Role of biochar surface characteristics in the adsorption of aromatic compounds: Pore structure and functional groups

Biochar (BC) are widely used as highly efficient adsorbents to alleviate aromatics-based contaminants due to their ease of preparation, wide availability, and high sustainability. The surface properties of BCs usually vary greatly due to their complex chemical constituents and different preparation processes and are reflected in the values of parameters such as the specific surface area (SSA), pore volume/size, and surface functional groups (SFGs). The effects of SSA and pore volume/size on the adsorption of aromatics have been widely reported. However, the corresponding mechanisms of BC SFGs towards aromatics adsorption remains unclear as the compositions of the SFGs are usually complex and hard to determine. To address in this gap in the literature, this review introduces a new perspective on the adsorption mechanisms of aromatics. Through collecting previously-reported results, the parameters logP (logarithm of the K_ow), polar surface area, and the positive/negative charges were carefully calculated using ChemDraw 3D, which allowed the hydrophobicity/hydrophilicity properties, electron donor-acceptor interactions, H-bonding, and electrostatic interactions between SFGs and aromatics-based contaminates to be inferred intuitively. These predictions were consistent with the reported results and showed that tailor-made BCs can be designed according to the molecular weights, chemical structures, and polarities of the target aromatics. Overall, this review provides new insight into predicting the physicochemical properties of BCs through revealing the relationship between SFGs and adsorbates, which may provide useful guidance for the preparing of highly-efficient, functional BCs for the adsorption of aromatics.     

Mechanism of adsorption of different humic acid fractions on mesoporous activated carbons with basic surface characteristics

The effects of the humic acid (HA) nature and the activated carbon (AC) surface chemistry on the effectiveness of HA removal were investigated. Brown (BHA) and gray (GHA) humic acid fractions of different structure and physicochemical properties were tested in the adsorption process using mesoporous ACs. The modification of chemical structure and surface properties of AC was achieved by ammonization (AC/N) and hydrogen treatment (AC/H). Both approaches led to a decrease in the oxygen content followed by an increase in the carbon basicity, maintaining the porous texture of AC nearly unaltered. Over twice higher removal degree of BHA and GHA was observed for the modified ACs. The kinetics of adsorption of HA fractions have been discussed using the pseudo-second-order model and the intraparticle diffusion model. All ACs showed a higher adsorption capacity toward BHA compared to GHA, which is mainly attributed to the lower molecular weight of BHA. The shape of the equilibrium isotherms indicates a strong competition between water and HA molecules for adsorption sites of the carbon surface.     

Microcolumn test and model analysis of activated carbon adsorption of dissolved organic matter after precoagulation: effects of pH and pore size distribution

Microcolumn adsorption experiments were conducted to generate breakthrough profiles of dissolved organic matter (DOM) remaining after coagulation treatment of a naturally colored surface water for three coal-based activated carbons (ACs) and four water pH levels. A plug-flow homogeneous surface diffusion model was applied to determine the intraparticle surface diffusivities of the DOM at different AC-pH combinations. It was found that, for all three ACs, the removal of DOM increased as pH decreased and the increasing extent changed with the ACs used. The pH dependency of the column performance seemed to be attributed more apparently to pH's capability in changing the zeta potential of AC particles. In addition, at all pH levels, the column performance varied markedly with the ACs used. Correlation analyses of the accumulated amounts of DOM onto all three ACs with corresponding pore volumes in several divided pore size regions clearly indicated that pores with sizes 30-100 A were more effective in adsorbing organic macromolecules. Furthermore, based on model simulations, the sensitivity of bed performance to equilibrium and kinetic parameters was quantified by conducting variance analyses with a four-way classification method.     

