Environmental remediation of emerging contaminants using subcritical water: A review.

The continuous rise of emerging contaminants (ECs) in the environment has been a growing concern due to their potentially harmful effects on humans, animals, plants, and aquatic life, even at low concentrations. ECs include human and veterinary pharmaceuticals, hormones, personal care products, pesticides, polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), organic dyes, heavy metals (HMs), and others. The world's growing population contributes to the release of many kinds of chemicals into the environment, which is estimated to be more than 200 billion metric tons annually and results in over 9 million deaths. The removal of these contaminants using conventional physical, chemical, and biological treatments has proven to be ineffective, highlighting the need for simple, effective, inexpesive, practical, and eco-friendly alternatives. Thus, this article discusses the utilization of subcritical water oxidation (SBWO) and subcritical water extraction (SBWE) techniques to remove ECS from the environment. Subcritical water (water below the critical temperature of 374.15 °C and critical pressure of 22.1 Mpa) has emerged as one of the most promising methods for remediation of ECs from the environment due to its non-toxic properties, simplicity and efficiency of application. Furthermore, the impact of temperature, pressure, treatment time, and utilization of chelating agents, organic modifiers, and oxidizing agents in the static and dynamic modes was investigated to establish the best conditions for high ECs removal efficiencies.

A coherent engineering assessment of ABS/biochar biocomposites in MEX 3D additive manufacturing.

Acrylonitrile butadiene styrene (ABS) composites were prepared in filament form compatible with the material extrusion (MEX) 3D printing method, using biochar as a filler at various loadings of up to 10.0 wt %. Samples were fabricated to experimentally investigate their mechanical performance. The ABS/biochar composites were characterized using thermogravimetric analysis, differential scanning calorimetry, Raman spectroscopy, and rheological tests. The electrical properties of the composites were investigated using broadband dielectric spectroscopy. Scanning electron microscopy was utilized to analyze the morphological features of the fabricated specimens by examining their side and fracture surfaces. The results indicate that the composite with 4.0 wt % biochar content compared to pure ABS showed the highest mechanical response between the prepared composites (24.9 % and 21 % higher than the pure ABS tensile and flexural strength respectively). The composites retained their insulating behavior. These findings contribute to expanding the utilization of the material extrusion (MEX) 3D printing method while also unlocking prospects for potential applications in microelectronics, apart from mechanical reinforcement.

Insight into the wheat residues-derived adsorbents for the remediation of organic and inorganic aquatic contaminants: A review.

Wheat is a major grain crop of the world that provides a stable food for human consumption. Large amounts of by-products/waste materials are produced after the harvesting and processing of wheat crop. Such materials can cause an environmental issue if not disposed of properly. Several studies have shown that wheat residues can be efficient precursors for adsorbents because of their availability, renewability, lignocellulosic composition, and surface active groups enriched structure. In the literature, there are few review articles that address wheat residues-based adsorbents. However, these reviews were specific in terms of adsorbate or adsorbent and did not provide detailed information about the modification, properties, and regeneration of these adsorbents. This article extensively reviews the utilization of wheat biomass/waste including straw, bran, husk, and stalk as precursors for raw or untreated, chemically treated, carbonaceous, and composite adsorbents against various environmental pollutants. The influences of inlet pollutant amount, adsorbent dose, pH, temperature, and time on the performance of adsorbents against pollutants were considered. The maximum uptakes, equilibrium time, and adsorption nature were identified from isotherms, kinetic, and thermodynamic studies. The highest adsorbed amounts of most tested contaminants were 448.20, 322.58, and 578.13 mg/g for lead, chromium, and copper, 1374.6 and 1449.4 mg/g for methylene blue and malachite green, and 854.75, 179.21, and 107.77 mg/g for tetracycline, phosphate, and nitrate, respectively. For the studied adsorbate/adsorbent systems the adsorption mechanism and regeneration were also discussed. Significant results and future directions are finally presented.

Removal of 241Am from Aqueous Solutions by Adsorption on Sponge Gourd Biochar.

