conditioning by pyrite acid eluent-activated peroxymonosulfate based on dewaterability, heavy metals risk and ore recovery

Wastewater activated sludge (WAS) has poor dewaterability and contains heavy metals (HMs), limiting its land application. Therefore, in this study, a novel pyrite acid eluent-activated peroxymonosulfate (Fe2+pyrite/PMS) conditioning approach that can completely recover the residual pyrite and greatly reduce acid use was developed to improve WAS dewaterability, and the HMs chemical speciation and risks of conditioned WAS were assessed. Our results showed that under the optimized operational parameters, the capillary suction time (CST) and water content (Wc) of WAS decreased by 46.03% and 7.75%, respectively. Furthermore, during Fe2+pyrite/PMS conditioning processing, sulfate radical (SO4-) destroyed the extracellular polymeric substances (EPS) matrix, causing bound water release and the decrease of proteins/polysaccharides in outer layered EPS, even the decomposition of some protein-N in tightly bound EPS (TB-EPS) into inorganic-N. In addition, although the total concentration of HMs in the conditioned WAS matrix increased, the Ni concentration decreased in the solid fraction. Further, the risk assessment code (RAC) levels did not increase, and the eco-toxicity of Cr became weakened after Fe2+pyrite/PMS conditioning. However, after acid extraction, the pyrite residue had worsened recycle performance because the passivation layer contained S0/Sn2- on its surface, and no additional elements were detected in the pyrite residue, which had almost no effect on its further usage.

Remediation of arsenic contaminated water and soil using mechanically (ball milling) activated and pyrite-amended electrolytic manganese slag

With the development of industry, heavy metal (HM) pollution of soil has become an increasingly serious problem. Using passivators made of industrial by-products to immobilize HMs in contaminated soil is a promising in-situ remediation technology. In this study, the electrolytic manganese slag (EMS) was modified into a passivator (named M-EMS) by ball milling, and the effects of M-EMS on adsorption of As(V) in aquatic samples and on immobilization of As(V) and other HMs in soil samples were investigated under different conditions. Results demonstrated that M-EMS had a maximum As(V) adsorption capacity of 65.3 mg/g in the aquatic samples. Adding M-EMS to the soil reduced the leaching of As (from 657.2 to 319.8 μg/L) and other HMs after 30 d of incubation, reduced the bioavailability of As(V) and improved the quality and microbial activity of the soil. The mechanism for M-EMS to immobilize As in the soil are complex reactions, ion exchange reaction with As and electrostatic adsorption. This work provides new ideas of using waste residue matrix composites for sustainable remediation of Arsenic in the aquatic environment and soil.

Role of carboxylates in the phase determination of metal sulfide nanoparticles

Techniques are well established for the control of nanoparticle shape and size in colloidal synthesis, but very little is understood about precursor interactions and their effects on the resultant crystalline phase. Here we show that oleate, a surface stabilizing ligand that is ubiquitous in nanocrystal synthesis, plays a large role in the mechanism of phase selection of various metal sulfide nanoparticles when thiourea is used as the sulfur source. Gas and solid-phase FTIR, 13C, and 1H NMR studies revealed that oleate and thiourea interact to produce oleamide which promotes the isomeric shift of thiourea into ammonium thiocyanate, a less reactive sulfur reagent. Because of these sulfur sequestering reactions, sulfur deficient and metastable nanoparticles are produced, a trend seen across four different metals: copper, iron, nickel, and cobalt. At low carboxylate concentrations, powder XRD indicated that the following phases formed: covellite (CuS); vaesite (NiS2); smythite (FeS1.3), greigite (FeS1.3), marcasite (FeS2) and pyrite (FeS2); and cattierite (CoS2). At high sodium oleate concentration, these phases formed: digenite (CuS0.55), nickel sulfide (NiS), pyrrhotite (FeS1.1), and jaipurite (CoS).

Study on the removal effect and mechanism of calcined pyrite powder on Cr(VI)

Pyrite exhibits considerable potential as an adsorbent in wastewater treatment. However, few pyrite adsorbents are directly obtained from natural pyrite, as most are composite materials that require a complex preparation process. To develop a pyrite-based adsorbent with a simple preparation process, pyrite was processed by calcination at 400, 600, and 800 °C for 4 h and ball-milled into a fine powder. The adsorption properties of the pyrite powder were systematically explored. The calcined pyrite powder was characterized by SEM-EDS and XRD. The results revealed that the pyrite calcined at 600 °C exhibited excellent adsorption properties and was primarily composed of Fe7S8. The optimum conditions for Cr(VI) removal were a temperature of 45 °C, an adsorbent dosage of 1 g, an equilibration time of 60 min, and an initial pH of 3. Moreover, the calcined pyrite powder exhibited excellent reusability, and the Cr(VI) removal rate exceeded 65% after three cycles. The Cr(VI) adsorption on pyrite can be well described by the Freundlich model and pseudo-second-order kinetic equation. The calcination temperature is the main factor affecting the adsorption performance of pyrite. Therefore, the calcined pyrite powder is expected to be an excellent adsorbent for Cr(VI) in the wastewater treatment industry.

