Role of surface functional groups in the adsorption behavior of microcystin-LR on graphene surfaces

Biochars are good adsorbents for removing microcystin from water but the molecular interactions responsible for microcystin adsorption are not understood. In this work, adsorption behavior of microcystin-LR (MC-LR) on three model surfaces that mimic biochar (bare graphene, graphene with ammonium, and with phosphate functional groups) is studied using well-tempered metadynamics in atomistic simulations. MC-LR is found to strongly adsorb on all the three surfaces. The adsorption free energy is most favorable for the bare graphene surface. On both bare graphene and the surface with phosphate groups, MC-LR adsorbs with its ring parallel to the surface. On the surface with ammonium groups, MC-LR adsorbs with its ring tilted with respect to the surface because of favorable Coulombic interactions between the ammonium groups and the glutamic acid in the MC-LR ring. On the bare graphene surface, the phenyl ring of the pendant Adda group shows a bimodal distribution with peaks at 0° and 40° with the surface normal, indicating that the phenyl ring forms π-π interactions with graphene in some adsorbed configurations. Such π-π interactions are not observed on the surfaces with ammonium and phosphate groups. Favorable adsorption free energy of MC-LR on the charge-neutral (bare graphene), positively charged (ammonium) and negatively charged (phosphate) surfaces suggest that the adsorption is dominated by van der Waals and hydrophobic interactions. Coulombic and π-π interactions affect the orientation of MC-LR in the adsorbed state.

Multiple roles of ferric chloride in preparing efficient magnetic hydrochar for sorption of methylene blue from water solutions

Highly efficient and cheap magnetic materials have application prospects in wastewater treatment. Herein, Fe₃O₄-loaded hydrochar (HC-Fe₃O₄) was obtained from hydrothermal carbonization (HTC) of bamboo with FeCl₃ and then added with FeCl₃ to form a magnetic sorbent via simple precipitation. The HC-Fe₃O₄ was characterized with various instruments. The characterizations show FeCl₃ plays at least two roles as a catalyst and an oxidant in HTC. The specific surface area of hydrochar enlarged from 39.9731 to 60.9887 m²·g^-1 after the addition of FeCl₃ during HTC, which showed FeCl₃ acted as a catalyst in HTC. XRD indicated Fe₃O₄ was formed by the structure of HC-Fe₃O₄, which indicated Fe(III) was reduced to Fe(II) during HTC. Sorption of methylene blue (MB) onto HC-Fe₃O₄ was better fitted by the Langmuir isotherm and pseudo-second-order kinetic models. Sorption is a spontaneous thermodynamic endothermic process and HC-Fe₃O₄ is easily separated by an applied magnetic field and reused.

Efficient removal of methylene blue by activated hydrochar prepared by hydrothermal carbonization and NaOH activation of sugarcane bagasse and phosphoric acid

Activated-hydrochar (AHC) derived from sugarcane bagasse was synthesized by hydrothermal carbonization (HTC) using phosphoric acid and sodium hydroxide (NaOH) as activators. The properties of AHC were systematically characterized by elemental analysis, BET, SEM, FTIR, XPS and zeta potential, and applied to evaluate the adsorption ability of methylene blue (MB) by batch adsorption tests. The MB adsorption isotherm and kinetics of AHC were well described by the Langmuir model and pseudo-second-order kinetic model. Characteristic analysis suggested electrostatic attraction, hydrogen bonding and π-π interactions were the main contributors to MB adsorption. Analysis of mass transfer mechanisms demonstrated the adsorption process towards MB by AHC involved intra-particle diffusion to some extent. Thermodynamic studies indicated MB adsorption was an endothermic, spontaneous process associated with a disorder increase at the solid-liquid interface. The maximum adsorption capacity of AHC for MB was 357.14 mg g-1 at 303 K. Thus, the combination of HTC in phosphoric acid and NaOH activation offered a facile, green and economical alternative for conversion of sugarcane bagasse into efficient adsorbents used in wastewater treatment.

Removal of anti-inflammatory drugs using activated carbon from agro-industrial origin: current advances in kinetics, isotherms, and thermodynamic studies

Nonsteroidal anti-inflammatory drugs (NSAIDs) are highly consumed around the world and consequently found as emerging pollutants in water; they are found in concentrations up to µg L−1 making their removal a priority. In this matter, adsorption is an efficient alternative for drug removal, so using activated carbon (AC) as an adsorbent is a highly explored subject. The current interest is to obtain AC from waste, for example, those of agro-industrial origin, reducing this way the overall costs of the process. Although information regarding the use of AC from agro-industrial origin in the removal of NSAIDs is limited, an exclusive compilation is required to understand the state of the art to date. This work aims to update information related to the adsorption of ibuprofen, diclofenac, and naproxen on agro-industrial AC, and it is focused on the period 2016–2021. It highlights the characteristics of agro-industrial AC responsible for efficient adsorption. Recent adsorption studies, including kinetics, isotherms, and thermodynamics, are analyzed and compared. Progress on removing NSAIDs from real wastewater is also presented and finally proposed adsorption mechanisms and costs related to these removal processes.