Luffa cylindrica biomass was converted to biochar and the removal of 241Am by pristine and oxidized biochar fibers was investigated in laboratory and environmental water samples. This species has the added advantage of a unique microsponge structure that is beneficial for the production of porous adsorbents. The main purpose of this study was to valorize this biomass to produce an efficient adsorbent and investigate its performance in radionuclide-contaminated waters. Following the preparation of Am3+ solutions at a concentration of 10-12 mol/L, the adsorption efficiency (Kd) was determined as a function of pH, adsorbent mass, ionic strength, temperature, and type of aqueous solution by batch experiments. At the optimum adsorbent dose of 0.1 g and pH value of 4, a log10Kd value of 4.2 was achieved by the oxidized biochar sample. The effect of temperature and ionic strength indicated that adsorption is an endothermic and entropy-driven process (ΔH° = -512 kJ mol-1 and ΔS° = -1.2 J K-1 mol-1) leading to the formation of inner-sphere complexes. The adsorption kinetics were relatively slow (24 h equilibrium time) due to the slow diffusion of the radionuclide to the biochar surface and fitted well to the pseudo-first-order kinetic model. Oxidized biochar performed better compared to the unmodified sample and overall appears to be an efficient adsorbent for the treatment of 241Am-contaminated waters, even at ultra-trace concentrations.

Subcritical Water Extraction of Onosma mutabilis: Process Optimization and Chemical Profile of the Extracts.

The aboveground and root parts of Onosma mutabilis were extracted using subcritical water and the process was optimized with response surface methodology. The composition of the extracts was determined by chromatographic methods and compared to that of conventional maceration of the plant. The optimum total phenolic contents for the aboveground part and the roots were 193.9 and 174.4 µg/g, respectively. These results were achieved at a subcritical water temperature of 150 °C, an extraction time of 180 min, and a water/plant ratio of 0.1, for both parts of the plant. Principal component analysis revealed that the roots contained mainly phenols, ketones, and diols, with the aboveground part mostly alkenes and pyrazines, whereas the extract from maceration contained mainly terpenes, esters, furans, and organic acids. The quantification of selected phenolic substances showed that subcritical water extraction compared favorably to maceration, especially with respect to pyrocatechol (1062 as compared to 10.2 µg/g) and epicatechin (1109 as compared to 23.4 µg/g). Furthermore, the roots of the plant contained twice as much of these two phenolics compared to the aboveground part. Subcritical water extraction of O. mutabilis is an environmentally friendly method that can extract selected phenolics at higher concentrations compared to maceration.

Bamboo-derived adsorbents for environmental remediation: A review of recent progress.

The bamboo family of plants is one of the fastest-growing species in the world. As such, there is an abundance of bamboo residues available for exploitation, especially in southeast Asian, central African and south American regions. The preparation of efficient adsorbents from bamboo residues is an emerging exploitation pathway. Biochars, activated carbons or raw bamboo fibers embedded with nanoparticles, each class of materials has been shown to be highly efficient in adsorption processes. This review aims to summarize recent findings in the application of bamboo-based adsorbents in the removal of organic, inorganic, or gaseous pollutants. Therefore, this review first discusses the preparation methods and surface modification methodologies and their effects on the adsorbent elemental content and other basic properties. The following sections assess the recent progress in the adsorption of heavy metals, organics, and gaseous substances by bamboo-based adsorbents, focusing on the optimum adsorption capacities, adsorption mechanisms and the optimum-fitting kinetic models and isotherms. Finally, research gaps were identified and directions for future research are proposed.

Biochar application as a soil potassium management strategy: A review.

The established practices of intensive agriculture, combined with inadequate soil Κ replenishment by conventional inorganic fertilization, results in a negative environmental impact through the gradual exhaustion of different forms of K reserves in soils. Although biochar application as soil amendment has been established as an approach of integrated nutrient management, few works have focused on the impact of biochar application to soil K availability and crop uptake. This review provides an up-to-date analysis of the published literature, focusing on the impact of biochar in the availability of potassium in soil and crop growth. First, the effect of biomass type and pyrolysis temperature on potassium content of biochar was assessed. Second, the influence of biochar addition to the availability of potassium in soil and on potassium soil dynamics was examined. Finally, alternative methods for estimating available K in soils were proposed. The most promising biomasses in terms of potassium content were grape pomace, coffee husk and hazelnut husk however, these have not been widely utilized for biochar production. Higher pyrolysis temperatures (>500 °C) increase the total potassium content whereas lower temperatures increase the water-soluble and exchangeable potassium fractions. It was also determined that biochar has considerable potential for enhancing K availability through several distinct mechanisms which eventually lead directly or indirectly to increased K uptake by plants. Indirect mechanisms mainly include increased K retention capacity based on biochar properties such as high cation exchange capacity, porosity, and specific surface area, while the direct supply of K can be provided by K-rich biochar sources through purpose-made biochar production techniques. Research based on biochar applications for soil K fertility purposes is still at an early stage, therefore future work should focus on elucidating the mechanisms that define K retention and release processes through the complicated soil-biochar-plant system.

