Conversion of char from pyrolysis of plastic wastes into alternative activated carbons for heavy metal removal.

The valorization of post-consumer mixed plastics in pyrolysis processes represents an abundant reservoir of carbon that can be effectively converted into useful chars. This process not only holds appeal in terms of improving plastic waste concerns but also contributes to the reduction of greenhouse gas emissions, thus aligning with the principles of a circular economy paradigm. In this study, the char produced from the pyrolysis of post-consumer mixed plastic waste has been activated with Na2CO3, KOH, NaOH, and K2CO3 to improve the textural, structural, and composition characteristics, leading to improved adsorption capability. These characteristics were studied by N2 adsorption-desorption isotherms, scanning electron microscopy, elemental and immediate analysis, and X-ray photoelectron spectroscopy. The developed surface area (SBET) was 573, 939, 704 and 592 m2 g-1 for Na2CO3, KOH, NaOH and K2CO3 activated carbons, respectively. These activated chars (ACs) were tested for the adsorption of heavy metals in both synthetic waters containing Pb, Cd, and Cu and industrial wastewater collected at an agrochemical production plant. Na2CO3-AC was the best performing material. The metal uptake in synthetic waters using a batch set-up was 40, 13 and 12 mg g-1 for Pb, Cd and Cu. Experiments in a column set-up using Na2CO3-AC resulted in a saturation time of 290, 16, and 80 min for Pb, Cd, and Cu synthetic waters, respectively, and metal uptakes of 26.8, 4.1, and 7.9 mg g-1, respectively. The agrochemical effluents, containing mainly Cr, Cu, Mn, and Zn were tested in a plug-flow column. The metal uptake notably decreased compared to synthetic water due to a competition effect for active sites.

Promoting the circular economy: Valorization of a residue from industrial char to activated carbon with potential environmental applications as adsorbents.

Pyrolysis of residues enriched with carbon, such as in agroforestry or industrial activities, has been postulated as an emerging technology to promote the production of biofuels, contributing to the circular economy and minimizing waste. However, during the pyrolysis processes a solid fraction residue is generated. This work aims to study the viability of these chars to develop porous carbonaceous materials that can be used for environmental applications. Diverse chars discharged by an industrial pyrolysis factory have been activated with KOH. Concretely, the char residues came from the pyrolysis of olive stone, pine, and acacia splinters, spent residues fuel, and cellulose artificial casings. The changes in the textural, structural, and composition characteristics after the activation process were studied by N2 adsorption-desorption isotherms, scanning electron microscopy, FTIR, elemental analysis, and XPS. A great porosity was developed, SBET within 776-1186 m2 g-1 and pore volume of 0.37-0.59 cm3 g-1 with 70-90% of micropores contribution. The activated chars were used for the adsorption of CO2, leading to CO2 maximum uptakes of 90-130 mg g-1. There was a good correlation between the CO2 uptake with microporosity and oxygenated surface groups of the activated chars. Moreover, their ability to adsorption of contaminants in aqueous solution was also evaluated. Concretely, there was studied the adsorption of aqueous heavy metals, i.e., Cd, Cu, Ni, Pb, and Zn, and organic pollutants of emerging concern such as caffeine, diclofenac, and acetaminophen.

Boosted Activity of g-C3N4/UiO-66-NH2 Heterostructures for the Photocatalytic Degradation of Contaminants in Water.

The combination of graphitic carbon nitride and the metal-organic framework UiO-66-NH2 has been developed with the aim to enhance the photocatalytic activity of pure semiconductors. Different proportions of g-C3N4 and UiO-66-NH2 were combined. Complete characterization analysis of the resulting photocatalytic materials was conducted, including N2 adsorption isotherms, XRD, FTIR, STEM-EDX microscopy, DRS-UV-visible, and photoluminescence. The photocatalytic activity was tested in an aqueous solution for the removal of acetaminophen as the target pollutant. From the obtained results, less than 50% of UiO-66-NH2 incorporated in the g-C3N4 structure enhanced the photocatalytic degradation rate of both bare semiconductors. Concretely, 75% of g-C3N4 in the final g-C3N4/UiO-66-NH2 heterostructure led to the best results, i.e., complete acetaminophen elimination initially at 5 mg·L-1 in 2 h with a pseudo-first order rate constant of ca. 2 h-1. The presence of UiO-66-NH2 in the g-C3N4 enhanced the optoelectronic properties, concretely, the separation of the photo-generated charges was improved according to photoluminescence characterization. The better photo-absorption uptake was also confirmed by the determination of the quantum efficiency values of the heterostructure if compared to either pure g-C3N4 or UiO-66-NH2. This photocatalyst with the best activity was further tested at different pH values, with the best degradation rate at a pH close to the pHpzc ~4.15 of the solid. Sequential recycling tests demonstrated that the heterostructure was stable after five cycles of use, i.e., 15 h. A high contribution of photo-generated holes in the process of the degradation of acetaminophen, followed marginally by superoxide radicals, was suggested by scavenger tests.

