Enhanced removal of cis-1,2-dichloroethene and vinyl chloride in groundwater using ball-milled sulfur- and biochar-modified zero-valent iron: From the laboratory to the field

Engineered biochar for in-situ and ex-situ remediation of contaminants from soil and water

Tailoring physical and chemical properties of biochar enhances its selectivity, treatability, and efficiency in contaminant remediation. Thus, engineered biochar has emerged as a promising remedy for both in-situ and ex-situ remediation of polluted soil and water. Several factors influence the effectiveness of engineered biochar, including feedstock sources, pyrolysis conditions, surface functionalization, mode of application, and site characteristics. The advantages and disadvantages of different modification approaches to engineered biochar and their specific treatability for in-situ and ex-situ remediation are obscure and must be adequately addressed. This review critically evaluates the application of engineered biochar for on/off-spot contamination management, taking into account the long-term stability and biocompatibility prospects. The properties of engineered biochar resulting from modification with clay minerals, nanoparticles, polymers, surfactants, and oxidants/reductants were critically reviewed. Recent progress and advances in remediation mechanisms and modes of application were elaborated for the effective removal of organic and inorganic contaminants, including heavy metals, pesticides, dyes, polycyclic aromatic hydrocarbons, per- and poly-fluoroalkyl substances, and agrochemicals. Several crucial parameters influence in-situ remediation, including the distribution of contaminants, background electrolytes, hydraulic conductivity, as well as dispersion and stability of adsorbents. Ex-situ remediation of pollutants relies heavily on adsorption or degradation kinetics, background electrolytes, adsorbent dose, and pollutant concentrations. In addition, factors restricting the application of engineered biochar were highlighted for long-term sustainable contaminant management and maintaining low environmental impact. Finally, the challenges and future perspectives of utilizing engineered biochar for field-scale demonstration of contaminated sites are proposed.

The clogging effect of nanoscale zero valent iron corrosion in bicarbonate anaerobic water on porous media: A real-time pore-scale visualization

The corrosion-induced permeability changes of nanoscale zero-valent iron (NZVI) are one of the crucial factors constraining the successful application of NZVI in the remediation of contaminated groundwater. It is of great significance to study the dynamic evolution of corrosion products of NZVI after NZVI is injected into porous media and its influence on pore plugging effect from the pore scale. Micro computed tomography (Micro-CT) imaging technology, mineralogical characterization and theoretical calculations were used to understand the details of NZVI corrosion plugging porous media at the pore scale. This study reveals the factors of NZVI corrosion plugging porous media, namely, gas production (H2) in the early and middle stages of corrosion (before 90 days) and solid phase changes (NZVI volume increase and migration) in the later stages (after 90 days). The permeability loss rate of the porous media was 66.8%, 87.3%, 79.4%, and 53.6% at the corrosion times of 30, 60, 90, and 120 days, respectively. After 90 d of corrosion, the particle size of NZVI increases by 7.9%, and the secondary minerals formed by corrosion are mainly Fe3O4/γ-Fe2O3 and FeOOH. In addition, this study also found that the migration of NZVI after 90 d was due to its corrosion reducing the magnetic attraction between particles, dissociating into smaller particles or agglomerates under the action of fluid dynamics, resulting in its redistribution in the porous medium and causing blockage. This study clarifies that NZVI corrosion plays a vital influence in affecting the permeability and clogging of porous media, providing valuable guidance for optimizing NZVI-based remediation technologies.

Responses of nitrobenzene removal performance and microbial community by modified biochar supported zerovalent iron in anaerobic soil

Biochar-supported ZVI have received increasing attention for their potential to remove nitrobenzene in groundwater and soil. However, the capacity of this material to enhance the biological reduction of nitrobenzene and alter microbial communities in anaerobic groundwater have not been explored. In this study, the nitrobenzene removal performance and mechanism of modified biochar-supported zerovalent iron (ZVI) composites were explored in anaerobic soil. The results showed that the 700 °C biochar composite enhanced the removal of nitrobenzene and inhibited its release from soil to the aqueous phase. NaOH-700-Fe50 had the highest removal rate of nitrobenzene, reaching 64.4%. However, the 300 °C biochar composite inhibited the removal of nitrobenzene. Microbial degradation rather than ZVI-mediated reduction was the main nitrobenzene removal pathway. The biochar composites changed the richness and diversity of microbial communities. ZVI enhanced the symbiotic relationship between microbial genera and weakened competition between soil microbial genera. In summary, the 700 °C modified biochar composite enhanced the removal of nitrobenzene by increasing microbial community richness and diversity, by upregulating functional genes, and by promoting electron transfer. Overall, the modified biochar-supported ZVI composites could be used for soil remediation, and NaOH-700-Fe50 is a promising composite material for the on-site remediation of nitrobenzene-contaminated groundwater.

