Effectiveness and potential of straw- and wood-based biochars for adsorption of imidazolium-type ionic liquids

Techniques for recovery and recycling of ionic liquids: A review

Due to beneficial properties like non-flammability, thermal stability, low melting point and low vapor pressure, ionic liquids (ILs) have gained great interest from engineers and researchers in the past decades to replace conventional solvents. The superior characteristics of ILs make them promising for applications in fields as wide-ranging as pharmaceuticals, foods, nanoparticles synthesis, catalysis, electrochemistry and so on. To alleviate the high cost and environmental impact of ILs, various technologies have been reported to recover and purify the used ILs, as well as recycling the ILs. The aim of this article is to overview the state-of-the-art research on the recovery and recycling technologies for ILs including membrane technology, distillation, extraction, aqueous two-phase system (ATPS) and adsorption. In addition, challenges and future perspectives on ILs recovery are discussed. This review is expected to provide valuable insights for developing effective and environmentally friendly recovery methods for ILs.

Preparation of wavy three-dimensional graphene-like biochar and its adsorption mechanism of embedded separation for dimethoate

In this study, graphene-like biochar (IZBC) was prepared by pyrolysis of wheat straw in the presence of catalyst and activator. The formation of graphene in IZBC could be divided into three stages: shell core generation, carburization, and carbon precipitation. When the pyrolysis temperatures were in the ranges of 500-600 ℃, 600-700 ℃, 700-800 ℃ and 800-900 ℃, 17%, 32%, 13% and 38% of graphene were produced, respectively. The contribution ratios of graphene by FeCl₃, ZnCl₂ and HCl were 64%, 23% and 13%, respectively. Moreover, IZBC was filled with porous wavy three-dimensional graphene nanosheets that enabled self-aggregation to be effectively prevented, which was superior to the striped two-dimensional structure. The adsorption of IZBC for dimethoate was a spontaneous exothermic reaction with the adsorption capacity of 980 μmol/g, which was consistent with the pseudo-second-order and intraparticle diffusion models. The adsorption was inhibited by coexisting cations, anions, and humic acid in water. Dimethoate was adsorbed on graphene through embedded separation, with pore filling, cation-π and electrostatic attraction as the key driving forces. In addition, the adsorbed saturated IZBC could be effectively regenerated for many times by 2 mol/L HCl solution.

Ionic liquid recovery and recycling via electrodialysis in biomass processing: An economical assessment

Extravagant price and lack of high-efficiency recovery technology limited scale-up utilization of ionic liquids. Ionic liquids recovery with electrodialysis-based techniques has caught wide concern due to membrane-based characteristic. Economical assessment for electrodialysis-based ionic liquid recovery and recycling in biomass processing was performed by determining influence of equipment-related and financial-related factors with sensitivity analysis for each factor. Overall recovery cost of 1-ethyl-3-methylimidazolium acetate, choline acetate, 1-butyl-3-methylimidazolium hydrogen sulphate and 1-ethyl-3-methylimidazolium hydrogen sulfate varied within 0.75-1.96 $/Kg, 0.99-3.00 $/Kg, 1.37-2.74 $/Kg and 1.15-2.89 $/Kg when factors changed within investigated range. Fold of membrane cost, factor of membrane stack cost, factor of auxiliary equipment cost, factor of annual maintenance cost and annual interest rate of loan were positively related with recovery cost. While percentage of annual elapsed time and loan period were negatively correlated with recovery cost. Economical assessment confirmed economic efficiency of electrodialysis for ionic liquids recovery and recycling in biomass processing.

