Remediation of phthalates in river sediment by integrated enhanced bioremediation and electrokinetic process

Enhancement of electrokinetic-phytoremediation by Ophiopogon japonicus: stimulation of electrokinetic on root system and improvement of polycyclic aromatic hydrocarbon degradation

Root systems are sensitive to voltage and tend to improve the degradation of organic pollutants by promoting the root exudates and increasing microbial enzyme activity in the rhizosphere under the effect of electrokinetic. In this study, electrokinetic-assisted phytoremediation (EKPR) was applied for the remediation of soil containing phenanthrene (PHE) and pyrene (PYR). Direct current (DC) voltage (1 V cm-1) was applied across the soils for 30 days following 3 treatment schedules (0 h, 4 h, and 12 h per day), referred to as treatments EK0, EK4, and EK12. Electrokinetic assistance improved phytoremediation. Compared to EK0, the removal of PHE and PYR increased by 51.79% and 45.07% for EK4 and by 43.18% and 38.75% for EK12. The applied voltage promoted root growth, stimulated the root exudate release, and increased accumulation of PHE and PYR by plants, and the effect was most pronounced in treatment EK4. Catalase and urease activities in rhizosphere soil also increased, by respective increments of 44.51% and 40.86% for EK4 and by 28.53% and 21.24% for EK12. In this study, we demonstrated that a low voltage applied for an appropriate duration (4 h per day) improves removal of PAHs by stimulating root growth, promoting the root exudate release and enhancing enzyme activity in the microbiome of rhizosphere soil.

Mechanism of bio-electrokinetic remediation of pyrene contaminated soil: Effects of an electric field on the degradation pathway and microbial metabolic processes

In this study, the mechanism of bio-electrokinetic (BIO-EK) remediation to improve the degradation of pyrene was evaluated based on an analysis of the intermediate products and the microbial community. The results show that BIO-EK remediation has a higher pyrene degradation efficiency on pyrene and its intermediate products than the bioremediation and electrokinetic (EK) remediation processes. A series of intermediate products were detected. According to the type of the intermediate products, two degradation pathways, biological metabolism and electrochemical oxidation, are proposed in the BIO-EK remediation of pyrene. Furthermore, the primary microbial taxa involved in the pollutant degradation changed, which led to variations in the functional gene components. The abundant and functional genes related to metabolism were specifically analyzed. The results indicate that the electric field promotes the expression of metabolisms associated with 14 carbohydrates, 13 lipids, 13 amino acids, five energies, and in particular, 11 xenobiotics. These results suggest that in addition to the promotion effect on the microbial metabolism caused by the electric field, BIO-EK remediation can promote the degradation of pollutants due to the coexistence of a microbial metabolic pathway and an electrochemical oxidation pathway.

Changes of microbial community and activity under different electric fields during electro-bioremediation of PAH-contaminated soil

Electro-bioremediation is a promising technology for remediation of soil contaminated with persistent organic compounds such as polycyclic aromatic hydrocarbons (PAHs). During electro-bioremediation, electrical fields have been shown to increase pollutant degradation. However, it remains unclear whether there is an optimal strength for the electrical field applied that is conductive to the maximum role played by microbes. This study aimed to determine the optimal strength of electric field through the analysis of the effects of different voltages on the microbial community and activity. Four bench-scale experiments with voltages of 0, 1, 2 and 3 V cm^-1 were conducted for 90 days in an aged PAH-contaminated soil. The spatiotemporal changes of the soil pH, moisture content and temperature, microbial biomass and community structure, and the degradation extent of PAHs were researched over 90 days. The results indicated that the total microbial biomass and degradation activity were highest at voltages of 2 V cm^-1. The concentration of total phospholipid fatty acids, used to quantify soil microbial biomass, reached 65.7 nmol g^-1 soil, and the mean degradation extent of PAHs was 44.0%. Similarly, the maximum biomass of actinomycetes, bacteria and fungus also occurred at the voltage of 2 V cm^-1. The Gram-positive/Gram-negative and (cy17:0+cy 19:0)/(16:1ω7+18:1ω7) ratios also showed that the intensity of electric field and electrode reactions strongly influenced the microbial community structure. Therefore, to optimize the electro-bioremediation of PAH-contaminated soil, the strength of electric field needs to be selected carefully. This work provides reference for the development of novel electrokinetically enhanced bioremediation processes.