Equilibrium,kinetics and thermodynamics study of phenols adsorption onto activated carbon obtained from lignocellulosic material (<Emphasis Type="Italic">Eucalyptus Globulus labill</Emphasis> seed)

Activated carbon was prepared from lignocellulosic material (Eucalyptus Globulus labill seed) by chemical activation with ZnCl₂ at two different concentrations (10 and 25 % m/v) named ACS25 and ACS10. The textural characteristics of the activated carbons (ACs) were determined by N₂ adsorption isotherms; these exhibit B.E.T. surface areas of 250 and 300 m² g^?1 for ACS25 and ACS10, respectively, with micropore volume contents of 0.140 and 0.125 cm³ g^?1 in the same order. In addition, the FTIR and Boehm methods were conducted for the chemical characterisation of ACs, where many groups with basic character were found, which favours the adsorption of phenols. The prepared carbonaceous adsorbents were used in the adsorption of wide pollutants monosubstituted phenol derivatives: phenol, 4-nitrophenol and 4-chlorophenol. The effect of temperature on the thermodynamics, kinetic and equilibrium of phenols adsorption on ACs was thoroughly examined. The adsorption kinetics adjusted properly for a pseudo-second-order kinetic model. However, the Elovich model (chemisorption) confirms that phenols adsorption did not occur via the sharing of electrons between the phenolic ring and basal plane of ACs because is not properly adjusted, so the process is given by physisorption. The thermodynamic parameters [i.e. Gibbs free energy change (ΔG°), enthalpy change (ΔH°) and entropy change (ΔS°)] were also evaluated. The overall adsorption process was exothermic and spontaneous in nature. The values found in the thermodynamic study, confirm that the adsorption process corresponds to a clearly physical process.     

Critical evaluation and comparison of enrichment efficiency of multi-walled carbon nanotubes, C18 silica and activated carbon towards some pesticides from environmental waters

In this work, optimization of multi-residue solid phase extraction (SPE) procedures coupled with high-performance liquid chromatography for the determination of Propoxur, Atrazine and Methidathion from environmental waters is reported. Three different sorbents were used in this work: multi-walled carbon nanotubes (MWCNTs), C18 silica and activated carbon (AC). The three optimized SPE procedures were compared in terms of analytical performance, application to environmental waters, cartridge re-use, adsorption capacity and cost of adsorbent. Although the adsorption capacity of MWCNT was larger than AC and C18, however, the analytical performance of AC could be made close to the other sorbents by appropriate optimization of the SPE procedures. A sample of AC was then oxidized with various oxidizing agents to show that ACs of various surface properties has different enrichment efficiencies. Thus researchers are advised to try AC of various surface properties in SPE of pollutants prior to using expensive sorbents (such as MWCNT and C18 silica).     

Adsorption of mercury from single and multicomponent metal systems on activated carbon developed from cherry stones

The adsorption of mercury from a single/multi-solute aqueous solution by activated carbon (AC) prepared from cherry stones (CS) by chemical activation with H₃PO₄, ZnCl₂ or KOH is studied. Three series of AC (i.e., P, H₃PO₄; Z, ZnCl₂; K, KOH) were prepared by controlling the impregnation ratio and carbonization temperature. The textural characterization of AC was carried out by gas adsorption, mercury porosimetry and density measurements. The surface chemistry was analyzed by the pH of the point of zero charge (pH_zpc), FT-IR spectroscopy and Boehm’s method. Experiments of mercury adsorption were conducted by the batch method, using aqueous solutions of mercury and of mercury, cadmium and zinc without pH adjustment. The ACs possess a wide range of pore volumes and sizes. Their microporosity is usually well developed. The meso- and macropore volumes are higher for the P carbons and K carbons, respectively. BET surface areas as a rule range between 1000 and 2000 m²?g^?1. The pH_zpc is much lower for the P carbons. The content of acidic oxygen surface groups is lower for the K carbons, whereas the content of basic groups is higher for these carbons. The kinetics of the adsorption process of mercury is faster for ACs with high volumes of large size pores. However, the surface groups have a marked unfavorable influence on the kinetics. The pseudo-second order rate constant (k₂×10^?3, g/mol?h) is higher by the order Z-4-800 (67.69)>K-3-800 (43.45)>P-3.44-400 (36.98). The incorporation of zinc and cadmium to the mercury solution usually decelerates the adsorption process for the P carbons and Z carbons and accelerates it for the K carbons. The amount adsorbed of mercury is much larger for the K carbons than for the other ACs. For the Z carbons, competition effects of zinc and cadmium on the adsorption of mercury are negligible, which indicates that mercury adsorbs specifically on surface active sites of these adsorbents.     