Effects of different feedstocks-based biochar on soil remediation: A review

As a promising amendment, biochar has excellent characteristics and can be used as a remediation agent for diverse types of soil pollution. Biochar is mostly made from agricultural wastes, forestry wastes, and biosolids (eg, sewage sludge), but not all the biochar has the same performance in the improvement of soil quality. There is a lack of guidelines devoted to the selection of biochar to be used for different types of soil pollution, and this can undermine the remediation efficiency. To shed light on this sensitive issue, this review focus on the following aspects, (i) how feedstocks affect biochar properties, (ii) the effects of biochar on heavy metals and organic pollutants in soil, and (iii) the impact on greenhouse gas emissions from soil. Generally, the biochars produced from crop residue and woody biomass which are composed of lignin, cellulose, and hemicellulose are more suitable for organic pollution remediation and greenhouse gas emission reduction, while biochar with high ash content are more suitable for cationic organic pollutant and heavy metal pollution (manure and sludge, etc.). Additionally, the effect of biochar on soil microorganisms shows that gram-negative bacteria in soil tend to use WB biochar with high lignin content, while biochar from OW (rich in P, K, Mg, and other nutrients) is more able to promote enzyme activity. Finally, our recommendations on feedstocks selection are presented in the form of a flow diagram, which is precisely intended to be used as a support for decisions on the crucial proportioning conditions to be selected for the preparation of biochar having specific properties and to maximize its efficiency in pollution control.

An environmental-friendly approach to remove cyanide in gold smelting pulp by chlorination aided and corncob biochar: Performance and mechanisms

In this study, a new process was developed using ClO^- and corncob biochar (CB) combined with HAS (a stabilizer) to remove cyanide from gold smelting pulp. The Box-Behnken design was employed to optimize the doses of treatment reagents during cyanide removal. Results showed that the optimal doses of the three reagents were as follows: ClO^- dose of 20 mg/g dry solid (DS), CB dose of 22 mg/g DS, and an HAS dose of is 24 mg/g DS. The cyanide concentration in the filtrate was the lowest (0.114 mg/L), with a 98.36% removal efficiency after a contact time of 2 h at 25 °C under optimized conditions. Compared with those of ClO^- and HAS, it was found that the dose of biochar was the dominant factor influencing cyanide removal. Batch sorption experiments of cyanide to biochar indicated that the Langmuir isotherm model fit the sorption data, and the maximum cyanide sorption capacity was expected to be 2.57 ± 0.06 mg/g. Density functional theory (DFT) calculations (interaction energy was -74.42 kcal/mol) indicated that the adsorption peak resulted from cation-π interactions between the cyanide and CB. This study could lead to a novel environmental-friendly approach for the removal of cyanide from gold smelting pulp.

Insight into the role of ion-pairing in the adsorption of imidazolium derivative-based ionic liquids by activated carbon

The association of the cation and anion of ionic liquids (ILs) dominates the absorbability of ILs by activated carbon (AC). Nevertheless, the mechanism behind the role of ion-pairs is largely unknown. In this study, the adsorption of a series of imidazolium derivative-based ILs by AC was involved in response to the octanol-water partition coefficient (K_OW), hydrogen bonding acidity (α), ion-pair binding constants (K_IP), binding energy of ion-pairs (E_binding) and density functional theory (DFT) calculation of ILs. A significant positive correlation between lg K_OW and K_d and between K_IP and lg K_OW was observed (p < 0.05). However, both E_binding and α was inversely proportional to K_IP. Hence, the substitution of oxygen-containing functional groups, such as carboxyethyl, 1-methoxyethyl, and 1-(ethoxycarbonyl)methyl, on imidazolium ring enhanced the hydrogen bond interaction with water molecules and then weakened the binding of imidazolium cation and [NTf₂]^-, thereby reducing the adsorption of ILs to AC. DFT calculation further revealed that the polar substitution improved the electron density and electronegativity of imidazolium skeleton. By contrast, the ILs functionalized with non-polar groups (e.g., butyl, allyl, and benzyl) generally displayed high K_IP values and low α values. Consequently, the formation of hydrogen bond between the oxygen-containing functional groups of IL cation and water would facilitate the dissociation of IL ion-pairs and then decrease the adsorption of ILs on AC.

