Dependence of particle concentration effect on pH and redox for arsenic removal by FeS-coated sand under anoxic conditions

A Critical Look at Colloid Generation, Stability, and Transport in Redox-Dynamic Environments: Challenges and Perspectives

Colloid generation, stability, and transport are important processes that can significantly influence the fate and transport of nutrients and contaminants in environmental systems. Here, we critically review the existing literature on colloids in redox-dynamic environments and summarize the current state of knowledge regarding the mechanisms of colloid generation and the chemical controls over colloidal behavior in such environments. We also identify critical gaps, such as the lack of universally accepted cross-discipline definition and modeling infrastructure that hamper an in-depth understanding of colloid generation, behavior, and transport potential. We propose to go beyond a size-based operational definition of colloids and consider the functional differences between colloids and dissolved species. We argue that to predict colloidal transport in redox-dynamic environments, more empirical data are needed to parametrize and validate models. We propose that colloids are critical components of element budgets in redox-dynamic systems and must urgently be considered in field as well as lab experiments and reactive transport models. We intend to bring further clarity and openness in reporting colloidal measurements and fate to improve consistency. Additionally, we suggest a methodological toolbox for examining impacts of redox dynamics on colloids in field and lab experiments.

Simultaneous adsorption of As(III) and Pb(II) by the iron-sulfur codoped biochar composite: Competitive and synergistic effects

Simultaneous elimination of As(III) and Pb(II) from wastewater is still a great challenge. In this work, an iron-sulfur codoped biochar (Fe/S-BC) was successfully fabricated in a simplified way and was applied to the remediate the co-pollution of As(III) and Pb(II). The positive enthalpy indicated that the adsorption in As-Pb co-pollution was an endothermic reaction. The mechanism of As(III) removal could be illustrated by surface complexation, oxidation and precipitation. In addition to precipitation and complexation, the elimination mechanism of Pb(II) also contained ion exchange and electrostatic interactions. Competitive and synergistic effects existed simultaneously in the co-contamination system. The suppression of As(III) was ascribed to competitive complexation of the two metals on Fe/S-BC, while the synergy of Pb(II) was attributed to the formation of the PbFe₂(AsO₄)₂(OH)₂. Batch experiments revealed that Fe/S-BC had outstanding ability to remove As(III) and Pb(II), regardless of pH dependency and interference by various coexisting ions. The maximum adsorption capacities of the Fe/S-BC for As(III) and Pb(II) were 91.2 mg/g and 631.7 mg/g, respectively. Fe/S-BC could be treated as a novel candidate for the elimination of As(III)-Pb(II) combined pollution.

A comprehensive evaluation of factors affecting the reactivity of FeS towards hexabromocyclododecane diastereoisomers

Reactivity of iron sulfide (FeS) towards hexabromocyclododecane (HBCD) was explored under conditions of varying temperature, pH, inorganic ion and dissolved organic matter (DOM) in this study. Results show that the reduction of HBCD by FeS has an activation energy of 29.2 kJ mol^-1, suggesting that the rate-limiting step in the reduction was a surface-mediated reaction. The reduction of HBCD by FeS was a highly pH-dependent process. The optimal rate for HBCD reduction by FeS was observed at a pH of 6.2. All the tested inorganic ions suppressed the reduction of HBCD by FeS. XPS analysis confirmed that both Fe(II)-S and bulk S(-II) on FeS surface could be impacted by solution pH and inorganic ions and were responsible for the regulation of HBCD reduction. Some DOMs (i.e., fulvic acid, humic acid, salicylic acid, catechol and sodium dodecyl sulfate) were found to hinder the reduction via competing with HBCD for active sites on FeS surface. However, the presence of 2,2'-bipyridine, triton X-100 and cetyltrimethyl ammonium bromide was able to significantly enhance the rate of HBCD reduction by 5.8, 9.0 and 20 times, respectively. Different factors could influence the reduction efficiency of HBCD diastereoisomers to different extent, but not the reaction orders of HBCD diastereoisomers (α-HBCD < γ-HBCD < β-HBCD). Moreover, FeS could completely remove HBCD diastereoisomers in sediments with multiple factors within 9 d reaction. Our results contribute to give a better understanding on the performance of FeS towards HBCD under real and complex conditions and facilitate the application of FeS in remediation sites.

Cr(VI) immobilization by FeS-coated alumina and silica: Effects of pH and surface coating density

We investigated the feasibility of using FeS-coated alumina and silica for permeable reactive barrier (PRB) applications. By both coated materials, Cr(VI) was reduced to Cr(III), which was immobilized via surface complexation/precipitation at acidic pH, and bulk precipitation at neutral to basic pH. Both pH and surface coating density (the amount of FeS deposits per unit surface area of a supporting matrix) controlled Cr(VI) reduction capacity and [Cr,Fe](OH)₃ composition. The reduction was higher at acidic pH due to lower passivation, as evidenced by the increased production of Fe(III) (oxyhydr)oxides over Fe(II)-Fe(III) phases. The coated alumina, despite the lower amount of FeS deposits than the coated silica, showed greater reduction capacities due to its higher surface coating density, which made Fe(III) closer together to favor Fe(III) (oxyhydr)oxide formation. Since Cr(III) was preferentially substituted for Fe(III) in Fe(III) (oxyhydr)oxides, lower pH and higher surface coating density led to lower Cr fractions in [Cr,Fe](OH)₃ because of the increased production of Fe(III) (oxyhydr)oxides. Given that Cr-poor [Cr,Fe](OH)₃ is more resistant to re-oxidation, FeS-coated alumina is better for PRB applications. This study reveals the significance of the surface coating density when evaluating the effectiveness of coated materials in redox-based treatments.