Hydrothermal Synthesis of Siderite and Application as Catalyst in the Electro-Fenton Oxidation of p-Benzoquinone.

A weak aspect of the electro-Fenton (EF) oxidation of contaminants is the dependence of the Fenton reaction on acidic pH values. Therefore, the rationale of this work was to develop a novel catalyst capable of promoting the EF oxidation process at near-neutral and basic pH values. In this framework, rhombohedral FeCO3 was synthesized hydrothermally and used as a catalyst in the EF oxidation of p-benzoquinone (BQ). The catalyst was characterized using various surface and spectroscopic methods. Moreover, the effects of applied current (100-500 mA), time (1-9 h), catalyst dosage (0.25-1.00 g L-1), and initial concentration of BQ (0.50-1.00 mM) on the total organic carbon removal efficiency were determined. The results indicated that a 400 mA current was sufficient for a 95% total organic carbon removal and that the increase in catalyst dosage had a positive effect on the mineralization of BQ. It was determined that at pH 3, FeCO3 behaved like a homogeneous catalyst by releasing Fe3+ ions; whereas, at the pH range of 5-7, it shifted to a homogeneous/heterogeneous catalyst. At pH 9, it worked solely as a heterogeneous catalyst due to the decrease of Fe ions passing into the solution. Finally, the spent catalyst did not undergo structural deformations after the EF treatment at higher pH values and could be regenerated and used several times.

A dual purpose aluminum-based metal organic framework for the removal of chloramphenicol from wastewater.

The presence of antibiotics in the aquatic environment can cause significant environmental and human health problems even at trace concentrations. Conventional treatment systems alone are ineffective in removing these resistant antibiotics. To address this problem, oxidation and adsorption techniques were used to explore the removal of recalcitrant antibiotic chloramphenicol (CAP). An aluminum-based metal-organic framework (Al-MIL) with high surface area and extended porosity, was prepared and used both as adsorbent and catalyst for the oxidation of CAP. Characterization of the Al-MIL revealed a large surface area of 1137 m2 g-1, a homogeneous microporous structure, good crystallinity, and particle size in the range of 200-400 nm. Adsorption of CAP on Al-MIL achieved equilibrium after 1 h, reaching a maximum adsorption capacity of 96.1 mg g-1 at the optimum pH value of 5.3. The combination of adsorption and oxidation did not improve the % TOC reduction considerably, indicating an antagonistic rather than synergistic effect between the two processes. Oxidation alone in the presence of persulfate, achieved a % TOC reduction of 71% after 2 h, compared to 56% achieved by adsorption alone at the same duration. The optimum persulfate concentration was determined as 2.5 mM. The Al-MIL structure did not demonstrate any substantial deterioriation after six repeated runs, according to the reusability experiments.

A web-based GIS platform supporting innovative irrigation management techniques at farm-scale for the Mediterranean island of Crete.

The aim of this paper is the creation of an integrated and free-access web platform for parcel irrigation water management on a large spatial scale (Water District of Crete, in Greece) in order to: a) accurately determine the irrigation needs of the main crops for Crete such as olives, citrus, avocados and vineyards, b) design strategies, for optimal adaptation of the agricultural sector in the context of climate change, and c) incorporate the dynamic integration of the above information through the creation of a digital platform. In the proposed decision-making system, essential factors are taken into account, such as real-time meteorological data, information about the type and spatial distribution of the agricultural parcels in Crete, algorithms for calculation crop evapotranspiration per development stage and age of the crops, satellite remote sensing techniques in combination with field surveys to depict accurate soil texture map for the whole island of Crete as well as sustainable cultivation practices for saving water per crop and parcel geomorphology. Based on the proposed decision-making system, users will have the opportunity in any specific location/farm in Crete to know the irrigation needs of the crops in real-time and obtain information about proper climate-water adaptation practices. The main novelty points of the proposed platform include the derivation of parcel-level soil texture data from Sentinel-2 satellite imagery and field samples, the comprehensiveness of the irrigation management information, the relatively low data requirements and the application interface simplicity provided to the end-user.