Ionic liquid-based water treatment technologies for organic pollutants: Current status and future prospects of ionic liquid mediated technologies

Water scarcity motivated the scientific researcher to develop efficient technologies for the wastewater treatment for its reuse. Ionic liquids have been applied to many industrial and analytical separation processes, but their applications in the wastewater treatment, especially in the removal of organic pollutants, are still not well explored. Potential applications of ionic liquids include solvent extraction, solvent membrane technologies and ionic liquid-modified materials that are mainly used as adsorbents. Aforementioned technologies have been examined for the abatement of phenol, chloro- and nitrophenols, toluene, bisphenol A, phthalates, pesticides, dyes, and pharmaceuticals etc. Present review enlightens the application of different ionic liquids in wastewater treatment and suggests the versatility of ionic liquids in the development of rapid, effective and selective removal processes for the variety of organic pollutants. Implementation of ionic liquid based technologies for wastewater treatment have lots of challenges including the selection of non-hazardous ionic liquids, technological applications, high testing requirements for individual uses and scaling-up of the entire pollutant removal, disposal, and ionic liquid regeneration process. Toxicity assessment of water soluble ionic liquids (ILs) is the major issue due to the widespread application of ILs and hence more exposure of environment by ILs. The development of effective technologies for the recovery/treatment of wastewater contaminated with ILs is necessary from the environmental point of view. Furthermore, the cost factor is the major challenge associated with ionic liquid-based technologies.

The adsorption, regeneration and engineering applications of biochar for removal organic pollutants: A review

In recent years, with the continuous development of industry and agriculture, the content of organic pollutants in the environment has been increasing, which has caused serious pollution to the environment. Adsorption has proven to be an effective and economically viable method of removing organic contaminants. Since biochar has many advantages such as various types of raw materials, low cost, and recyclability, it can achieve the effect of turning waste into treasure when used for environmental treatment. This paper summarizes the source and production of biochar, points out its research status in the removal of organic pollutants, expounds its adsorption mechanism on organic pollutants, introduces the relevant adsorption parameters, summarizes its regeneration methods, studies its application of engineering, and finally analyses of benefits and describes the development prospects.

Recovery and purification of ionic liquids from solutions: a review

With low melting point, extremely low vapor pressure and non-flammability, ionic liquids have been attracting much attention from academic and industrial fields. Great efforts have been made to facilitate their applications in catalytic processes, extraction, desulfurization, gas separation, hydrogenation, electronic manufacturing, etc. To reduce the cost and environmental effects, different technologies have been proposed to recover the ionic liquids from different solutions after their application. This review is mainly focused on the recent advances of the recovery and purification of ionic liquids from solutions. Several methods for recovery of ionic liquids including distillation, extraction, adsorption, membrane separation, aqueous two-phase extraction, crystallization and external force field separation, are introduced and discussed systematically. Some industrial applications of ionic liquid recovery and purification methods are selected for discussion. Additionally, considerations on the combined design of different methods and process optimization have also been touched on to provide potential insights for future development of ionic liquid recovery and purification.

The environmental characteristics and applications of biochar

The environmental deterioration is in a grave situation, and it is urgent to restore the environment. Biochar is a carbon-rich pyrolysis product of feedstock, which has aroused extensive concern due to its broad application potential for getting rid of pollutants and rehabilitating environment. This review generalizes three aspects on biochar, including production and properties, applications and mechanisms, and its modifications. Firstly, the production and characteristics have been summarized, because the practical applications of biochar are highly related to the special characteristics of biochar. Secondly, this paper outlines the latest applications of biochar for environmental remediation, and further provides a critical review on the application mechanisms in environmental restoration. Thirdly, the modification methods and applications of modified biochar are summarized, and all of the ways can be classified into two types: pretreatment of feedstock and modification of primitive biochar. Furthermore, the possible improvements and outlooks of applying biochar in environmental remediation are proposed. This review provides useful information for the application of biochar in environmental restoration.

Counteranion-dependent sorption of imidazolium- and benzimidazolium-based ionic liquids by soot