Biosynthesized iron sulfide nanoparticles by mixed consortia for enhanced extracellular electron transfer in a microbial fuel cell

The bioanode of mixed consortia was for the first time used to in-situ synthesize iron sulfide nanoparticles in a microbial fuel cell (MFC) over a long-term period (46 days). These poorly crystalline nanoparticles with an average size of 29.97 ± 7.1 nm, comprising of FeS and FeS₂, significantly promoted extracellular electron transfer and thus the electricity generation of the MFC. A maximum power density of 519.00 mW/m² was obtained from the MFC, which was 1.92 times as high as that of the control. The cell viability was promoted by a small amount of iron sulfide nanoparticles but inhibited by the thick nanoparticle "shell" covered on the bacterial cells. Some electroactive and sulfur reducing bacteria (eg. Enterobacteriaceae, Desulfovibrio, and Geobacter) were specifically enriched on the anode. This study provides a novel insight for improving the performance of bioelectrochemical systems through in-situ sustainable nanomaterials biofabrication by mixed consortia.

Spectroscopic study on biological mackinawite (FeS) synthesized by ferric reducing bacteria (FRB) and sulfate reducing bacteria (SRB): Implications for in-situ remediation of acid mine drainage

Mackinawite (FeS), widespread in low temperature aquatic environments, is generally considered to be the first Fe sulfide formed in sedimentary environments which has shown effective immobilization of heavy metals and toxic oxyanions through various sorption reactions. The spectroscopic study researches on mackinawite formed by FRB and SRB and its environmental implication for in-situ remediation of acid mine drainage where contains large amounts of Fe³⁺ and SO₄^2-. The XRD result of biologically synthetic particles shows that these particles are mainly composed of mackinawite (FeS_0.9). The Raman peaks observed at 208, 256, 282, 298cm^-1 are attributed to FeS stretching vibrations of mackinawite. The Attenuated Total Reflection-Fourier Transform Infrared Spectroscopy (ATR-FTIR) reveals that the diagnostic bands of low intensity for these FeS particles occur at 412-425cm^-1 and 607-622cm^-1, which are assigned to the stretching vibrations of SS and FeS bonds. The Raman and IR vibrations from organic components both confirm that these particles are biogenic origin. The IR spectra of biologically synthesized mackinawite for different aging times show that the nano-sized particles mackinwate will be completely oxidized within 10h. All these findings have good implications for in-situ remediation of acid mine drainage.

Immobilization of heavy metals in electroplating sludge by biochar and iron sulfide

Electroplating sludge (ES) containing large quantities of heavy metals is regarded as a hazardous waste in China. This paper introduced a simple method of treating ES using environmentally friendly fixatives biochar (BC) and iron sulfide (FeS), respectively. After 3 days of treatment with FeS at a FeS-to-ES mass ratio of 1:5, the toxicity characteristic leaching procedure (TCLP)-based leachability of total Cr (TCr), Cu(II), Ni(II), Pb(II), and Zn(II) was decreased by 59.6, 100, 63.8, 73.5, and 90.5 %, respectively. After 5 days of treatment with BC at a BC-to-ES mass ratio of 1:2, the TCLP-based leachability was declined by 35.1, 30.6, 22.3, 23.1, and 22.4 %, respectively. Pseudo first-order kinetic model adequately simulated the sorption kinetic data. Structure and morphology analysis showed that adsorption, electrostatic attraction, surface complexation, and chemical precipitation were dominant mechanisms for heavy metals immobilization by BC, and that chemical precipitation (formation of metal sulfide and hydroxide precipitates), iron exchange (formation of CuFeS2), and surface complexation were mainly responsible for heavy metals removal by FeS. Economic costs of BC and FeS were 500 and 768 CNY/t, lower than that of Na2S (940 CNY/t). The results suggest that BC and FeS are effective, economic, and environmentally friendly fixatives for immobilization of heavy metals in ES before landfill disposal.

Removal of Arsenite from Water by Ce-Al-Fe Trimetal Oxide Adsorbent: Kinetics, Isotherms, and Thermodynamics

Ce-Al-Fe trimetal oxide adsorbent was prepared. The morphology characteristics of the new adsorbent were analysed by the transmission electron microscope (SEM) method. The SEM results implied its ability in the adsorption of As (III). To verify the analyses, bench-scale experiments were performed for the removal of As (III) from water. In the experiments of adsorption, As (III) adsorption capacity of the trimetal oxide adsorbent was presented significantly higher than activated aluminium oxide and activated carbon. As (III) adsorption kinetics resembled pseudo-second-order adsorption mode. When initial As (III) concentration was 3, 8, and 10 mg·L−1, the maximum adsorption capacity achieved was 1.48, 3.73, and 5.12 mg·g−1, respectively. In addition, the experimental adsorption data were described well by the Freundlich adsorption isotherm model at 20, 30, and 40°C. The enthalpy change(ΔS), the standard free energy(ΔG), and entropy change(ΔH)indicated that the nature of As (III) adsorption was exothermic and spontaneous with increasing randomness on the interface of solid and liquid. And the adsorption mechanism can be interpreted as chemisorption with As (III) multilayer coverage formation on the adsorbent surface.