Natural iron minerals in an electrocatalytic oxidation system and in situ pollutant removal in groundwater: Applications, mechanisms, and challenges

Natural iron-bearing minerals are widely distributed in the environment and show prominent catalytic performance in pollutant removal. This work provides an overview of groundwater restoration technologies utilizing heterogeneous electro-Fenton (HEF) techniques with the aid of different iron forms as catalysts. In particular, applications of natural iron-bearing minerals in groundwater in the HEF system have been thoroughly summarized from either the view of organic pollutant removal or degradation. Based on the analysis of the catalytic mechanism in the HEF process by pyrite (FeS₂), goethite (α-FeOOH), and magnetite (Fe₃O₄) and the geochemistry analysis of these natural iron-bearing minerals in groundwater, the feasibility and challenges of HEF for organic degradation by using typical iron minerals in groundwater have been discussed, and natural factors affecting the HEF process have been analyzed so that appropriate in situ remedial measures can be applied to contaminated groundwater.

Spatial confinement: A green pathway to promote the oxidation processes for organic pollutants removal from water

Organic pollutants removal from water is pressing owing to the great demand for clean water. Oxidation processes (OPs) are the commonly used method. However, the efficiency of most OPs is limited owing to the poor mass transfer process. Spatial confinement is a burgeoning way to solve this limitation by use of nanoreactor. Spatial confinement in OPs would (i) alter the transport characteristics of protons and charges; (ii) bring about molecular orientation and rearrangement; (iii) cause the dynamic redistribution of active sites in catalyst and reduce the entropic barrier that is high in unconfined space. So far, spatial confinement has been utilized for various OPs, such as Fenton, persulfate, and photocatalytic oxidation. A comprehensive summary and discussion on the fundamental mechanisms of spatial confinement mediated OPs is needed. Herein, the application, performance and mechanisms of spatial confinement mediated OPs are overviewed firstly. Subsequently, the features of spatial confinement and their effects on OPs are discussed in detail. Furthermore, environmental influences (including environmental pH, organic matter and inorganic ions) are studied with analyzing their intrinsic connection with the features of spatial confinement in OPs. Lastly, challenges and future development direction of spatial confinement mediated OPs are proposed.

Guest Molecule Insertion-Optimized d-Band Center Position in MoS2 with Improved Sulfite Activation Ability Inspired by Sulfite Oxidase

As a prospective member in the family of advanced oxidation processes (AOPs), heterogeneous sulfite activation shows low cost and high safety for poisonous organic pollutants' degradation. To obtain an efficient sulfite activator, sulfite oxidase (SuO_x), a molybdenum-based enzyme that can prompt oxidation and activation of sulfite, inspired us greatly. Based on the structure of SuO_x, MoS₂/BPE (BPE = 1, 2-bis-(4-pyridyl)-ethylene) is synthesized successfully. In MoS₂/BPE, the BPE molecule is inserted between the MoS₂ layers as a pillar and the N atom links with Mo⁴⁺ directly. MoS₂/BPE shows excellent SuO_x mimic activity. Theoretical calculation implies that BPE insertion optimizes the d-band center position of MoS₂/BPE, which regulates the interaction between MoS₂ and *SO₄^2-. This prompts ^•SO₄^- generation and organic pollutants' degradation. At pH 7.0, its tetracycline degradation efficiency achieved is 93.9% in 30 min. Furthermore, its sulfite activation ability also endows MoS₂/BPE with excellent antibiofouling performance because ^•SO₄^- can kill the microorganisms in water effectively. This work develops a new sulfite activator based on SuO_x. The connection between structure and SuO_x mimic activity and sulfite activation ability is clarified in detail.