Recent progress on the phytotoxic effects of hydrochars and toxicity reduction approaches.

Hydrothermal carbonization of wet biomasses has been known to produce added-value materials for a wide range of applications. From catalyst substrates, to biofuels and soil amendments, hydrochars have distinct advantages to offer compared to conventional materials. With respect to the agricultural application of hydrochars, both positive and negative results have been reported. The presence of N, P and K in certain hydrochars is appealing and may contribute to the reduction of chemical fertilizer application. However, regardless of biomass, hydrothermal carbonization results in the production of phytotoxic organic compounds. Additionally, hydrochars from sewage sludge often contain heavy metal concentrations which exceed the regulatory limits set for agricultural use. This review critically discusses the phytotoxic aspects of hydrochar and provides an account of the substances commonly responsible for these. Furthermore, phytotoxicity reduction approaches are proposed and compared with each other, in view of field-scale applications.

Degradation of emerging contaminant coumarin based on anodic oxidation, electro-Fenton and subcritical water oxidation processes.

The degradation of emerging contaminant coumarin was separately investigated in anodic, electro-Fenton and subcritical water oxidation processes. With respect to the anodic and electro-Fenton oxidation, the influence of constant current, treatment time and initial concentration of coumarin was studied. Regarding subcritical water oxidation, the effect of the oxidant concentration, temperature, treatment time and initial coumarin concentration was investigated. In anodic and electro-Fenton oxidation processes, coumarin degradation proceeded in a similar manner, achieving 99% degradation, after 180 min at a constant current of 200 mA. In both set-ups, further increasing the applied current lowered the degradation efficiency due to the formation of by-products and the increasing occurrence of side-reactions. The highest degradation of 88% was achieved in subcritical conditions, specifically at 200 °C, using 150 mM H2O2 and after 37.5 min of treatment. Under subcritical conditions, temperature was the most prominent parameter, followed by the H2O2 concentration. Under all methodologies, increasing treatment time had a small positive effect on coumarin degradation, indicating that time is not the most influential parameter. A comparison of the three methodologies in terms of performance as well as energy consumption and simplicity of operation highlighted the advantages of subcritical water oxidation.

Comparative degradation of 5-fluorouracil in aqueous solution by using H2O2-modified subcritical water, photocatalytic oxidation and electro-Fenton processes.

This study investigated the degradation of the antineoplastic agent 5-fluorouracil (5-FU) widely applied to treat different cancers using different advanced oxidation processes such as electro-Fenton (EF), photocatalysis with TiO2, and H2O2-modified subcritical water oxidation. The treatment with the EF process was the most efficient compared to others. Interestingly, in the EF process, the oxidative degradation of 5-FU behaved differently depending on the anode used. At low currents (20 and 40 mA), Pt and DSA anodes performed better than BDD and Ti4O7 anodes. In contrast, at the higher current of 120 mA, the production of heterogeneous hydroxyl radicals (M(•OH)) became important and contributed significantly to the oxidation of 5-FU in addition to homogeneous •OH generated in the bulk solution. These latter have high O2-evolution overpotential leading to the high amount of physisorbed M(•OH) compared to Pt and DSA. The oxidative degradation of 5-FU was then performed by titanium dioxide-based photocatalytic oxidation and subcritical water oxidation processes, both of which showed a lower degradation efficiency and failed to achieve complete mineralization. Finally, a comparison was performed in laboratory-scale, taking into account the following performance indicators: the degradation efficiency, the mineralization power, the cost of equipment and reagents, and the energy required for the treatment of 5-FU.

Preparation and Application of a Hydrochar-Based Palladium Nanocatalyst for the Reduction of Nitroarenes.

