Development of novel persulfate tablets for passive trichloroethylene (TCE)-contaminated groundwater remediation

Insight into the role of reactive species on catalyst surface underlying peroxymonosulfate activation by P-Fe2MnO4 loaded on bentonite for trichloroethylene degradation

In this study, bentonite supporting phosphorus-doped Fe2MnO4 (BPF) was synthesized and applied for PMS activation to degrade TCE. Morphology and structure characterization results indicated the successfully synthesized of BPF, and the BPF/PMS system not only featured high TCE removal (97.4%) but also high reaction rate constant (kobs = 0.0554 min-1) and PMS utilization (70.4%, kobs = 0.0228 min-1). According to the results of various experiments, massive oxygen vacancies on P-Fe2MnO4 alter its charge balance and facilitate the electron transfer process named adjacent transfer (direct electron capture by adsorbed PMS from adjacent TCE). Mn(III) is the main adsorption site for PMS, and the hydroxyl groups on the catalyst (Fe sites of P-Fe2MnO4, Si and Al sites of bentonite) can also offer binding sites for PMS. The hydrogen-bonded PMS on Fe(III) and Mn(III) sites will subsequently accept the discharged electrons to generate free radicals and high-valent metal species. Meanwhile, electron loss of HSO5- that chemically bonded to hydroxyl groups on bentonite will generate SO5•-, which will further produce 1O2 through self-bonding. the active species on the catalyst surface contribute 65% of TCE degradation in the heterogeneous catalytic oxidation system.

Tannic acid promotes the activation of persulfate with Fe(ii) for highly efficient trichloroethylene removal

Trichloroethylene (TCE) is an Environmental Protection Agency (EPA) priority pollutant that is difficult to be removed by some remediation methods. For instance, TCE removal using persulfate (PS) activated by ferrous iron (Fe(ii)) has been tested but is limited by the unstable Fe(ii) concentration and the initial pH of contaminated water samples. Here we reported a new TCE removal system, in which tannic acid (TA) promoted the activation of PS with Fe(ii) (TA-Fe(ii)-PS system). The effect of initial pH, temperature, and concentrations of PS, Fe(ii), TA, inorganic anions and humic acid on TCE removal was investigated. We found that the TA-Fe(ii)-PS system with 80 mg L-1 of TA, 1.5 mM of Fe(ii) and 15 mM of PS yielded about 96.2-99.1% TCE removal in the pH range of 1.5-11.0. Radical quenching experiments were performed to identify active species. Results showed that SO4˙- and ˙OH were primarily responsible for TCE removal in the TA-Fe(ii)-PS system. In the presence of TA, the Fe-TA chelation and the reduction of TA could regulate Fe(ii) concentration and activate persulfate for continuously releasing reactive species under alkaline conditions. Based on the excellent removal performance for TCE, the TA-Fe(ii)-PS system becomes a promising candidate for controlling TCE in groundwater.

High-efficient removal of arsenic(III) from wastewater using combined copper ferrite@biochar and persulfate

Arsenic (As) is a potentially toxic element with variable valence states. Due to high toxicity and bioaccumulation, As can pose a severe threat to the quality of the ecology as well as human health. In this work, As(III) in water was effectively removed by biochar-supported copper ferrite magnetic composite with persulfate. The copper ferrite@biochar composite exhibited higher catalytic activity than copper ferrite and biochar. The removal of As(III) could reach 99.8% within 1 h under the conditions of initial As(III) concentration at 10 mg/L, initial pH at 2-6, and equilibrium pH at 10. The maximum adsorption capacity of As(III) by copper ferrite@biochar-persulfate was 88.9 mg/g, achieving superior performance than mostly reported the metal oxide adsorbents. By means of a variety of characterization techniques, it was found that ∙OH acted as the main free radical for removing As(III) in the copper ferrite@biochar-persulfate system and the major mechanisms were oxidation and complexation. As a natural fibre biomass waste-derived adsorbent, ferrite@biochar presented a high catalytic efficiency and easy magnetic separation for As(III) removal. This study highlights the great potential of copper ferrite@biochar-persulfate application in As(III) wastewater treatment.