Heterogeneous Electro-Fenton-Catalyzed Degradation of Rhodamine B by Nano-Calcined Pyrite

The use of natural pyrite as a catalyst for the treatment of recalcitrant organic wastewater by an electro-Fenton system (pyrite-EF) has recently received extensive attention. To improve the catalytic activity of natural pyrite (Py), magnetic pyrite (MPy), and pyrrhotite (Pyr), they were obtained by heat treatment, and the nanoparticles were obtained by ball milling. They were characterized by X-ray diffraction, X-ray electron spectroscopy, and scanning electron microscopy. The degradation performance of rhodamine B (Rhb) by heterogeneous catalysts was tested under the pyrite-EF system. The effects of optimal pH, catalyst concentration, and current density on mineralization rate and mineralization current efficiency were explored. The results showed that the heat treatment caused the phase transformation of pyrite and increased the relative content of ferrous ions. The catalytic performance was MPy > Py > Pyr, and the Rhb degradation process conformed to pseudo-first-order kinetics. Under the optimum conditions of 1 g L^-1 MPy, an initial pH of five, and a current density of 30 mA cm^-2, the degradation rate and TOC removal rate of Rhb wastewater reached 98.25% and 77.06%, respectively. After five cycles of recycling, the chemical activity of MPy was still higher than that of pretreated Py. The main contribution to Rhb degradation in the system was •OH radical, followed by SO4•-, and the possible catalytic mechanism of MPy catalyst in the pyrite-EF system was proposed.

Rational design of cobalt sulfide anchored on nitrogen-doped carbon derived from cyanobacteria waste enables efficient activation of peroxymonosulfate for organic pollutants oxidation

With the increasing of eutrophication in water body, algae blooms have become one of the global environmental problems. The cyanobacteria waste has placed a severe burden on the environment and transforming cyanobacteria into functional materials may be a wise approach. Herein, cobaltous sulfide/nitrogen-doped biochar (N-BC/CoS_x) composite was synthesized by pyrolysis of cyanobacteria waste. The N-BC/CoS_x showed excellent performance in peroxymonosulfate (PMS) activation for enrofloxacin (ENR) degradation, which could remove more than 90% ENR within 60 min. The influencing factors of pH and catalyst dosage on ENR removal efficiency were studied. The N-BC/CoS_x showed good recyclability in the cycle runs. The radicals (O₂^•-, OH andSO₄^•-) and the non-radical species (charge transfer and ¹O₂) were generated in the ENR degradation. The cycle of Co(II)/Co(III) m ay contribute to the radical generation process. This work proved that metal sulfide modified cyanobacteria biochar has a specific application value in water pollution control and provides a new method for resource utilization of cyanobacteria.

Release characteristics of heavy metals in high-sulfur coal gangue: Influencing factors and kinetic behavior

High-sulfur coal gangue (HS-CG) is extremely unstable in the environment, releasing acid mine drainage with high concentrations of harmful heavy metals (HMs). The effects of HS-CG particle size, leaching solution pH, Fe³⁺ and acidophilic microorganisms on the release of HMs from the HS-CG and their kinetic behavior were studied using static leaching tests. The results showed that the smaller the particle size of HS-CG and the more acidic the leaching solution, the greater the release of HMs. As the chemical catalyst, the external addition of 300 mg/L Fe³⁺ can make the leaching amount of Fe, Mn, Cu, Zn, Ni, Cr reached 10,224.93, 93.88, 52.25, 11.56, 7.55, 2.97 mg/kg respectively, and the release of HMs was 1.36-2.60 times of the tests without the addition of iron. However, the concentration of Fe³⁺ above 800 mg/L promoted the production of jarosite on the surface of HS-CG, which led to decrease in the release of HMs. The HMs forms in HS-CG were different, while the effect of microorganisms on the leaching of Zn (54.99%) and Mn (52.35%) in the higher acid soluble fraction was more obvious, their leaching amount reached 87.21 and 107.58 mg/kg respectively. The kinetic analysis indicated that the rate-controlling step was mainly redox reaction at first, and then gradually controlled by the diffusion of ash layer. So, the kinetic equation controlled jointly by two rate-controlling stages has been proposed to describe the dissolution of HS-CG. This work help develop pertinent strategies for mine area remediation via controlling the HMs generation path.

Evaluation of pyrite/sodium hypochlorite for activating purification of arsenic from fractured-bedrock groundwater

In this work, the effectiveness of pyrite/sodium hypochlorite (FeS₂/NaClO) treatment to eliminate arsenic (As) from fractured-bedrock groundwater via oxidative adsorption was evaluated. The As concentration in the tested reactors decreased sharply during the initial 5 min, as the addition of NaClO effectively increased the As removal efficiency, attaining 98.6% removal within 60 min in the presence of 0.05 M NaClO. There was no coexisting anion effect (Cl^-, CO₃^-, HCO₃^-, NO₃^-, and F^-) on the As removal capacity of FeS₂/NaClO, except for the PO₄^3- which resulted in less removal of As. X-ray spectroscopy analysis of As(III)-sorbed FeS₂ surfaces revealed that a portion of As(III) was oxidized into As(V) during the adsorption process. Scanning electron microscopy-energy-dispersive spectrometer results of FeS₂ exhibited the distribution of adsorbed As on the newly formed iron (oxy) hydroxide surfaces, with an As element ratio of 1.27%. A continuous flow-bed column study further demonstrated the efficiency of FeS₂/NaClO treatment to lower the contamination level of As at the removal rates of 0.66-3.02 mg/L·day for 160 h. These results suggest that FeS₂/NaClO treatment can be considered an effective strategy for removing As in groundwater of bedrock aquifers.

