Exploring the environmental pathways and challenges of fluoroquinolone antibiotics: A state-of-the-art review

Fluoroquinolone (FQ) antibiotics have become a subject of growing concern due to their increasing presence in the environment, particularly in the soil and groundwater. This review provides a comprehensive examination of the attributes, prevalence, ecotoxicity, and remediation approaches associated with FQs in environmental matrices. The paper discusses the physicochemical properties that influence the fate and transport of FQs in soil and groundwater, exploring the factors contributing to their prevalence in these environments. Furthermore, the ecotoxicological implications of FQ contamination in soil and aquatic ecosystems are reviewed, shedding light on the potential risks to environmental and human health. The latter part of the review is dedicated to an extensive analysis of remediation approaches, encompassing both in-situ and ex-situ methods employed to mitigate FQ contamination. The critical evaluation of these remediation strategies provides insights into their efficacy, limitations, and environmental implications. In this investigation, a correlation between FQ antibiotics and climate change is established, underlining its significance in addressing the Sustainable Development Goals (SDGs). The study further identifies and delineates multiple research gaps, proposing them as key areas for future investigational directions. Overall, this review aims to consolidate current knowledge on FQs in soil and groundwater, offering a valuable resource for researchers, policymakers, and practitioners engaged in environmental management and public health.

A critical review of sustainable application of biochar for green remediation: Research uncertainty and future directions

Biochar, a carbon-rich material produced from the pyrolysis of organic biomass, has gained significant attention as a potential solution for sustainable green remediation practices. Several studies analyze biomass-derived biochar techniques and environmental applications, but comprehensive assessments of biochar limitations, uncertainty, and future research directions still need to be improved. This critical review aims to present a comprehensive analysis of biochar's efficacy in environmental applications, including soil, water, and air, by sequentially addressing its preparation, application, and associated challenges. The review begins by delving into the diverse methods of biochar production, highlighting their influence on physical and chemical properties. This review explores the diverse applications of biochar in remediating contaminated soil, water, and air while emphasizing its sustainability and eco-friendly characteristics. The focus is on incorporating biochar as a remediation technique for pollutant removal, sequestration, and soil improvement. The review highlights the promising results obtained from laboratory-scale experiments, field trials, and case studies, showcasing the effectiveness of biochar in mitigating contaminants and restoring ecosystems. The environmental benefits and challenges of biochar production, characterization, and application techniques are critically discussed. The potential synergistic effects of combining biochar with other remediation methods are also explored to enhance its efficacy. A rigorous analysis of the benefits and drawbacks of biochar for diverse environmental applications in terms of technical, environmental, economic, and social issues is required to support the commercialization of biochar for large-scale uses. Finally, future research directions and recommendations are presented to facilitate the development and implementation of biochar-based, sustainable green remediation strategies.

Remediation of phenol contaminated soil using persulfate activated by ball-milled colloidal activated carbon

The degradation of phenolic compounds through persulfate (PS) activation is a valuable approach for soil/groundwater remediation. Several reports have been made related to PS activation and contaminant degradation using carbo-catalysts; however, there is no detailed study on soil remediation by colloidal activated carbon. This study demonstrates the phenol (PhOH) degradation efficiency in spiked and field-contaminated soils by a novel and low-cost ball-milled colloidal activated carbon (CAC_BM) catalyst. The CAC_BM/PS system exhibited outstanding degradation performance for PhOH in both spiked and field-contaminated soils. Optimum condition for degradation of 5.63 mmol PhOH kg soil^-1 was achieved at 2.5 mg CAC_BM g soil^-1, 5 mM PS, and a solid-liquid ratio of 1:5 at 25 °C in the wide pH range of 3-11. Radical scavenger experiments and electron spin resonance (ESR) spectroscopy revealed that both radical (^•OH and SO₄^•-) and non-radical (¹O₂) species were involved in the CAC_BM/PS system. PhOH degradation in soil phase followed several degradation pathways, resulting in various intermediate byproducts such as acetic acid, maleic acid, p-benzoquinone, fumaric acid, and ferulic acid as analyzed by ultra-high-performance liquid chromatography with mass spectroscopy (UPLC-MS). The CAC_BM/PS system showed a promising potential in the remediation of organic-contaminated soil.

Colloidal activated carbon as a highly efficient bifunctional catalyst for phenol degradation

A preparation of colloidal activated carbon (CAC) for phenol remediation from groundwater was introduced. The CAC prepared by a simple pulverization technique was an excellent metal-free catalyst for persulfate (PS) activation due to high contact surface area. The removal efficiency of phenol in the PS/CAC system (~100%) was higher than that in the PS/activated carbon (AC) system (90.1%) and was superior to the conventional PS/Fe²⁺ system (27.9%) within 30 min. The phenol removal reaction occurred both in bulk solution and at the surface of the CAC, as confirmed by Langmuir-Hinshelwood (L-H) kinetic model fitting, FT-IR, and electron spin resonance (ESR) analyses. The downsizing of particle size from AC to CAC played a critical role in the radical oxidation mechanism by leading to the formation of predominant superoxide radical (O₂^•-) species in the PS/CAC system. Anions NO₃^-, SO₄^2-, and Cl^- slightly inhibited the phenol removal efficiency, whereas CO₃^2-, HCO₃^- and PO₄3^- did not. Ferulic acid (C₁₀H₁₀O₄) was detected as an organic byproduct of phenol oxidation. The use of CAC as a metal-free bifunctional catalyst has an important implication in the PS activation for phenol degradation in groundwater.