Sorption of ionic liquids (ILs) to soil and porous materials as affected by anions was observed, but scarce effort has been focused on addressing the role of counteranions in sorption and the associated underlying mechanisms. In this work, two series of 1-butyl-3-methylimidazolium- (Bmim-) and N-butyl, methyl-benzimidazolium-based (Bmbim-based) ILs coupled with different counteranions were prepared to investigate the effect of anions on IL sorption by soot. The octanol-water partition coefficient (K_ow) and the ion-pair formation constant at infinite dilution in water (K_IP^°) of ILs were independently measured to explore the contribution of counteranion-dependent hydrophobicity and ion-pair. A wide range of sorption coefficients (K_d) of ILs were achieved with values varying from 59.8 to 344.3 L·kg^-1 for Bmbim-based ILs and from 253.4 to 489.7 L kg^-1 for Bmbim-based ILs. Compared with other anions, bis(trifluoromethanesulphonyl)imide ([Tf₂N]^-) and hexafluorophosphate ([PF₆]^-) exhibit tighter association with IL cations in aqueous solution due to their larger K_ow and higher K_IP^°. Positive linear relationships between log K_IP^° and K_d and between log K_ow and K_d evidenced that the counteranion-dependent sorption of ILs relies on the association strengths of IL cations and counteranions, which further influence the hydrophobicity/hydrophilicity of ion pairs. Compared with that of strongly coordinating anions (such as [CH₃SO₃]^-, [CF₃COO]^-, [BF₄]^-, [CF₃SO₃]^-, and [Cl]^-), the addition of weakly coordinating anions (such as [Tf₂N]^- and [PF₆]^-) in solution contributes to markedly large sorption enhancement of ILs. Consequently, the contribution of different counteranions on IL sorption is essentially based on the formation of ion pair with different K_IP^° and K_ow in aqueous solution.

Phytotoxicity of ionic liquids with different structures on wheat seedlings and evaluation of their toxicity attenuation at the presence of modified biochar by adsorption effect

The toxic effects of eight common ionic liquids (ILs) on wheat seedlings was evaluated with specific emphasis on the influence of concentration range, anion species and cation chain length of ILs. The growth of wheat seeds was significantly inhibited by ILs, especially under higher concentration, presence of the fluoride anion and the longer alkyl chain length of the cation. The modified biochar (PB-K-N) efficiently removed the ILs from aqueous solutions, the order of the adsorption capacities was as follows: [Bmim]OAc [Bmim]C₇H₅O₂ [Bmim]BF₄ [Bmim]Br, [Domim]Br [BPy]Br [Omim]Br [Bmim]Br [Emim]Br. Furthermore, the wheat growth of all ILs groups except [Bmim]BF₄ group in the presence of PB-K-N was also similar to that of the control groups, which clearly demonstrated that PB-K-N could decrease or alleviate toxicity of ILs toward wheat by adsorption effect. Therefore, the biochar application was effective in improving plant resistance to ILs stress by adsorption, to reduce the phytotoxicity of ILs and provide an alternative approach for the utilization of PB-K-N in ILs contaminated water and soils.

Transport of imidazolium-based ionic liquids with different anion/cation species in sand/soil columns

Ionic liquids (ILs) have been widely used as environmentally friendly solvents to replace volatile organic solvents in the chemistry industries. They have a high water solubility and potential risk to organisms in the soil-water environment. At present, most studies focused on the batch sorption of ILs in soil and neglected the investigation of IL transports in soil, which results in a lack of understanding of the structure-dependent mobility of ILs in the environment. Laboratory-scale sand/soil column experiments were performed to study the transport of imidazolium-based ILs, such as [C₄mim][OTF], [C₄mim][TOS], [C₄mim][MeSO₃], [C₄mim][BF₄], [C₂mim][BF₄], and [C₆mim][BF₄] including different counteranions and alkyl chain lengths of IL cations. Batch experiments were also carried out to compare the difference of sorption distribution coefficient (K_d) between the batch and column experiments. A one-dimensional convective-dispersive model using CXTFIT code was created based on the measured breakthrough curves (BTCs) to estimate the column transport parameters. For the anion, [BF₄^-], the K_d of ILs in both batch and column experiments increased with increasing alkyl chain lengths of the IL cation. In batch tests, counteranions showed no influence on the K_d of [C₄mim][OTF], [C₄mim][TOS], [C₄mim][MeSO₃], [C₄mim][BF₄], [C₂mim][BF₄], and [C₆mim][BF₄]. However, in column tests, the BTCs of 1-butyl-3-methylimidazolium-based ILs were anion dependent as evidenced by the change of retardation factor (R) for different counteranions. Furthermore, the effects of transport distance (11cm, 15cm, 19cm, and 24cm) on the mobility of ILs were estimated. The longer distances signified an increase in the contact time and more binding sites for ILs and therefore, the smoother shapes of BTCs in column experiments.