The impact of heteroaggregation between nZVI and SNPs on the co-transport of Cd(II) in saturated sand columns

This study investigated the co-transport behaviors of nano zero-valent iron (nZVI) and Cd(II) in the presence of soil nanoparticles (SNPs) under various SNPs/nZVI mass ratios. It was illustrated that the mobility of colloidal Cd(II) was highly dependent on the nZVI-SNPs heteroaggregation behavior. In the case of 40 mg/L nZVI with SNPs/nZVI mass ratios > 1, the formation of stable SNPs-nZVI heteroaggregates with hydrodynamic diameters (Dh) < 500 nm facilitated the nZVI and colloidal Cd(II) transport at their effluent mass recoveries of 34.76-37.82 % and 9.81-17.17 %, respectively. However, in the case of 100 mg/L nZVI with SNPs/nZVI mass ratios of 0.4-2, the interception of nZVI-SNPs heteroaggregates with Dh > 1500 nm by quartz sands led to almost complete retention of nZVI and colloidal Cd(II) in the columns. Combined with analytical results of zeta potentials and XRD spectrum, it was revealed that the Cd(II) ions could accelerate nZVI corrosion. The positively charged Fe3O4 and γ-FeOOH on corroded nZVI surface could facilitate the heteroaggregation of nZVI-SNPs by the patch-charge attraction, which further reduced the environmental risk of colloidal Cd(II) transport. These findings revealed the important effects of heteroaggregation between nZVI and SNPs on the transport risk of Cd(II) in groundwater.

Fast degradation of vinyl chloride by green rust and nitrogen-doped graphene

Carbonaceous materials catalyze reductive dechlorination of chlorinated ethylenes (CEs) by iron(II) materials providing a new approach for the remediation of CE polluted groundwater. While most CEs are reduced via β-elimination, vinyl chloride (VC), the most toxic and recalcitrant CE, degrades by hydrogenolysis. The significance of carbon catalysts for reduction of VC is well documented for iron(0) systems, but hardly investigated with iron(II) materials as reductants. In this study, a layered iron(II)‑iron(III) hydroxide sulfate (green rust) was used as reductant for VC, with an N-doped graphene (NG), prepared by co-pyrolysis of graphene and urea, as catalyst. VC (80 μM) was completely reduced to ethylene within 336 h in the presence of 5 g Fe/L GR and 5 g/L NG pyrolyzed at 950 °C, following pseudo-first-order kinetics with a rate constant of 0.017 h-1. Dosing experiments demonstrated that dechlorination of VC takes place on the NG phase. Monitoring of hydrogen formation, cyclic voltammetry, and quenching experiments demonstrated that atomic hydrogen contributes significantly to the dehalogenation reaction, where NG is critical for formation of atomic hydrogen. CE competition experiments demonstrated the presence of specific VC reduction sites with hydrogenolysis being unaffected by concurrent β-elimination reactions. The system exhibited excellent performance in natural groundwaters and in comparison with iron(0) systems. This study demonstrates that GR + NG is a promising system for remediation of VC contaminated groundwater, and the mechanistic part of the study can be used as a reference for subsequent studies.

Performance of field demonstration nanoscale zero-valent iron in groundwater remediation: A review

Nanoscale zero-valent iron (nZVI) has gained widespread usage in groundwater remediation due to its exceptional reactivity. Since its initial deployment in field demonstrations in 2001, nZVI has proven to be an effective nanomaterial for addressing groundwater contaminants. Subsequent research has highlighted the versatility of nZVI, showcasing its potential to overcome critical limitations associated with conventional remediation technologies. The effectiveness of nZVI in remediation varies, contingent on factors such as the type of nZVI, contaminant nature, site conditions, and injection methodologies employed. This review aims to present a comprehensive progress report on the field application of nZVI spanning 22 years across eight countries. Drawing from a database encompassing 32 pilot or full-scale remediation sites, the study delineates the various types of nZVI, modification methods, demonstration sites, and primary contaminants targeted in field tests. Specific attention is given to the application effects and mechanisms of unmodified nZVI, Pd, surfactants, and carbon-modified nZVI in diverse field demonstrations. An analysis of the key factors influencing their performance is provided, and potential future applications of nZVI in groundwater remediation are discussed.