In the present study, a novel heterogeneous catalyst was successfully fabricated through the decoration of palladium nanoparticles on the surface of designed Fe3O4-coffee waste composite (Pd-Fe3O4-CWH) for the catalytic reduction of nitroarenes. Various characterization techniques such as XRD, FE-SEM and EDS were used to establish its nano-sized chemical structure. It was determined that Pd-Fe3O4-CWH is a useful nanocatalyst, which can efficiently reduce various nitroarenes, including 4-nitrobenzoic acid (4-NBA), 4-nitroaniline (4-NA), 4-nitro-o-phenylenediamine (4-NPD), 2-nitroaniline (2-NA) and 3-nitroanisole (3-NAS), using NaBH4 in aqueous media and ambient conditions. Catalytic reactions were monitored with the help of high-performance liquid chromatography. Additionally, Pd-Fe3O4-CWH was proved to be a reusable catalyst by maintaining its catalytic activity through six successive runs. Moreover, the nanocatalyst displayed a superior catalytic performance compared to other catalysts by providing a shorter reaction time to complete the reduction in nitroarenes.

Towards a Soil Remediation Strategy Using Biochar: Effects on Soil Chemical Properties and Bioavailability of Potentially Toxic Elements.

Soil contamination with potentially toxic elements (PTEs) is considered one of the most severe environmental threats, while among remediation strategies, research on the application of soil amendments has received important consideration. This review highlights the effects of biochar application on soil properties and the bioavailability of potentially toxic elements describing research areas of intense current and emerging activity. Using a visual scientometric analysis, our study shows that between 2019 and 2020, research sub-fields like earthworm activities and responses, greenhouse gass emissions, and low molecular weight organic acids have gained most of the attention when biochar was investigated for soil remediation purposes. Moreover, biomasses like rice straw, sewage sludge, and sawdust were found to be the most commonly used feedstocks for biochar production. The effect of biochar on soil chemistry and different mechanisms responsible for PTEs' immobilization with biochar, are also briefly reported. Special attention is also given to specific PTEs most commonly found at contaminated soils, including Cu, Zn, Ni, Cr, Pb, Cd, and As, and therefore are more extensively revised in this paper. This review also addresses some of the issues in developing innovative methodologies for engineered biochars, introduced alongside some suggestions which intend to form a more focused soil remediation strategy.

Single-stage production of miscanthus hydrochar at low severity conditions and application as adsorbent of copper and ammonium ions.

In the framework of bio-circular economy, miscanthus biomass was valorized through a single-stage, low severity hydrothermal carbonization process. The produced hydrochars were characterized using elemental and spectroscopic methodologies. It was determined that as the temperature increased so did the C content (47.9 and 68.9% for the samples prepared at 180 and 260 °C, respectively), whereas the O content decreased (from 44.2 to 25.5%, respectively). The adsorption behaviour of the hydrochars was investigated in the adsorption of Cu2+ and NH4+ and MIS-180 was determined as the optimum sample, achieving qmax values of 310 and 71 mg g-1, respectively. Isotherm and kinetic analysis indicated the higher number of O-containing functional groups of MIS-180 as the main reason for its higher adsorption capacities. Furthermore, Cu2+ adsorption followed the 2nd-order kinetic model, whereas NH4+ adsorption followed the 1st-order kinetic model, due to the different mechanisms involved, inner-sphere and outer-sphere complex formation, respectively.

Degradation, solubility and chromatographic studies of Ibuprofen under high temperature water conditions.

Ibuprofen (IBP) is an emerging environmental contaminant having low aqueous solubility which negatively affects the application of advanced oxidation and adsorption processes. It was determined that as the temperature increased to 473 K, the mole fraction solubility increased considerably from 0.02 × 10-3 to 212.88 × 10-3 (10600-fold). Calculation of the thermodynamic properties indicated an endothermic process, ΔsolH > 0, with relatively high ΔsolS values. Spectroscopic, thermal and chromatographic analyses established the IBP stability at subcritical conditions. In the second part of the study, the degradation of IBP in H2O2-modified subcritical was studied and the effect of each process variable was investigated. The optimum degradation of 88% was reached at an IBP concentration of 15 mg L-1, temperature of 250 °C, 105 min treatment time and 250 mM H2O2. The process was optimized by response surface methodology and a mathematical model was proposed and validated. Temperature was determined as the most influential parameter, followed by H2O2 concentration. At temperatures higher than 230 °C, a small but noticeable reduction in degradation % suggested that the OH· radicals are consumed at a higher rate than they are produced, through side reactions with other radicals and/or IBP by-products. Finally, potential by-products were determined by gas chromatographic-mass spectrometric analysis and potential by-products were proposed.