Roles of polar groups and aromatic structures of biochar in 1-methyl-3-octylimidazolium chloride ionic liquid adsorption: pH effect and thermodynamics study

Adsorption mechanisms of 1-methyl-3-octylimidazolium chloride ([OMIM]Cl) on rice straw-derived biochars produced at 400, 500, and 700 °C (referred as RB400, RB500, and RB700, respectively) were evaluated. Adsorption affinity followed the order of RB700 > RB400 > RB500. Electrostatic attraction and hydrogen bond controlled adsorption of [OMIM]Cl on RB400, while π-π EDA interaction between [OMIM]Cl and the aromatic rings of biochar dominated adsorption of RB500 and RB700. With increasing solution pH, -COOH and -OH on biochar became deprotonated. Consequently, [OMIM]Cl binding to these sites changed from hydrogen bond to electrostatic attraction. Adsorption capacity of [OMIM]Cl increased with increasing pH during the adsorption process. Solid concentration induced by -OH of [OMIM]Cl was higher than that of -COOH. Thermodynamics study indicated that adsorption process was spontaneous and endothermic. ∆H ⁰ values indicated that [OMIM]Cl adsorption on biochars was a physisorption.

Facile preparation of 3D GO/CNCs composite with adsorption performance towards [BMIM][Cl] from aqueous solution

A novel three-dimensional crumpled graphene oxide/cellulose nanocrystals (GO/CNCs) composite was successfully synthesized and firstly used as adsorbent for the removal of ionic liquid [BMIM][Cl] from aqueous solution. The 3D crumpled structure and abundant oxygen of the functional groups on GO/CNCs composite can provide more chance for the sorption of [BMIM][Cl] compared with CNCs and GO, respectively. Therefore, a series of batch experiments were carried out to evaluate the adsorptive property of 3D GO/CNCs composite towards [BMIM][Cl], such as the GO mass content, the pH value and contact time. The results showed that pseudo-second-order kinetic model and Eovlich model were well fitted with the sorption kinetic. The isotherm adsorption data indicated that it was better described by Langmuir model, with the maximum sorption capacity of 0.455mmol/g. This work provides a facile method for the preparation of 3D structure adsorbent from graphene oxide and cellulose nanocrystals which has high adsorption capacity of [BMIM][Cl] in aqueous solution.

Environmental Impact of Ionic Liquids: Recent Advances in (Eco)toxicology and (Bio)degradability

This Review aims to integrate the most recent and pertinent data available on the (bio)degradability and toxicity of ionic liquids for global and critical analysis and on the conscious use of these compounds on a large scale thereafter. The integrated data will enable focus on the recognition of toxicophores and on the way the community has been dealing with them, with the aim to obtain greener and safer ionic liquids. Also, an update of the most recent biotic and abiotic methods developed to overcome some of these challenging issues will be presented. The review structure aims to present a potential sequence of events that can occur upon discharging ionic liquids into the environment and the potential long-term consequences.

Mesoporous activated carbon prepared from NaOH activation of rattan (Lacosperma secundiflorum) hydrochar for methylene blue removal

Hydrothermal carbonization of biomass wastes presents a promising step in the production of cost-effective activated carbon. In the present work, mesoporous activated carbon (HAC) was prepared by the hydrothermal carbonization of rattan furniture wastes followed by NaOH activation. The textural and morphological characteristics, along with adsorption performance of prepared HAC toward methylene blue (MB) dye, were evaluated. The effects of common adsorption variables on performance resulted in a removal efficiency of 96% for the MB sample at initial concentration of 25mg/L, solution pH of 7, 30°C, and 8h. The Langmuir equation showed the best isotherm data correlation, with a maximum uptake of 359mg/g. The adsorbed amount versus time data was well fitted by a pseudo-second order kinetic model. The prepared HAC with a high surface area of 1135m²/g and an average pore size distribution of 35.5Å could be an efficient adsorbent for treatment of synthetic dyes in wastewaters.