Can biochar and hydrochar be used as sustainable catalyst for persulfate activation?

Over the past decade, there has been a surge of interest in using char (hydrochar or biochar) derived from biomass as persulfate (PS, either peroxymonosulfate or peroxydisulfate) activator for anthropogenic pollutants removal. While extensive investigation showed that char could be used as a PS activator, its sustainability over prolonged application is equivocal. This review provides an assessment of the knowledge gap related to the sustainability of char as a PS activator. The desirable char properties for PS activation are identified, include the high specific surface area and favorable surface chemistry. Various synthesis strategies to obtain the desirable properties during biomass pre-treatment, hydrochar and biochar synthesis, and char post-treatment are discussed. Thereafter, factors related to the sustainability of employing char as a PS activator for anthropogenic pollutants removal are critically evaluated. Among the critical factors include performance uncertainty, competing adsorption process, char stability during PS activation, biomass precursor variation, scalability, and toxic components in char. Finally, some potential research directions are provided. Fulfilling the sustainability factors will provide opportunity to employ char as an economical and efficient catalyst for sustainable environmental remediation.

Low Temperature One-Pot Hydrothermal Carbonization of Corn Straw into Hydrochar for Adsorbing Cadmium (II) in Wastewater

Corn straw, a typical agricultural waste, was directly converted into hydrochar with a yield of 77.56% by hydrothermal carbonization at 140–230 °C for 2 h with a solid–liquid ratio of 1:20. The morphology and surface properties were characterized by elemental analysis, specific surface area and pore size analysis and Fourier transform infrared spectroscopy. The results showed that with the increase of hydrothermal reaction temperature, some physical and chemical properties such as the increase of hydrocarbon content, crystallinity, and specific surface area of hydrochar changed significantly. A series of chemical reactions such as dehydration, decarboxylation, and aromatization occurred in the hydrothermal carbonization process so that the prepared hydrochar had rich oxygen-containing functional groups (-HO, C-O-C, C=O) and unique porous structure made the hydrochar prepared at 170 °C had the best removal effect on Cd2+ in solution (5.84 mg/g). These specific conditions could remove Cd2+ and greatly improve the adsorption performance. The pseudo-second-order kinetic model and Freundlich isotherm model could better describe the adsorption behavior of Cd2+. Therefore, corn straw hydrochar as a potential adsorbent for removing Cd2+ from water.

Removal of aqueous Cr(VI) by Zn- and Al-modified hydrochar

Pristine hydrochar is a carbonaceous material that can sorb hexavalent chromium (Cr(VI)), a kind of toxic pollutants and difficult to removal, from aqueous solution but its capacity is limited. With the goal of improving this ability, two modified hydrochars were produced by co-hydrothermal carbonization (200 °C, 7h) of bamboo sawdust with zinc chloride (ZnCl₂) or aluminum chloride (AlCl₃). Compared to the pristine hydrochar, the ZnCl₂-and AlCl₃-modified hydrochars were more fully carbonized (higher C content and lower H/C) and had higher surface area (increased by 26 and 4.3 times, respectively) and larger pore volume (increased by 43 and 5.5 times, respectively). Due to these improved properties, the Cr(VI) maximum adsorption capacity (modeled via Langmuir isotherms) of ZnCl₂-and AlCl₃-modified hydrochar increased by 3.4 and 2.8 times, respectively. In addition, Cr(VI) adsorption kinetic of modified hydrochar was well fitted by the pseudo-second-order model. Cr sorption capacity increased at low pH and ion strengths, suggesting the potential roles of electrostatic interaction and ion exchange mechanisms. These results indicate that hydrochars modified by ZnCl₂ and AlCl₃ treatment are promising in environmental applications that require Cr(VI) removal.

Intriguing Carbon Flake Formation during Microwave-Assisted Hydrothermal Carbonization of Sodium Lignosulfonate

Elongated carbon structures, here denoted as carbon flakes (CF), are revealed after microwave-assisted hydrothermal carbonization of sodium lignosulfonate. The morphology of formed CF is investigated by transmission electron microscopy and atomic force microscopy. Interestingly, a wide range of length distributions (between 100 and 700 nm) and a relatively constant aspect ratio and thickness are observed, indicating structures clearly different from the carbon spheres commonly formed during hydrothermal carbonization of lignocellulosic biomass. Moreover, X-ray diffraction, Raman spectroscopy, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy provide further information of the chemical structure, which consist mainly of nanographitic domains with a high degree of defects such as oxygenated functional groups, hybridized sp3 carbon, and aliphatic side chains. Furthermore, new insights into the formation mechanisms are uncovered and the formation is speculated to proceed through the combined effect of microwave irradiation and a heterogeneous solid-solid conversion. The formed CF are anticipated as highly interesting products for a variety of material applications.