Molecular Oxygen Activation by Citric Acid Boosted Pyrite-Photo-Fenton Process for Degradation of PPCPs in Water

Pyrite has been used in photo-Fenton reactions for the degradation of pollutants, but the application of photo-Fenton processes with extra H₂O₂ in real water/wastewater treatment has still been limited by the economic cost of H₂O₂ and artificial light sources. Herein, citric acid (CA) and simulated/natural sunlight are used to develop a pyrite-based photo-Fenton system (pyrite-CA-light) in situ generating H₂O₂ through the enhanced activation of molecular oxygen. The degradation of pharmaceuticals and personal care products (PPCPs), especially acetaminophen (APAP) as the main target pollutant, in the pyrite-CA-light system was investigated. The effects of influencing factors such as various organic acids, APAP concentration, pH, pyrite dosage, CA concentration and co-existing anions (HCO₃^-, Cl^-, NO₃^-, SO₄^2- and H₂PO₄^-) were examined. At a pyrite dosage of 0.1 g L^-1, CA concentration of 0.6 mM and an initial pH of 6.0, the degradation efficiency of APAP (30 μM) was 99.1% within 30 min under the irradiation of xenon lamp (70 W, λ ≥ 350 nm). Almost the same high efficiency of APAP degradation (93.9%) in the system was achieved under natural sunlight irradiation (ca. 650 W m^-2). The scavenging experiments revealed that the dominant active species for degrading APAP was hydroxyl radical (HO^•). Moreover, a quantitative structural-activity relationship (QSAR) model for pseudo-first-order rate constants (k_obs) was established with a high significance (R² = 0.932, p = 0.001) by using three descriptors: octanol-water partition coefficient (logKow), dissociation constant (pK_a) and highest occupied molecular orbital (HOMO). This work provides an innovative strategy of the photo-Fenton process for the degradation of PPCPs using natural minerals and ordinary carboxylic acid under sunlight.

Single atom Mn anchored on N-doped porous carbon derived from spirulina for catalyzed peroxymonosulfate to degradation of emerging organic pollutants

Highly efficient single atom catalysts are critical to substantially promote for peroxymonosulfate (PMS) activation to organic pollutant degradation, but it remains a challenge at present. Herein, single atom Mn anchored on N-doped porous carbon (SA-Mn-NSC) was synthesized by ball milling of Mn-doped carbon nitride and spirulina biochar to dominantly activate PMS. The precursor of carbon nitride and spirulina possessed a strong coordinating capability for Mn(II), facilitating the formation of highly dispersed nitrogen-coordinated Mn sites (Mn-N₄). The SA-Mn-NSC catalyst exhibited high activity and stability in the heterogeneous activation of PMS to degrade a wide range of pollutants within 10 min, showing an outstanding degradation rate constant of 0.31 min^-1 in enrofloxacin (ENR) degradation. The high surface density of Mn-N₄ sites and abundant interconnected meso-macro pores were highly favorable for activating PMS to produce ¹O₂ and high-valent manganese (Mn(IV)) for pollutant degradation. This work offers a new pathway of using a low-cost and easily accessible single-atom catalysts (SACs) and could inspire more catalytic oxidation strategies.

Effects of activated carbon, biochar, and carbon nanotubes on the heterogeneous Fenton oxidation catalyzed by pyrite for ciprofloxacin degradation