Evaluation of sewage sludge biochar and modified derivatives as novel SPE adsorbents for monitoring of bisphenol A.

Sewage sludge is abundant biomass, the sustainable management of which remains a big issue worldwide. It was demonstrated that pyrolysis of sewage sludge using simple and cost-effective apparatus can produce biochars, suitable for solid-phase extraction applications of hydrophobic analytes. Detailed characterization showed that modification lead to three more hydrophobic and one more hydrophilic sample, compared to the original biochar. All samples were evaluated in the solid-phase extraction of the emerging contaminant Bisphenol A from aqueous solutions. KOH-SSB and KOH/MeOH-SSB exhibited the most promising behavior, with the latter achieving recoveries of 88.1%, at a quantity of 0.1 g at the natural pH of the BPA solution (6.5). The effect of solution pH was insignificant in the range of 4-7, whereas the initial BPA concentration had no effect in the recovery within the range of 1-100 µg L-1. The mechanism of interaction between the optimum sample and BPA was based on hydrogen bonding and π-π interactions, establishing earlier observations that the type (and not concentration) of individual surface groups and the total surface area play a significant role in the process.

Synthesis of ZrO2 nanoparticles on pumice and tuff for sonocatalytic degradation of rifampin.

ZrO2-pumice and ZrO2-tuff nanocomposites were synthesized via a modified sol-gel method and used as efficient catalysts for sonocatalytic degradation of rifampin (RIF). The physico-chemical properties of the prepared catalysts were examined using XRF, SEM, EDX, FT-IR and BET analyses and compared to pure pumice and tuff samples. Subsequently, the efficacy of catalysts in degradation of RIF was assessed under various experimental conditions. Both ZrO2-pumice and ZrO2-tuff (1.5 g L-1) exhibited promising catalytic activity for sonocatalytic degradation of RIF at its initial concentration of 20 mg L-1, natural pH and under ultrasonic irradiation power of 300 W. In this condition, about 95% and 83% of RIF was removed through US/ZrO2-pumice and US/ZrO2-tuff processes, respectively. Furthermore, the influence of the addition of a number of scavengers, enhancers and gases on the degradation of RIF was studied. The pronounced degradation effectiveness of the catalysts under ultrasound irradiation could be assigned to their synergetic ability to produce reactive species and subsequent radical reactions. The intermediate products formed in the solution from degradation of RIF were also identified and a decomposition pathway was proposed using GC-MS, COD, TOC and IC analyses.

Preparation of novel CeO2-biochar nanocomposite for sonocatalytic degradation of a textile dye.

The sonocatalytic performance of CeO2 nanoparticles synthesized by a hydrothermal method (CeO2-H) and CeO2@biochar (CeO2-H@BC) nanocomposite, were evaluated for the degradation of Reactive Red 84 (RR84) under ultrasonic irradiation. For comparison purposes the corresponding performance of bare biochar (BC) and commercial CeO2 (CeO2-C) samples were also assessed. A complementary characterization study, involving scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), Transmission electron microscopy (TEM), dynamic light scattering (DLS), X-ray diffraction (XRD), N2 adsorption at -196°C (Brunauer-Emmett-Teller (BET) method) and Fourier transform infrared spectroscopy (FT-IR) was undertaken to gain insight into the structure-performance relationships. The effect of various parameters such as initial RR84 concentration, solution pH, catalyst amount and ultrasonic power on the sonodegradation of RR84 was studied in detail. The results indicated that the CeO2-H@BC nanocomposite exhibited the best RR84 degradation efficiency, which is enhanced with the increase of CeO2-H@BC amount and ultrasonic power but diminished with the increment in RR84 concentration and pH value. A 98.5% degradation was obtained with a CeO2-H@BC amount of 1g/L, ultrasonic power of 450 W, pH of 6.5 and initial RR84 concentration of 10mg/L. The quenching effects of various scavengers proposed that OH radical plays the key role in the process. Analyses of intermediates by Gas chromatography-Mass spectroscopy (GC-MS) identified several by-products and accordingly the main pathway was proposed.