Simulated solar driven photolytic ozonation for the oxidation of aqueous recalcitrant-to-ozone tritosulfuron. Transformation products and toxicity.

This work reports the combination of ozone and solar radiation as an advanced oxidation process to remove the herbicide tritosufuron (TSF) in water. Firstly, the recalcitrance of TSF has been assessed, obtaining an ozonation second order rate constant of 5-154 M-1 min-1 in the range of pH from 5 to 8; while the rate constant with HO was found to be (1.8-3.1)·109 M-1 s-1. Secondly, the simultaneous application of simulated solar radiation in between 300 and 800 nm and ozone resulted positive in the oxidation rate of TSF. Mineralization extent was also higher. Less effective oxidation was achieved after limiting the radiation to the range 360-800 nm or 390-800 nm; also completely inappropriate for mineralization. Thirdly, the detected transformation products (TPs) demonstrated the vulnerability of TSF molecule to be attacked by HO in the sulfonylurea bridge. The combination of ozone and radiation of 300-800 nm led to the most effective removal of the TPs. Finally, after the photolytic ozonation treatment toxicity was also evaluated in terms of phytotoxicity towards the germination and root elongation of Lactuca Sativa seeds, and toxicity by immobilization tests of Daphnia Magna.

Integrated aerobic biological-chemical treatment of winery wastewater diluted with urban wastewater. LED-based photocatalysis in the presence of monoperoxysulfate.

The oxidation of Winery Wastewater (WW) by conventional aerobic biological treatment usually leads to inefficient results due to the presence of organic substances, which are recalcitrant or toxic in conventional procedures. This study explores the combination of biological and chemical processes in order to complete the oxidation of biodegradable and non-biodegradable compounds in two sequential steps. Thus, a biological oxidation of a diluted WW is carried out by using the activated sludge process. Activated sludge was gradually acclimated to the Diluted Winery Wastewater (DWW). Some aspects concerning the biological process were evaluated (kinetics of the oxidation and sedimentation of the sludge produced). The biological treatment of the DWW led to a 40-50% of Chemical Oxygen Demand (COD) removal in 8 h, being necessary the application of an additional process. Different chemical processes combining UVA-LEDs radiation, monoperoxysulfate (MPS) and photocatalysts were applied in order to complete the COD depletion and efficient removal of polyphenols content, poorly oxidized in the previous biological step. From the options tested, the combination of UVA, MPS and a novel LaCoO3-TiO2 composite, with double route of MPS decomposition through heterogeneous catalysis and photocatalysis, led to the best results (95% of polyphenol degradation, and additional 60% of COD removal). Initial MPS concentration and pH effect in this process were assessed.

Monopersulfate photocatalysis under 365 nm radiation. Direct oxidation and monopersulfate promoted photocatalysis of the herbicide tembotrione.

Oxone(®) (potassium monopersulfate, MPS) has been used to oxidize the herbicide tembotrione in aqueous solution. Tembotrione elimination kinetics by MPS direct oxidation has been studied. The influence of the main operating variables affecting the process (MPS concentration, temperature and pH) has been evaluated. The process follows 2/3 and first orders in MPS and tembotrione concentrations, respectively. Optimal pH is located around circumneutral conditions. MPS decomposition in the presence of 365 nm UVA radiation and titanium dioxide has also been studied. A kinetic mechanism that simulates MPS decomposition has been proposed, showing the positive effect of titania load and MPS concentration. The system MPS/UVA/TiO2 significantly improves tembotrione and mineralization rate abatement if compared to runs conducted in the absence of MPS. Tembotrione total abatement was achieved in 20 min when 0.05 g L(-1) of titania and 10(-4) M of Oxone(®) were used. TOC conversion was roughly 70% in 90 min under similar operating conditions. An experimental design (Plackett-Burman) has been considered to study the influence of the main variables affecting tembotrione photocatalytic oxidation promoted by MPS.