Influence of KMnO4 oxidation on the electrochemical performance of pitch-based activated carbons

In this work, activated carbons (ACs) are obtained from petroleum pitch by the combination of a chemical treatment with different potassium permanganate (KMnO₄) amounts, i.e., 0, 0.5, 1.0, and 2.0 g, and a chemical activation with KOH at a constant KOH/pitch ratio of 3/1. The effects of the chemical activating agent on the surface morphology and porosity are evaluated with scanning electron microscopy and N₂ adsorption isotherms at 77 K, respectively. The specific surface area of the pitch-based ACs is increased with increasing the amount of KMnO₄ pre-treatment and showed the highest value of 2,334 m² g^?1 at 2 g of KMnO₄ amount. The electrochemical performance of AC electrodes is examined by cyclic voltammetry and galvanostatic charge/discharge characteristics in 6 M KOH electrolyte. Among the prepared ACs, 2.0 K-ACs possesses a specific capacitance as high as 237 F g^?1 and showed excellent electrochemical performance due to its suitable porous structure and low interface resistance.     

Process Parameters Optimization,Characterization, and Application of KOH-Activated Norway Spruce Bark Graphitic Biochars for Efficient Azo Dye Adsorption

In this work, Norway spruce bark was used as a precursor to prepare activated biochars (BCs) via chemical activation with potassium hydroxide (KOH) as a chemical activator. A Box–Behnken design (BBD) was conducted to evaluate and identify the optimal conditions to reach high specific surface area and high mass yield of BC samples. The studied BC preparation parameters and their levels were as follows: pyrolysis temperature (700, 800, and 900 °C), holding time (1, 2, and 3 h), and ratio of the biomass: chemical activator of 1: 1, 1.5, and 2. The planned BBD yielded BC with extremely high SSA values, up to 2209 m²·g⁻¹. In addition, the BCs were physiochemically characterized, and the results indicated that the BCs exhibited disordered carbon structures and presented a high quantity of O-bearing functional groups on their surfaces, which might improve their adsorption performance towards organic pollutant removal. The BC with the highest SSA value was then employed as an adsorbent to remove Evans blue dye (EB) and colorful effluents. The kinetic study followed a general-order (GO) model, as the most suitable model to describe the experimental data, while the Redlich–Peterson model fitted the equilibrium data better. The EB adsorption capacity was 396.1 mg·g⁻¹. The employment of the BC in the treatment of synthetic effluents, with several dyes and other organic and inorganic compounds, returned a high percentage of removal degree up to 87.7%. Desorption and cyclability tests showed that the biochar can be efficiently regenerated, maintaining an adsorption capacity of 75% after 4 adsorption–desorption cycles. The results of this work pointed out that Norway spruce bark indeed is a promising precursor for producing biochars with very promising properties.     

Combination effect of activated carbon with TiO2 for the photodegradation of binary pollutants at typical indoor air level

Previously we identified that residence time and water vapor are the vital parameters that affect the photodegradation of indoor air pollutants at parts-per-billion (ppb) levels using TiO₂. The presence of water vapor competes with pollutants for adsorption sites on TiO₂ thus reducing the pollutant removal rate. By immobilizing TiO₂ on activated carbon (AC), a satisfactory pollutant removal rate is achieved even at high water vapor levels. This study further examines the effect of TiO₂ immobilized on AC by co-injecting binary pollutants simultaneously since the presence of other pollutants might have an inhibition effect on the photocatalytic activity under different humidity levels. 200 parts-per-billion NO, 20 ppb BTEX and 200 ppb SO₂ were co-injected under different residence time and humidity levels to investigate their mutual effect on TiO₂ and TiO₂ immobilized on AC. Results showed that no significant pollutant removal difference was observed between TiO₂ and TiO₂ immobilized on AC at longer residence time. The presence of BTEX only reduced NO conversion by 5%. At evaluated humidity levels, however, a significant different pollutant removal rate was observed. The presence of BTEX reduced NO conversion by more than 10%. The use of TiO₂ on AC, reduced both the competition effect of the pollutant and water vapor on TiO₂. The inhibition effect of BTEX and SO₂ on NO conversion was significantly reduced when TiO₂ immobilized on AC compared to TiO₂ only. The by-product, NO₂, from the photodegradation of NO, was also reduced despite the presence of SO₂ and BTEX under high humidity level.     