Fast degradation of phthalate acid esters by polyoxometalate nanocatalysts through adsorption, esterolysis and oxidation

A novel route was created to facilitate the degradation of diethyl phthalate (DEP) upon micellar polyoxometalate (POM) catalysts and H₂O₂. The best catalytic activity was obtained using [C₁₆H₃₃N(CH₃)₃]H₄PMo₁₀V₂O₄₀ (N-hexadecyl-N,N,N-trimethylammonium tetrahydrogen decamolybdo-divanadophosphate, abbreviated as (CTA)H₄PMoV) with 90.2% degradation efficiency within 30 min, while the chemical oxygen demand (COD) and total organic carbon (TOC) removal efficiency were about 77.7% and 74.3% within 40 min. The highest efficiency was attributed to the concentration of DEP by amphiphilic POM catalyst, coupling with its strong Brønsted acidity and higher redox potential to catalyze esterolysis and oxidation of DEP. This allowed the phthalate acid esters (PAEs) with long carbon chains in super low concentration of 0.03 μM to be efficiently decomposed. The above synergistic effects explored DEP being degraded into ethanol, lactic acid and CO₂, which were non-toxic to the water surroundings. And the reaction activation energy (Ea) of 12.49 kJ/mol was obtained upon the degradation of DEP with (CTA)H₄PMoV followed first-order kinetics. Meanwhile, (CTA)H₄PMoV acted as a heterogeneous catalyst, which showed long duration and higher stability with only 3.7% loss amount during ten recycles.

Integrated electrokinetic processes for the remediation of phthalate esters in river sediments: A mini-review

Concerning the contamination of phthalate esters (PAEs) in river sediments, this mini-review introduces four recently reported novel "integrated electrokinetic (EK) processes" for the remediation purpose, namely two combined technologies of the EK process and advanced oxidation process (EK-AOP Processes) and two combined technologies of the EK process and biological process (EK-BIO Processes). The following is a comprehensive summary for these remediation processes: (1) the EK process coupled with nano-Fe₃O₄/S₂O₈^2- oxidation process - Test results have shown that nanoscale Fe₃O₄ played a significant role in activating persulfate oxidation. Even a recalcitrant compound like di(2‑ethylhexyl)phthalate (DEHP), its concentration in test sediment was reduced to 1.97 mg kg^-1, far below the regulatory levels set by Taiwan EPA; (2) the EK process integrated with a novel Fenton-like process catalyzed by nanoscale schwertmannite (nano-SHM) - Test results have revealed that simultaneous injection of nano-SHM slurry and H₂O₂ into the anode reservoir and sediment compartment is a good practice. 70-99% in removal efficiency was obtained for various target PAEs; (3) enhanced in situ bioremediation coupled with the EK process for promoting the growth of intrinsic microorganisms by adding H₂O₂ as an oxygen release compound (ORC) - Test results have demonstrated that an intermittent mode of injecting lab-prepared ORC directly into the contaminant zone would be beneficial to the growth of intrinsic microorganisms in test sediment for in situ bioremediation of target PAEs; and (4) coupling of a second-generation ORC (designated 2G-ORC) with the EK-biological process - Test results have proved that 2G-ORC is long-lasting and can be directly utilized as the carbon source and oxygen source for microbial growth resulting in an enhanced biodegradation of PAEs. Except DEHP having a residual concentration of 4 μg kg^-1, all other target PAEs in test sediment were totally removed by this novel combined remediation process.

A statistical approach of zinc remediation using acidophilic bacterium via an integrated approach of bioleaching enhanced electrokinetic remediation (BEER) technology

The aim of the present study was to isolate an indigenous acidophilic bacterium from tannery effluent contaminated sludge (TECS) sample and evaluate its potentiality towards the removal of zinc using an integrated approach of bioleaching enhanced electrokinetic remediation (BEER) technology in zinc spiked soil at an initial concentration of 1000 mg/kg. The isolated acidophilic bacterium was characterized by biochemical and 16S rRNA molecular identification and was named as Serratia marcescens SMAR1 bearing an accession no. MG742410 in NCBI database. The effect of pH and inoculum dosage of SMAR 1 strain showed an optimal growth at pH 5.0 and 4% (v/v) respectively. Based on these experimental data, a statistical analysis was done using Design Expert computer software, v11 to study the interaction between the process parameters with respect to zinc reduction as an output response. Electrokinetic experiments were conducted in a customised EK cell under optimised process conditions, employing titanium electrodes. Experiments for zinc removal were demonstrated for bioleaching, electrokinetic (EK) and BEER technology. On comparing, the integrated process was found to evidence as an excellent metal remediation option with a maximum zinc removal of 93.08% in 72 h than plain bioleaching (72.86%) and EK (56.67%) in 96 h. This is the first report of zinc removal in a short period of time using Serratia marcescens. It is therefore concluded that the BEER approach can be regarded as an effective technology in cleaning up the metal contaminated environment with an easy recovery and reuse option within short period of time.