Solid residue and by-product yields from acid-catalysed conversion of poplar wood to levulinic acid

This study examines the yields of solid residue and by-product from the microwave-assisted acid hydrolysis of lignocellulosic poplar wood for levulinic acid production. The aim of this study was to optimise levulinic acid production via response surface methodology (RSM) and also investigate the effect of reaction conditions on other products such as furfural, solid residue, formic acid and acetic acid yields. A maximum theoretical levulinic acid yield of 62.1% (21.0 wt %) was predicted when reaction conditions were 188 °C, 126 min and 1.93 M sulphuric acid, with a corresponding solid residue yield of 59.2 wt %. Furfural from the hydrolysis of hemicellulose was found to have significantly degraded at the optimum levulinic acid yield conditions. The investigation of formic acid yields revealed lower formic acid yields than stoichiometrically expected, indicating the organic acid reactions under microwave-assisted hydrolysis of lignocellulose. The solid residue yields were found to increase significantly with increasing reaction time and temperature. The solid residue yields under all conditions exceeded that of levulinic acid and, therefore, should be considered a significant product alongside the high-value compounds. The solid residue was further examined using IR spectra, elemental analysis and XRF for potential applications. The overall results show that poplar wood has great potential to produce renewable chemicals, but also highlight all by-products must be considered during optimization.

Conductive Carbon Materials from the Hydrothermal Carbonization of Vineyard Residues for the Application in Electrochemical Double-Layer Capacitors (EDLCs) and Direct Carbon Fuel Cells (DCFCs)

This study investigates the production of bio-based carbon materials for energy storage and conversion devices based on two different vineyard residues (pruning, pomace) and cellulose as a model biomass. Three different char categories were produced via pyrolysis at 900 °C for 2 h (biochars, BC), hydrothermal carbonization (HTC) (at 220, 240 or 260 °C) with different reaction times (60, 120 or 300 min) (hydrochars, HC), or HTC plus pyrolysis (pyrolyzed hydrochars, PHC). Physicochemical, structural, and electrical properties of the chars were assessed by elemental and proximate analysis, gas adsorption surface analysis with N₂ and CO₂, compression ratio, bulk density, and electrical conductivity (EC) measurements. Thermogravimetric analysis allowed conclusions to be made about the thermochemical conversion processes. Taking into consideration the required material properties for the application in electrochemical double-layer capacitors (EDLC) or in a direct carbon fuel cell (DCFC), the suitability of the obtained materials for each application is discussed. Promising materials with surface areas up to 711 m² g^-1 and presence of microporosity have been produced. It is shown that HTC plus pyrolysis from cellulose and pruning leads to better properties regarding aromatic carbon structures, carbon content (>90 wt.%), EC (up to 179 S m^-1), and porosity compared to one-step treatments, resulting in suitable materials for an EDLC application. The one-step pyrolysis process and the resulting chars with lower carbon contents and low EC values between 51 and 56 S m^-1 are preferred for DCFC applications. To conclude, biomass potentials can be exploited by producing tailored biomass-derived carbon materials via different carbonization processes for a wide range of applications in the field of energy storage and conversion.

Hydrothermal carbonization of yard waste for solid bio-fuel production: Study on combustion kinetic, energy properties, grindability and flowability of hydrochar

Yard waste is either dumped or is being openly burned to get rid of it, instead of using it as a valuable renewable energy source. In this study, hydrothermal carbonization of yard waste was conducted to valorize it as a solid bio fuel, using a batch reactor. The effect of process parameter on yield, energy and physicochemical properties of the valorized solid bio fuel (hydrochar) was examined in this study by varying reaction temperature (160-200 °C for 2 h) and reaction time (2-24 h at 200 °C). The calorific value of hydrochar was within a range of 17.72-24.59 MJ/kg as compared to 15.37 MJ/kg for untreated yard waste. Hydrochar mass yield decreased from 78.6% at operating temperature - time of 160 °C -2 h to 45.6% at 200 °C -24 h. The plot of atomic ratios (H/C and O/C) demonstrates improvement in the coalification process which was mainly governed by decarboxylation and dehydration reactions. The grindability of the prepared hydrochar was comparable to that of coal. Hydrochar produced at lower reaction condition (160-200 °C at 2 h) have better flowability as compared to that produced at higher reaction condition (4-24 h at 200 °C). The reaction time longer than 12 h has a minimal effect on the yield, energy and physicochemical properties of hydrochar. Increasing reaction time and temperature improved the ignition and burnt temperature of hydrochar. All reaction condition has an energy ratio (energy output to energy input) of more than one making HTC process a net energy producer.