Hydrothermal modification of carbon adsorbents

The effects of high-pressure autoclave treatments on porous structure and surface properties were studied for a variety of activated carbons (AC, synthetic and produced from plum stones) treated with water vapour, hydrogen peroxide (10–50%) or 10% aqueous ammonia solution at relatively low temperatures (250, 350, 400^○C). Surface and structural parameters of modified ACs were determined using nitrogen, water, ammonia and benzene adsorption isotherms. It was found that the effects of AC modification resulting in changes in their porous structure and surface chemistry depend on the kind of initial ACs, modifier type and concentration of modifier and treatment temperature. At the same conditions synthetic ACs are modified to a larger extent than ACs prepared using natural raw materials. Repeated treatment of a given carbon intensifies changes in its porous structure.     

Effects of activation conditions on the structural and adsorption characteristics of pinecones derived activated carbons

This study was conducted to understand and optimize the activation process for the production of a low-cost activated carbon (AC) using a renewable and plentiful biomass waste, pinecones. This was achieved by tracking the changes in porous structure, surface chemistry and adsorption properties of the AC produced using different activating agents, activation temperatures, holding times and heating rates. Generally, produced ACs were predominantly microporous with small external surface area and were different in terms of H/C and O/C ratios. Study of Pb²⁺ cations adsorption on these samples proved the high affinity of the pinecones derived ACs to this cation. The best adsorption behaviour was recognized in sample prepared by impregnation with H₃PO₄ at weight ratio of 2, then heating at 400?°C for 2?h at 5?°C/min heating rate. This sample possessed the highest BET surface area (1335 m²/g). The adsorption process obeyed the pseudo-first-order and Freundlich model slightly better than the pseudo-second-order kinetics and Langmuir model. The high Langmuir maximum adsorption capacity of 418?mg/g supports the applicability of the produced AC for the removal of Pb²⁺ cations from wastewater.     

孔结构对活性炭吸附水溶液中铅离子的影响

选取三种表面化学性质相近的活性炭(AC),通过等温吸附实验考察活性炭对水溶液中铅离子的吸附性能,利用扫描电子显微镜(SEM)观察活性炭的表面微观形貌,通过低温(77 K)液氮吸附测定活性炭的比表面积和孔容,并分别以密度泛函理论(DFT)和Barrett-Joyner-Halenda (BJH)法计算微孔和中孔的孔径分布.结果表明:选用的三种活性炭AC1、AC2、AC3在比表面积和总孔容上呈依次下降的趋势,但表面开放孔均匀分布的AC2,具有最高的饱和吸附量,孔结构类似颗粒堆积孔的AC3,具有与表面开放孔分布集中的AC1相近的饱和吸附量;通过对孔结构与吸附量的关联分析可知,在活性炭吸附铅离子的过程中, 0.4-0.6 nm的孔是有效吸附孔, 10.5-20.6 nm、20.6-55.6 nm、5.2-10.5 nm三个区间的孔则会对吸附产生阻碍作用.     

The removal of H2S derived from livestock farm on activated carbon modified by combinatory method of high-pressure hydrothermal method and impregnation method

H₂S is considered as the main gas pollutant from livestock farm and activated carbon (AC) is widely used as adsorbent for H₂S. This paper focuses on the influence of modification conditions and operation conditions on the H₂S adsorption performance on AC samples. The H₂S adsorption performance on modified AC (MAC) samples by single and combinatory method has been investigated. It is concluded that the MAC by combinatory method of high-pressure hydrothermal method followed by alkaline solution impregnation method could promote the H₂S adsorption performance remarkably. The H₂S adsorption performance differs with different operation conditions. Meanwhile the samples of fresh AC and exhausted AC have been characterized using BET, FTIR, TPD and Boehm titration method. The experimental results are confirmed that the characteristics of AC have significant influence on the adsorption ability for H₂S.     

