Bone Char Mediated Dechlorination of Trichloroethylene by Green Rust.

Biochars function as electron transfer mediators and thus catalyze redox transformations of environmental pollutants. A previous study has shown that bone char (BC) has high catalytic activity for reduction of chlorinated ethylenes using layered Fe(II)-Fe(III) hydroxide (green rust) as reductant. In the present study, we studied the rate of trichloroethylene (TCE) reduction by green rust in the presence of BCs obtained at pyrolysis temperatures (PTs) from 450 to 1050 °C. The reactivity increased with PT, yielding a maximum pseudo-first-order rate constant (k) of 2.0 h-1 in the presence of BC pyrolyzed at 950 °C, while no reaction was seen for BC pyrolyzed at 450 °C. TCE sorption, specific surface area, extent of graphitization, carbon content, and aromaticity of the BCs also increased with PT. The electron-accepting capacity (EAC) of BC peaked at PT of 850 °C, and EAC was linearly correlated with the sum of concentrations of quinoid, quaternary N, and pyridine-N-oxide groups measured by XPS. Moreover, no TCE reduction was seen with graphene nanoparticles and graphitized carbon black, which have high degrees of graphitization but low EAC values. Further analyses showed that TCE reduction rates are well correlated with the EAC and the C/H ratio (proxy of electrical conductivity) of the BCs, strongly indicating that both electron-accepting functional groups and electron-conducting domains are crucial for the BC catalytic reactivity. The present study delineates conditions for designing redox-reactive biochars to be used for remediation of sites contaminated with chlorinated solvents.

Sulfidation extent of nanoscale zerovalent iron controls selectivity and reactivity with mixed chlorinated hydrocarbons in natural groundwater.

Sulfidated nanoscale zerovalent iron (S-nZVI) exhibits low anoxic oxidation and high reactivity towards many chlorinated hydrocarbons (CHCs). However, nothing is known about S-nZVI reactivity once exposed to complex CHC mixtures, a common feature of CHC plumes in the environment. Here, three S-nZVI materials with varying iron sulfide (mackinawite, FeSm) shell thickness and crystallinity were exposed to groundwater containing a complex mixture of chlorinated ethenes, ethanes, and methanes. CHC removal trends yielded pseudo-first order rate constants (kobs) that decreased in the order: trichloroethene > trans-dicloroethene > 1,1-dichlorethene > trichloromethane > tetrachloroethene > cis-dichloroethene > 1,1,2-trichloroethane, for all S-nZVI materials. These kobs trends showed no correlation with CHC reduction potentials based on their lowest unoccupied molecular orbital energies (ELUMO) but absolute values were affected by the FeSm shell thickness and crystallinity. In comparison, nZVI reacted with the same CHCs groundwater, yielded kobs that linearly correlated with CHC ELUMO values (R2 = 0.94) and that were lower than S-nZVI kobs. The CHC selectivity induced by sulfidation treatment is explained by FeSm surface sites having specific binding affinities towards some CHCs, while others require access to the metallic iron core. These new insights help advance S-nZVI synthesis strategies to fit specific CHC treatment scenarios.

Effects of common groundwater ions on the transformation and reactivity of sulfidized nanoscale zerovalent iron.

Sulfidized nanoscale zerovalent iron (S-nZVI) is an Fe-based reactant widely studied for its potential use for groundwater remediation. S-nZVI reactivity has been widely investigated testing various contaminants in various water matrices, but studies on S-nZVI corrosion behaviour and reactivity upon exposure to complex groundwater chemistries are limited. Here, we show that anoxic aging of S-nZVI for 7 days in the absence and presence of key groundwater solutes (i.e., Cl-, SO42-, Mg2+, Ca2+, HCO3-, CO32-, NO3-, or HPO42-) impacts Fe0 corrosion extent, corrosion product and reduction rates with trichloroethene (TCE). White rust was the dominant corrosion product in ultrapure water and in SO42-, Cl-, Mg2+ or Ca2+ solutions; green rust and/or chukanovite formed in HCO3- and CO32- solutions; magnetite, formed in NO3- solutions and vivianite in HPO42- solutions. The aged S-nZVI materials expectedly showed lower reactivities with TCE compared to unaged S-nZVI, with reaction rates mainly controlled by ion concentration, Fe0 corrosion extent, type(s) of corrosion product, and solution pH. Comparison of these results to observations in two types of groundwaters, one from a carbonate-rich aquifer and one from a marine intruded aquifer, showed that S-nZVI corrosion products are likely controlled by the dominant GW solutes, while reactivity with TCE is generally lower than expected, due to the multitude of ion effects. Overall, these results highlight that S-nZVI corrosion behaviour in GW can be manifold, with varied impact on its reactivity. Thus, testing of S-nZVI stability and reactivity under expected field conditions is key to understand its longevity in remediation applications.

Sorption of chlorinated hydrocarbons from synthetic and natural groundwater by organo-hydrotalcites: Towards their applications as remediation nanoparticles.

Chlorinated hydrocarbons (CHCs) are recalcitrant compounds frequently found as contaminants in groundwater. Hydrotalcites (HT) have emerged as promising sorbents due to their tunable properties and anion exchange capacity. Here, two types of organo-HT were synthesized, via coprecipitation, by intercalation of two different anionic surfactants, sodium dodecyl sulfate and sodium 1-dodecane sulfonate. These compounds were first characterized by a suite of techniques to quantify surfactant intercalation and to evaluate their physico-chemical properties. Next, the sorption affinity of these organo-HT towards a suite of CHCs was tested under various conditions, including interlayer surfactant type, single and multiple CHCs systems, and different water chemistry (pH, ionic composition). Sorption coefficients (Kd) and organic-matter-normalized partition coefficient (Kom) derived from linear sorption isotherms for individual CHC were inversely correlated to their hydrophobicity in the order of: tetrachloroethylene > tetrachloromethane > trichloroethylene> 1,1,2-trichloroethane > trichloromethane. Kom values were further affected by the organo-HT drying process. In contrast, varying water chemistry and pH, and the co-existence of multiple CHCs had little effect on Kom values, indicating that competition between CHCs and ionic strength have a marginal effect on the sorption affinity. The inverse linear relationship between CHC hydrophobicity and Kom is shown to be a suitable tool to predict organo-HT's sorption efficiency in complex CHCs contaminated groundwaters. Overall, organo-HT's might be used as potential sorbents for ex situ treatment of CHCs in groundwater.