Oxidative removal of sulfadiazine using synthetic and natural manganese dioxides

Studies on oxidation kinetics of sulfadiazine (SDZ) using δ-MnO₂ (birnessite) and natural MnO₂ are limited. Reaction order at different SDZ speciation was determined based on the effects of initial H⁺, MnO₂ and SDZ concentrations using initial rate method, which would be useful to determine the optimum pH and MnO₂ concentration. Birnessite and natural MnO₂ with different physico-chemical properties such as BET surface area, pH_PZC, d-spacing, and crystal size similarly showed good efficiencies in oxidizing neutral SDZ (pH 5) and anionic SDZ (pH 8). Activation energy (E_a) and thermodynamic parameters indicated the similar oxidation efficiencies in the temperature range of 10-40°C. The SO42- was produced from the SDZ oxidation coupled to the reduction of MnO₂ to Mn²⁺. The effect of co-solute ciprofloxacin (CIP) on the oxidation kinetics of SDZ was also studied. The rates of SDZ oxidation by both birnessite and natural MnO₂ were reduced by the presence of CIP due to competition in oxidation between SDZ and CIP. The SDZ was more rapidly oxidized than CIP in both single- and bi-solute systems, as indicated by the presence of CIP intermediate, whereas the intermediate of SDZ was not detected.

Slow-release persulfate candle-assisted electrochemical oxidation of 2-methylnaphthalene: Effects of chloride, sulfate, and bicarbonate

Slow-release persulfate candle (PSC)-assisted electrochemical oxidation (ECO) of 2-methylnaphthalene (2-MNA) in an undivided cell using graphite-sheet electrodes was investigated using Fe(II) as an activator. The effects of anions (Cl^-, SO₄^2-, and HCO₃^-) were investigated. In the PSC/ECO/Fe(II), the highest pseudo-first-order rate constant (k_obs) and % removal was achieved by adding Cl^- (2.723 h^-1, 75.2%) followed by SO₄^2- (1.753 h^-1, 63.9 %) and HCO₃^- (0.047 h^-1, 3.3%). Addition of Cl^- played a critical role in improving the removal efficiency by inducing OH and SO₄^- oxidations, while SO₄^2- reduced the efficiency due to non-radical oxidation, as elucidated by electron spin resonance (ESR). Furthermore, in the PSC/ECO/Fe(II) + Cl^-, dominant radical was changed from SO₄^- to OH. Scavenger experiments also confirmed that Cl^- and SO₄^2- ions are controlling the oxidation reaction. Two chlorinated byproducts analyzed by LC-MS were identified in PSC/ECO/Fe(II) + Cl^- system.

Removal of sulfadiazine and ciprofloxacin by clays and manganese oxides: Coupled sorption-oxidation kinetic model

Sorption onto clays (montmorillonite and kaolinite), oxidation and sorption by manganese oxides (synthesized MnO and natural MnO), and coupled sorption-oxidation experiments were conducted for the removal of antibiotics sulfadiazine (SDZ) and ciprofloxacin (CIP) at pH 5 and 8. Individual sorption and oxidation modelling were carried out using the first-order kinetic model. A coupled sorption-oxidation kinetic model was developed to predict the simultaneous sorption and oxidation process. The coupled sorption-oxidation enhanced the antibiotic sorption, with the first-order sorption rate constants in the simultaneous presence of clays and manganese oxides (k_sorp) being higher than those with clays only (k_sorp⁰). In contrast, a depression was observed; the first-order oxidation and sorption combination rate constants in the simultaneous presence of manganese oxides and clays (k_MnO) were lower than those with manganese oxides only (k_MnO⁰). In the coupled sorption-oxidation reaction, 13.5-62.5% of SDZ and CIP removal was attributed to the sorption. The SDZ and CIP species distributions at pH 5 affected the coupled sorption and oxidation systems more than those at pH 8. The best removal efficiency was achieved by the montmorillonite-synthesized MnO combination, mainly due to the higher surface area (A_BET) and pore size of montmorillonite and synthesized MnO combination compared to other clays and manganese oxides combinations.

Sorption of antibiotics onto montmorillonite and kaolinite: competition modelling

Antibiotic contaminants, which are generally present in bi-solute systems, can be competitively adsorbed onto clays. Single- and bi-solute sorptions of sulfadiazine (SDZ) and ciprofloxacin (CIP) onto montmorillonite and kaolinite were investigated at pH values of 5 and 8. Freundlich and Langmuir models were used and fit the experimental data well for single-solute sorption. The sorption isotherms were nonlinear (N_F = 0.265-0.730), and the maximum sorption capacities (q_mL) of the SDZ and CIP onto montmorillonite were higher than those onto kaolinite. The octanol-water distribution ratio (D_ow), cation exchange capacity (CEC), Brunauer-Emmett-Teller (BET) surface area (A_BET), pore size, point of zero charge (pH_PZC), and basal spacing predominantly affected the Freundlich constant (K_F) and q_mL of SDZ⁰ and CIP⁺ at pH 5 more than SDZ^- and CIP^± at pH 8. For bi-solute sorption, the presence of CIP inhibited the SDZ sorption onto montmorillonite and kaolinite. Competitive sorption models such as Sheindorf-Rebhun-Sheintuch (SRS), Murali-Aylmore (M-A) and the modified extended Langmuir model (MELM) were used; of these, the MELM provided the best prediction with SDZ sorption onto montmorillonite at pH 8 and CIP onto kaolinite at pH 5 and 8 in SDZ/CIP system occurring synergistically, whereas others occurred antagonistically. The distribution coefficient (K_d) of the bi-solute sorption decreased with increasing pH in the order cationic > neutral > anionic for SDZ and cationic > zwitterionic > anionic for CIP, which resembled the K_d of single-solute sorption. Fourier transform infrared spectroscopy (FT-IR) spectra indicated that amine in SDZ and keto oxygen in CIP were responsible for the interactions with the montmorillonite and kaolinite.