Comparative study on composition, structure, and adsorption behavior of activated carbons derived from different synthetic waste polymers

The composition, structure, and adsorption behavior of activated carbons (ACs) derived from three different types of waste polymers, i.e., tire rubber (TR), polyvinyl chloride (PVC), and polyethyleneterephtalate (PET), by KOH activation were compared. The AC derived from PET exhibited the largest surface area (2831 m(2)/g) and pore volume (1.68 cm(3)/g) due to the homogenous aromatic composition of PET. The AC derived from PVC exhibited relatively lower surface area (2666 m(2)/g) but more narrowed pore size distribution (2-3 nm). The complex composition and high ash content of tire particles resulted in AC product with significantly lower surface area (398.5 m(2)/g) and heterogeneous pore width. Adsorption data of methylene blue (MB) were fitted well by Langmuir equation, indicating monolayer coverage on the ACs. The high oxygen content of PET-derived AC heavily affected its adsorption to MB and iodine. Due to the remarkable surface area and highly mesoporous structures, ACs based on both PET and PVC exhibited much higher adsorption capacities than that of TR and commercial coal-based AC (F400). This study demonstrates that the properties of ACs are highly dependent on their starting polymers and the potential of converting synthetic polymer waste into effective adsorbents for environmental remediation and cleanup.     

Development of activated carbon from vine shoots by physical and?chemical activation methods. Some insight into activation mechanisms

Activated carbons (ACs) are prepared from vine shoots (VS) by the method of physical activation in air, CO₂ and steam atmospheres and by the method of chemical activation with H₃PO₄, ZnCl₂ and KOH aqueous solutions. The ACs were characterized texturally by N₂ adsorption at −196 °C, mercury porosimetry, and density measurements. The method of chemical activation has been proved to be more effective than the method of physical activation to prepare ACs with a well-developed porosity. ACs with high micro- and mesopore volumes are prepared with ZnCl₂ and H₃PO₄. Using ZnCl₂, the volume of micropores is 0.62 cm³ g⁻¹ and the volume of mesopores is 0.81 cm³ g⁻¹. A greater development of macroporosity is obtained by KOH activation. The volume of macropores is as high as 1.13 cm³ g⁻¹ for the resulting AC. Yield of the process of preparation of the ACs is low for the method of chemical activation. Some insights into the performance of the activating agents in the activation process are provided.     

Surface modification of date palm activated carbonaceous materials for heavy metal removal and CO2 adsorption

In this study, high surface area activated carbon (AC) was prepared from a local palm tree (Phoenix Dactylifera) using a variety of metal carbonates activators and finally achieved an excellent S_BET of 2700 m²/g when Cs₂CO₃ was used as an activating agent at a temperature of 600 °C. Surface modification of AC was carried out using various nitrogen transporting agents, resulting in N-doped ACs with nitrogen content varying from 4.0 to 11.4 %, depending on the functionalizing agents and activators used. The bimodal (presence of micro- as well as meso-porosity) ACs with such excellent surface properties were studied for their CO₂ uptake capacity at two different temperatures (0 and 25 °C) by isotherms recorded at pressure 1 bar and showed a remarkable uptake ability of 3.52 mmol/g (at 25 °C) and 5.6 mmol/g (at 0 °C), respectively. Also, batch experiments with variable pH, contact time, adsorbate concentrations, adsorbent dose, and temperatures were evaluated to understand the mechanism of sorption phenomena of Cr(VI) and Pb(II) achieving > 99.9 % removal capacity by the prepared ACs. Depending on the heavy metal ions being investigated, it was revealed that the pH of the solution and the amount of adsorbent had a direct impact on the total adsorption ability. Nitrogen atoms doped into the carbon frameworks were found to enhance the adsorption in the case of Pb(II) while the removal of Cr(VI) appeared to be unaffected. Maximum adsorption for Cr(VI) was observed at pH 2 and was determined to follow Freundlich isotherm while that of Pb(II) was observed at pH 7 and follows Langmuir isotherm. Best adsorption was found at an adsorbate concentration of 10 ppm and an adsorbent dose of 10 g/L. Kinetic modeling parameters showed the applicability of pseudo-second-order model perfectly.     