Pyrite and engineering carbon materials have received increasing attention for their catalytic potential in Fenton reactions due to their extensive sources and low cost. However, effects of carbon materials on the degradation of pollutants by pyrite-catalyzed heterogeneous Fenton oxidation have not been fully understood. In this study, the performance of pyrite-catalyzed heterogeneous Fenton system on the degradation of ciprofloxacin (CIP) was investigated in the presence of activated carbon (AC), biochar (BC), and carbon nanotubes (CNTs). Synchronous and asynchronous experiments (adsorption and catalysis) were conducted to elucidate the roles of the carbon materials in pyrite-catalyzed Fenton reactions. The results demonstrated that all the three carbon materials accelerated the pyrite-catalyzed Fenton oxidation of CIP. Under the experimental conditions, the reaction rates, which were obtained by fitting the synchronous experimental results with the pseudo-first-order kinetic model, of pyrite/AC, pyrite/BC and pyrite/CNTs with H₂O₂ for the removal of CIP were 8.28, 3.40 and 3.37 times faster than that of pyrite alone. Adsorption experiments and characterization analysis showed that AC had a higher adsorption capacity than BC and CNTs for CIP, which enabled it to distinguish itself in assisting the pyrite-catalyzed Fenton oxidation. In the presence of the carbon materials, the adsorption effect should not be neglected when studying the catalytic performance of pyrite. Free radical quenching experiments and electron spin-resonance spectroscopy (ESR) were used to detect and identify free radical species in the reactions. The results showed that hydroxyl radicals (•OH) contributed significantly to the degradation of CIP. The addition of carbon materials promoted the production of •OH, which favored the degradation of CIP. The results of this study suggested that the synergistic effect of oxidation and adsorption promoted the removal of CIP in pyrite/carbon materials/H₂O₂ systems, and coupling pyrite and carbon materials shows great potential in treating antibiotic wastewater.

Efficient naphthalene degradation in FeS2-activated nano calcium peroxide system: Performance and mechanisms

Naphthalene (NAP) has received increasing concern due to frequent detection in groundwater and harm to humans. In this study, FeS₂ was selected as a novel catalyst to activate nano calcium peroxide (nCP) for NAP degradation. Batch experiments were conducted in a 250 mL glass reactor containing 0.1 mM NAP solution to investigate the effect of reagents dosage, pH, air conditions (with or without N₂ purge), and different solution matrixes on NAP degradation. Scavenging tests, electron paramagnetic resonance (EPR) spectrum, and radical probe tests were conducted to identify the main radicals. Results indicated that over 96% NAP was removed in a wide pH range (3.0-9.0) within 180 min at optimal dosage of nCP = 1.0 mM and FeS₂ = 5.0 g L^-1 in nCP/FeS₂ system. Aerobic condition was more beneficial to NAP degradation and the system could tolerate complex solution conditions. Moreover, HO• was determined to be responsible for NAP degradation. NAP degradation intermediates were detected by gas chromatography-mass spectrometry (GC-MS) and the possible degradation pathways were revealed. Finally, the efficient degradation of other organic pollutants confirmed the broad-spectrum reactivity of the nCP/FeS₂ system. Overall, these findings strongly demonstrated the potential applicability of nCP/FeS₂ system in remediating organic contaminated groundwater.

Co-catalysis of trace dissolved Fe(iii) with biochar in hydrogen peroxide activation for enhanced oxidation of pollutants

Activation of hydrogen peroxide (H₂O₂) with biochar is a sustainable and low-cost approach for advanced oxidation of organic pollutants, but faces the challenge of a low yield of hydroxyl radical (˙OH). Herein, we hypothesize that the activation efficiency of H₂O₂ can be enhanced through co-catalysis of trace dissolved iron (Fe) with biochar. Two biochar samples derived from different feedstock, namely LB from liquor-making residue and WB from wood sawdust, were tested in the co-catalytic systems using trace Fe(iii) (0.3 mg L⁻¹). The cumulative ˙OH production in [Fe(iii) + LB]/H₂O₂ was measured to be 3.28 times that in LB/H₂O₂, while the cumulative ˙OH production in [Fe(iii) + WB]/H₂O₂ was 11.9 times that in WB/H₂O₂. No extra consumption of H₂O₂ was observed in LB/H₂O₂ or WB/H₂O₂ after addition of trace Fe(iii). Consequently, the reaction rate constants (k_obs) for oxidation of pollutants (2,4-dichlorophenoxyacetic acid and sulfamethazine) were enhanced by 3.13–9.16 times. Other iron species including dissolved Fe(ii) and iron minerals showed a similar effect on catalyzing 2,4-D oxidation by biochar/H₂O₂. The interactions involved in adsorption and reduction of Fe(iii) by biochar in which the defects acted as electron donors and oxygen-containing functional groups bridged the electron transfer. The fast regeneration of Fe(ii) in the co-catalytic system resulted in the sustainable ˙OH production, thus the efficient oxidation of pollutants comparable to other advanced oxidation processes was achieved by using dissolved iron at a concentration as low as the concentration that can be found in natural water.

The yield of ˙OH and oxidation of pollutants by biochar/H₂O₂ were enhanced dramatically by trace dissolved Fe(iii).