Dredged-Sediment-Promoted Synthesis of Boron-Nitride-Based Floating Photocatalyst with Photodegradation of Neutral Red under Ultraviolet-Light Irradiation

A novel floating photocatalyst (BN-DS-7) has been successfully synthesized by calcining the mixture of boron nitride (BN) and dredged sediment (DS) with a specific mass ratio (3:7) at 1100 °C for a half hour. BN is synthesized for the first time using an oxygen-limited method, which consists of a nanoplate ∼30 nm in size and has a bandgap at 3.94 eV. The as-synthesized BN can degrade NR under ultraviolet (UV) light irradiation. For BN-DS-7, X-ray photoelectron spectroscopy analysis suggests that BN mainly interacts with DS through the strong coordination between these N atoms in BN and these Si and Al atoms in DS. This leads to BN-DS-7 having good compression strength (∼9 MPa). Thermogravimetric analysis for BN shows that a few BN (∼13%) synthesized via an oxygen-limited method will pyrolyze at 1100 °C and the released gas can be sealed in the inside of DS at 1100 °C, resulting in that BN-DS-7 can float on the water surface. Photodegradation results show that BN-DS-7 can degrade 84% of NR (20 mg/L) under UV-light irradiation for 5 h, and the active species are •OH and photoinduced hole. Total organic carbon analysis for NR solution before and after photodegradation show that ∼70% of NR has been mineralized into inorganic carbons. This work is helpful to develop a new type of BN-based floating material and enlarge the application field of DS.

Various causes behind the desorption hysteresis of carboxylic acids on mudstones

Adsorption desorption is a key factor for leaching, migration and (bio)degradation of organic pollutants in soils and sediments. Desorption hysteresis of apolar organic compounds is known to be correlated with adsorption/diffusion into soil organic matter. This work focuses on the desorption hysteresis of polar organic compounds on a natural mudstone sample. Acetic, citric and ortho-phthalic acids displayed adsorption-desorption hysteresis on Callovo-Oxfordian mudstone. The non-reversible behaviours resulted from three different mechanisms. Adsorption and desorption kinetics were evaluated using 14C- and 3H-labelled tracers and an isotopic exchange method. The solid-liquid distribution ratio of acetate decreased using a NaN₃ bactericide, indicating a rapid bacterial consumption compared with negligible adsorption. The desorption hysteresis of phthalate was apparent and suppressed by the equilibration of renewal pore water with mudstone. This confirms the significant and reversible adsorption of phthalate. Finally, persistent desorption hysteresis was evidenced for citrate. In this case, a third mechanism should be considered, such as the incorporation of citrate in the solid or a chemical perturbation, leading to strong desorption resilience. The results highlighted the different pathways that polar organic pollutants might encounter in a similar environment. Data on phthalic acid is useful to predict the retarded transport of phthalate esters and amines degradation products in sediments. The behaviour of citric acid is representative of polydentate chelating agents used in ore and remediation industries. The impact of irreversible adsorption on solid/solution partitioning and transport deserves further investigation.

Degradation of phthalate esters and acetaminophen in river sediments using the electrokinetic process integrated with a novel Fenton-like process catalyzed by nanoscale schwertmannite

The main objective of this study was to develop and establish an in situ remediation technology coupling nano-schwertmannite/H2O2 process and electrokinetic (EK) process for the removal of phthalates (PAEs) and acetaminophen in river sediments. Test results are given as follows: (1) injection of nano-schwertmannite slurry and H2O2 (collectively, "novel oxidant") into the anode reservoir would yield ·OH radicals that then will be diffused into the sediment compartment and further transported by the electroosmotic flow and/or electrophoresis from the anode end toward the cathode to degrade PAEs and pharmaceuticals in the sediment if any; (2) an electric potential gradient of 1.5 V cm(-1) would help the removal of PAEs and acetaminophen in the blank test, which no "novel oxidants" was added to the remediation system; (3) the practice of electrode polarity reversal would maintain neutral pH for sediment after remediation; (4) injection of equally divided dose of 10 mL novel oxidant into the anode reservoir and four injection ports on the top of sediment chamber would further enhance the removal efficiency; and (5) an extension of treatment time from 14 d to 28 d is beneficial to the removal efficiency as expected. In comparison, the remediation performance obtained by the EK-assisted nano-SHM/H2O2 oxidation process is superior to that of the batch degradation test, but is comparable with other EK integrated technologies for the treatment of same contaminants. Thus, it is expected that the EK-assisted nano-SHM/H2O2 oxidation process is a viable technology for the removal of phthalate esters and pharmaceuticals from river sediments in large-scale operations.