Adsorption of oxygen-containing aromatics used in petrochemical,pharmaceutical and food industries by means of lignin based active carbons

The adsorption processes of three aromatic chemicals onto activated carbons (ACs) from aqueous solutions have been studied. Eucalyptus kraft lignin obtained from cellulose industry as a residual biomass has been used to prepare activated carbons by physical activation with CO₂. The influences of the activation time on the surface areas and pore volumes of the ACs were analyzed. The physicochemical properties and the surface chemical structure of the adsorbents have been studied by means of N₂ and CO₂ adsorption, ultimate analysis, XPS, TPD and SEM. XPS and TPD spectra of the ACs have suggested the presence of aromatic rings and carbon-oxygen functional groups in the solid surfaces. The potential use of the ACs for the removal of acetaminophen (paracetamol), salicylic acid and benzoic acid has been investigated at different pH, temperature and contact time. The adsorption equilibrium data have been correlated to Langmuir isotherm model. The thermodynamic study has been developed, the values of ΔH, ΔG, and ΔS have been calculated and they indicated that the processes are endothermic for acetaminophen and exothermic for salicylic and benzoic acids. The analysis of the kinetic experiments showed that the effective diffusivities are low; 10⁻¹² to 10⁻¹¹ cm²/s, and they are the corresponding to intraparticle mass transfer, which appears as the controlling step for the net adsorption processes.     

Powdered activated carbons as effective phases for bar adsorptive micro-extraction (BAμE) to monitor levels of triazinic herbicides in environmental water matrices

Bar adsorptive micro-extraction using three powdered activated carbons (ACs) as adsorbent phases followed by liquid desorption and high performance liquid chromatography with diode array detection (BAμE(ACs)-LD/HPLC-DAD), was developed to monitor triazinic herbicides (atrazine, simazine and terbutylazine) in environmental water matrices. ACs used present apparent surface areas around 1000 m² g⁻¹ with an important mesoporous volume and distinct surface chemistry characteristics (pH_PZC ranging from 6.5 to 10.4). The textural and surface chemistry properties of the ACs adsorbent phases were correlated with the analytical data for a better understanding of the overall enrichment process. Assays performed on 10 mL water samples spiked at the 10.0 μg L⁻¹ levels under optimized experimental conditions yielded recoveries around 100% for the three herbicides under study. The analytical performance showed good precision (RSD < 15.0%), convenient detection limits (≈0.1 μg L⁻¹) and suitable linearity (1.0-12.0 μg L⁻¹) with good correlation coefficients (r² > 0.9914). By using the standard addition method, the application of the present method on real water matrices, such as surface water and wastewater, allowed very good performances at the trace level. The proposed methodology proved to be a suitable sorptive extraction alternative for the analysis of priority pollutants with polar characteristics, showing to be easy to implement, reliable, sensitive and requiring a low sample volume to monitor triazinic compounds in water matrices.     

Surface structural and energetic heterogeneity of carbon materials prepared from corn grains using steam and H3PO4‐steam activations

The porous activated carbons (ACs) were prepared from corn grains through physical (steam) and chemical–physical (H₃PO₄‐steam) activations. The effects of steam activation temperature (700–900 °C) on pore development, surface roughness, and energetic heterogeneity were investigated in both activations. Also, the effect of prior carbonization on H₃PO₄‐steam activation was studied. The physical properties, surface fractal dimensions, and adsorption energy distributions of ACs were determined from nitrogen adsorption–desorption isotherm data. Both physical and chemical–physical activations show that the AC with higher surface area, relatively smoother surface, and energetically heterogeneous surface could be produced at a maximum steam activation temperature (900 °C). Copyright © 2015 John Wiley & Sons, Ltd.     

Experimental and Theoretical Study of the Effect of Moisture on Methane Adsorption and Desorption by Activated Carbon at 273.5 K

Adsorption and desorption of methane by activated carbon (AC) at constant temperature and at various pressures were investigated. The effect of moisture was also studied. A volumetric method was used, up to 40 bar, at a temperature of 273.5 K. Results of a dry AC sample were compared with those obtained from a moist sample and two different ACs with different physical and surface properties were used. As expected, the results showed that the existence of moisture, trapped in the AC pores, could lead to a decrease in the amount of methane adsorbed and a decrease in the amount of methane delivered during desorption. To model the experimental results, a large variety of adsorption isotherms were used. The regressed parameters for the adsorption isotherms were obtained using the experimental data generated in the present study. The accuracy of the results obtained from the different adsorption isotherms was favorably compared.