Coffee-waste templated CeOx/TiO2 nanostructured materials for selective photocatalytic oxidations.

An environmentally friendly solvent-free approach was tested using spent coffee as a biomass sacrificial template for the preparation of TiO2 modified with CeOx. The use of coffee as a template pursues the preparation of a nanostructured heterojunction without the need for a solvent. Two variables were optimized in the synthesis process, i.e. calcination temperature and proportion of CeOx. Firstly, bare coffee-template titania was prepared to explore the effect of the calcination temperature, within 500-650 °C. The anatase phase was obtained up to 600 °C. Higher temperatures, i.e. 650 °C, led to the appearance of rutile (10%) and efficient removal of the sacrificial agent (0.6% residue). The maximum photocatalytic activity in terms of conversion, in the oxidation of benzyl alcohol, was achieved employing the bare coffee-template TiO2 at 650 °C, and it was found comparable to the benchmarked P25. The incorporation of ceria in the solvent-free approach considerably improved photocatalytic benzaldehyde production. No changes in the XRD pattern of TiO2 were appreciated in the presence of ceria due to the low amount added, within 1.5-6.0%, confirmed by XPS as superficial Ce3+/Ce4+. The UV-visible absorption spectra were considerably redshifted in the presence of Ce, reducing the bandgap values of bare titania. An optimum amount of ceria in the structure within 3-0% was found. In this case, the selectivity towards benzaldehyde was ca. 75%, 3 times higher than the selectivity value registered for the benchmarked P25 or the bare prepared TiO2.

Activation of inorganic peroxides with magnetic graphene for the removal of antibiotics from wastewater.

Magnetic graphene catalysts were prepared for the removal of antibiotics (sulfamethoxazole, norfloxacin, tetracycline and flumequine) from water. Different proportions of magnetite-graphene from 1:0 to 0:1 were considered to study the catalytic activation of inorganic peroxides, i.e. peroxymonosulfate (PMS), peroxydisulfate and hydrogen peroxide. The presence of graphene was mainly responsible for the activation, which was most effective in the presence of PMS. A ratio of 20% of magnetite in the solid was enough to achieve complete degradation of antibiotics with high recovery by application of a magnetic field. The performance of the catalyst was further evaluated in a simulated urban wastewater, studying the main parameters affecting the process and the stability in sequential reuses. The non-radical mechanism during the catalytic activation of PMS was hypothesized from kinetic scavenging probes tests. The electron transfer was suggested as the mechanism of the reaction from electron paramagnetic resonance analysis in the presence of D2O. The prepared magnetic catalyst showed high catalytic activity and stability to remove antibiotics from water.

Adsorptive interaction of peroxymonosulfate with graphene and catalytic assessment via non-radical pathway for the removal of aqueous pharmaceuticals.

Graphene has been applied as a catalyst in peroxymonosulfate (PMS) activation for the removal of pharmaceuticals in water. Firstly, a kinetic adsorption study of PMS was developed, fitting the results to the Elovich's equation. Moreover, the influence of the main variables in the adsorptive process such as pH, initial PMS concentration, and graphene dose were assessed. Secondly, the degradation of diclofenac as a target compound was studied comparing PMS-catalytic versus adsorption processes. PMS-catalytic process enhanced the removal of the micropollutant if compared to adsorption when using a low dose of graphene (less than 50 mg L-1) or after surface saturation. Studies using radical scavengers suggested the lack of radicals in the process, suggesting the non-radical activation of PMS. Thirdly, the adsorption versus PMS-catalytic processes were also compared for the oxidation of a mixture of three antibiotics (norfloxacin, tetracycline and sulfamethoxazole) with different chemical structure. PMS-catalytic activation was more effective for the removal of those compounds that presented less affinity towards adsorption onto the graphene surface. Finally, characterization of the fresh and PMS-treated material was performed. Graphene demonstrated to be stable after its use as catalysts in PMS activation, suffering only slight transformation of the surface oxidation groups.