Remediation of groundwater contaminated with trichloroethylene (TCE) using a long-lasting persulfate/biochar barrier

Groundwater contamination by chlorinated solvents causes potential threats to water resources and human health. Therefore, it is important to develop effective technologies to remediate contaminated groundwater. This study uses biodegradable hydrophilic polymers, hydroxypropyl methylcellulose (HPMC), hydroxyethyl cellulose (HEC) and polyvinyl pyrrolidone (PVP) as binders to manufacture persulfate (PS) tablets for the sustained release of persulfate to treat trichloroethylene (TCE) in groundwater. The release time for different tablets decreases in the order: HPMC (8-15 days) > HEC (7-8 days) > PVP (2-5 days). The efficiency with which persulfate is released is: HPMC (73-79%) > HEC (60-72%) > PVP (12-31%). HPMC is the optimal binder for the manufacture of persulfate tablets and persulfate is released from a tablet of HPMC/PS ratio (wt/wt) of 4/3 for 15 days at a release rate of 1127 mg/day. HPMC/PS/biochar (BC) ratios (wt/wt/wt) between 1/1/0.02 and 1/1/0.0333 are suitable for PS/BC tablets. PS/BC tablets release persulfate for 9-11 days at release rates of 1243 to 1073 mg/day. The addition of too much biochar weakens the structure of the tablets, which results in a rapid release of persulfate. TCE is oxidized by a PS tablet with an efficiency of 85% and a PS/BC tablet eliminates more TCE, with a removal efficiency of 100%, due to oxidation and adsorption during the 15 days of reaction. Oxidation is the predominant mechanism for TCE elimination by a PS/BC tablet. The adsorption of TCE by BC fits well with the pseudo-second-order kinetics and the pseudo-first-order kinetics, which describes the removal of TCE by PS and PS/BC tablets. The results of this study show that a PS/BC tablet can be used in a permeable reactive barrier for long-term passive remediation of groundwater.

Trichloroethylene remediation using zero-valent iron with kaolin clay, activated carbon and bacteria

Nanoscale particles of zero-valent iron were used to form a permeable reactive barrier whose performance in dechlorinating a solution of trichloroethylene was compared with that of a barrier formed from limestone. The iron was combined with kaolin by calcination. The test liquid contained sewage sludge, and also added NH₄Cl and KH₂PO₄. The average removal rates of trichloroethylene and phosphorus over 365 days both exceeded 94%. Chemical oxygen demand was reduced by 92% and ammonium nitrogen by 43.6%. All were significantly greater than the removals with the limestone barrier. The ceramsite barrier retained 85% of its effectiveness even after 365 days of use. Dechloromonas sp. was the main dechlorinating bacterium, but its removal ability is limited. The removal of trichloroethylene in such a barrier mainly depends on reduction by the zero-valent iron and biodegradation. The results show that the prepared ceramsite is stable and effective in removing trichloroethylene from water. It is a promising in-situ remediation material for groundwater.

Removal of chlorine-contaminated groundwater by two-stage ozonation and biostimulation methods

Trichloroethene (TCE) contamination is a critical environmental hazard, and the substrate options for its biostimulated remediation are limited. This study applied an ozonation-and-biostimulation process to remove TCE from groundwater. The substrate used, denoted as Transferred Energy Element (TEE), was composed of natural organic materials and had a low viscosity (2.914 cP). Ten batch experiments were conducted through the application of micro-nano bubbles (MNBs) and substrates (TEE and EOS® [emulsified oil substrate]). MNBs with an average diameter of 157.5-180.8 nm effectively degraded TCE and dichloroethane within 6 min. Biostimulation using the TEE substrate effectively degraded both TCE and vinyl chloride pollutants and reached a steady state after 25 days. The two-stage dechlorination procedure with MNB treatment as the first stage enhanced TCE removal via biostimulation. MNBs reduced the TCE concentration in the first 20 min, but increased the chloride (Cl^-) concentration over the following five days (∼80 mg/L). The procedure with biostimulation as the first stage and 20 min ozonation as the second stage reduced the Cl^- concentration by ∼10 mg/L. The Cl^- concentrations rebounded after day 25 in the EOS environment. X-ray diffraction revealed that the released Na⁺ from the TEE settled with Cl^- as minerals in the soil. The novel two-stage method for TCE removal was found to be more effective than solo MNB treatment or biostimulation.