Hydrothermal Carbonization Kinetics of Lignocellulosic Agro-Wastes: Experimental Data and Modeling

Olive trimmings (OT) were used as feedstock for an in-depth experimental study on the reaction kinetics controlling hydrothermal carbonization (HTC). OT were hydrothermally carbonized for a residence time τ of up to 8 h at temperatures between 180 and 250 °C to systematically investigate the chemical and energy properties changes of hydrochars during HTC. Additional experiments at 120 and 150 °C at τ = 0 h were carried out to analyze the heat-up transient phase required to reach the HTC set-point temperature. Furthermore, an original HTC reaction kinetics model was developed. The HTC reaction pathway was described through a lumped model, in which biomass is converted into solid (distinguished between primary and secondary char), liquid, and gaseous products. The kinetics model, written in MATLABTM, was used in best fitting routines with HTC experimental data obtained using OT and two other agro-wastes previously tested: grape marc and Opuntia Ficus Indica. The HTC kinetics model effectively predicts carbon distribution among HTC products versus time with the thermal transient phase included; it represents an effective tool for R&D in the HTC field. Importantly, both modeling and experimental data suggest that already during the heat-up phase, biomass greatly carbonizes, in particular at the highest temperature tested of 250 °C.

Removal of methylene blue from aqueous solution by modified bamboo hydrochar

Four hydrochars (labeled as HC, AHC, MHC, and MAHC, respectively) were prepared by hydrothermal carbonization of bamboo with and without chlorane or followed by NaOH modification. Various techniques were adopted to characterize the physicochemical properties of hydrochars and the removal of methylene blue (MB) by these hydrochars from aqueous solution was investigated. The MB adsorption isotherm and kinetic onto MAHC and MHC can be preferably interpreted by Langmuir adsorption mode and the pseudo-second-order model, respectively. Thermodynamic parameters implied that adsorption was a spontaneous and exothermic process. The adsorption capacity of MAHC was 655.76 mg g^-1, which was 2-3 times compared to that of MHC (268.93 mg g^-1) at 303 K. The high adsorption capacity of MAHC for MB suggests that hydrothermal carbonization in acidic medium followed by alkaline treatment has the potential application to produce efficient MB adsorbents used in wastewater treatment.

Hydrochars from bamboo sawdust through acid assisted and two-stage hydrothermal carbonization for removal of two organics from aqueous solution

A series of bamboo hydrochars were prepared through acid-assisted and two-stage hydrothermal carbonization, characterized and evaluated for the adsorption of Congo red and 2-naphthol in aqueous solutions. The hydrochars have rough surfaces with BET surface areas of 6.77-57.74 m²/g and oxygen-rich functional groups. The additives in feed water revealed critical influences on the physical-chemical and adsorption properties of the hydrochars. The hydrochars can adsorb the two organics effectively, the highest adsorption capacities for Congo red and 2-naphthol are 90.51 and 72.93 mg/g, respectively, at 0.1 mg/mL and 298 K. The adsorption of the two organics on the selected hydrochars is a spontaneous and mainly physical adsorption process. 95% and 92% of the adsorption equilibrium could be accomplished in one hour for Congo red and 2-naphthol on the selected hydrochars, respectively. This study provides references for the production and application of hydrochars as efficient adsorbents in wastewater treatment.

A Novel Nitrogen Enriched Hydrochar Adsorbents Derived from Salix Biomass for Cr (VI) Adsorption

Hydrochars were prepared from Salix by hydrothermal carbonization, and characterized by FT-IR, ¹³C NMR, XPS, UV-vis, TG, SEM and BET techniques. The results showed that the hydrochars with molecular sieve-type open pore structure contained numbers of oxygen and nitrogen functional groups, which benefited the adsorption and diffusion of adsorbent Cr (VI). The hydrochar obtained from 26 h reaction (HC-26) was indicated an excellent adsorbent compared to the commercial activated carbon, and its maximum removal efficiency for Cr (VI) reaches up to 99.84% at pH 1. Langmuir´s model is well fitted the experimental equilibrium adsorption data of total Cr. The bath experiment results showed that Cr (VI) could be removed rapidly in the first 300 min. Furthermore, the adsorption kinetics process of HC-26 could be described by pseudo-second-order model. Based on the above results, HC-26 could be acted as a potential efficient adsorbent for removal of Cr (VI) from aqueous solution.

Influence of interfering anions on Cu2+ and Zn2+ ions removal on chestnut outer shell-derived hydrochars in aqueous solution

Hydrothermal carbonization method was used to produce different hydrochars from chestnut outer shell at various temperatures while resolving the environmental issues of agricultural bio-waste.