Peroxymonosulfate/solar radiation process for the removal of aqueous microcontaminants. Kinetic modeling, influence of variables and matrix constituents.

New technologies to address the presence of pharmaceutical and personal care products (PPCPs) in wastewater are needed, especially in those cases in which water will be reused. In this work, the activation of peroxymonosulfate (PMS) with simulated solar radiation has been applied to the oxidation of a mixture of six PPCPs, i.e. caffeine, primidone, N,N-diethyl-3-methylbenzamide (DEET), methylparaben, clofibric acid and ibuprofen. The sole application of solar radiation, i.e. solar photolysis, only led to the oxidation of clofibric acid (complete degradation in 90 min). The combination of PMS and solar radiation resulted in the degradation of all target micropollutants. The complete degradation of this mixture at initial 100 ppb was achieved with 0.5 mM of initial PMS after 90 min. A kinetic study that acceptably simulates the experimental data under different conditions has been proposed. The effects of initial PPCP concentration (1 mg L-1-100 µg L-1), PMS dose (0.1-5 mM), and pH (3-9) were tested and kinetically simulated. Finally, the PPCPs removal study was carried out in two real water matrices (river and a secondary effluent of an urban wastewater treatment plant). A higher dose of PMS, ten times higher, was required to achieve complete degradation of the micropollutants if compared to ultrapure water.

Sunlight driven photolytic ozonation as an advanced oxidation process in the oxidation of bezafibrate, cotinine and iopamidol.

This study investigates the efficacy of the system O3/sunlight radiation compared to dark ozonation when treating pharmaceuticals compounds of different reactivity, namely bezafibrate, cotinine, and iopamidol. Results show the beneficial effects of simulated sunlight radiation (300-800 nm) when treating ozone recalcitrant compounds such as cotinine and iopamidol. The system O3/sunlight radiation increased mineralization extent in all cases if compared to dark ozonation. Transformation products identified in individual runs suggest that amine oxidation and further alkyl chain attack is the main route of bezafibrate ozonation. Hydroxylation seems to be the preferential path in cotinine abatement while H abstraction from alcoholic moieties is suggested in the case of iopamidol. Toxicity of intermediates was approximately evaluated by QSAR methodologies and experimentally through Daphnia Magna survival after 24 h. As a rule of thumb, initial intermediates generated are even more toxic than parent compounds, however, after 120 min of treatment, toxicity significantly decreased. Amongst the most toxic compounds generated: 4-Chlorobenzoyltyramine, and 4-Chloro-N-[2-(3,4-dihydroxy-phenyl)-ethyl]-benzamide (from bezafibrate), and N-(2-Hydroxy-1-hydroxymethyl-ethyl)-N'-(1-hydroxymethyl-2-oxo-ethyl)-5-(2-hydroxy-propionylamino)-2,4,6-triiodo-isophthalamide, N,N'-Bis-(1-hydroxymethyl-2-oxo-ethyl)-5-(2-hydroxy-propionylamino)-2,4,6-triiodo-isophthalamide, and N-(1-Hydroxymethyl-2-oxo-ethyl)-5-(2-hydroxy-propionylamino)-2,4,6-triiodo-isophthalamide (from iopamidol) were identified.

Simulated solar photo-assisted decomposition of peroxymonosulfate. Radiation filtering and operational variables influence on the oxidation of aqueous bezafibrate.

This work investigates the potential of the combination of peroxymonosulfate (PMS) and simulated solar radiation (λ > 300 nm) to remove bezafibrate from aqueous solution. Different solar light filters indicate a higher removal efficiency as the wavelength range used moves to the more energetic region of the solar spectrum. The system PMS/Daylight (300-800 nm) eliminates bezafibrate (1 mg L-1) in less than 30 min under the best conditions used in this study (CPMS = 4·10-4 M) with no pH control (acidic pH). The efficiency of the process significantly improves under alkaline conditions (pH = 10), likely due to a higher PMS photolysis rate. Experiments conducted at different initial concentration of PMS and bezafibrate suggest first order regarding PMS and different from 1 in the case of bezafibrate. Intermediates generated at the beginning of the process have been tentatively identified to propose a hypothetical reaction pathway and to estimate their toxicity.