Environmental Research 200 (2021) 111492 Contents lists available at ScienceDirect Environmental Research journal homepage: www.elsevier.com/locate/envres Metal salt-modified biochars derived from agro-waste for effective congo red dye removal Dang Le Tri Nguyen a, b, Quach An Binh c, Xuan Cuong Nguyen d, e, **, Thi Thanh Huyen Nguyen d, e, Quang Nha Vo f, Trung Duong Nguyen f, Thi Cuc Phuong Tran g, Thi An Hang Nguyen h, Soo Young Kim i, ***, Thang Phan Nguyen j, Jaehan Bae j, Il Tae Kim j, ****, Quyet Van Le i, * a Division of Computational Physics, Institute for Computational Science, Ton Duc Thang University, Ho Chi Minh City, Viet Nam Faculty of Applied Sciences, Ton Duc Thang University, Ho Chi Minh City, Viet Nam c Department of Academic Affairs and Testing, Dong Nai Technology University, Dong Nai, Viet Nam d Laboratory of Energy and Environmental Science, Institute of Research and Development, Duy Tan University, Da Nang, 550000, Viet Nam e Faculty of Environmental and Chemical Engineering, Duy Tan University, Da Nang, 550000, Viet Nam f Department of Electrical Engineering, Hue University, Quang Tri Campus, Viet Nam g Faculty of Environmental Engineering Technology, Hue University, Quang Tri Campus, Viet Nam h Vietnam Japan University (VNU-VJU), Vietnam National University, Hanoi, Luu Huu Phuoc St., Nam Tu Liem Dist., Hanoi, 101000, Viet Nam i Department of Materials Science and Engineering, Korea University, 145, Anam-ro Seongbuk-gu, Seoul, 02841, Republic of Korea j Department of Chemical and Biological Engineering, Gachon University, Seongnam-si, Gyeonggi-do, 13120, South Korea b A R T I C L E I N F O A B S T R A C T Keywords: Adsorption Agro-waste Congo red Anionic dye Metal salt Modified biochar Anionic Congo red dye (CR) is not effectively removed by conventional adsorbents Three novel biochars derived from agro-waste (Acacia auriculiformis), modified with metal salts of FeCl3, AlCl3, and CaCl2 at 500 ◦ C pyrolysis have been developed to enhance CR treatment These biochars revealed significant differences in effluents compared to BC, which satisfied initial research expectations (P < 0.05) The salt concentration of M realized optimal biochars with the highest CR removal of 96.8%, for AlCl3-biochar and FeCl3-biochar and 70.8% for CaCl2-biochar The modified biochars were low in the specific surface area (137.25–380.78 m2 g− 1) compared normal biochar (393.15 m2 g− 1), had more heterogeneous particles and successfully integrated metal oxides on the surface The CR removal increased with a decrease in pH and increase in biochar dosage, which established an optimal point at an initial loading of 25 mg g− Maximum adsorption capacity achieved 130.0, 44.86, and 30.80 mg g− for BFe, BCa, and BAl, respectively As magnetic biochar, which is easily separated from the so lution and achieves a high adsorption capacity, FeCl3-biochar is the preferred biochar for CR treatment application Introduction Dyes have been widely used in the textile industry and their effluents have caused severe issues for the water environment and humans (Chandra, 2016; Saravanan et al., 2013) Several recommended methods are available for treating the dyes, including membrane filtration, advanced oxidation processes, photocatalytic technology (Lei et al., 2021), coagulation, and adsorption (Bulgariu et al., 2019; Suhas et al., 2016) Although each technology has its advantages and limitations, most of them are costly, complicated, and produce secondary pollution (Barka et al., 2011; Katheresan et al., 2018) Therefore, it is necessary to find and investigate dye removal methods that can overcome the dis advantages of the current technologies The adsorption approach is gaining interest for the decolorization of dyes due to their advantages such as simplicity, high proficiency, un * Corresponding author ** Corresponding author *** Corresponding author **** Corresponding author E-mail addresses: nguyenletridang@tdtu.edu.vn (D.L.T Nguyen), nguyenxuancuong4@duytan.edu.vn (X.C Nguyen), sooyoungkim@korea.ac.kr (S.Y Kim), itkim@gachon.ac.kr (I.T Kim), quyetbk88@korea.ac.kr (Q Van Le) https://doi.org/10.1016/j.envres.2021.111492 Received March 2021; Received in revised form 29 May 2021; Accepted 31 May 2021 Available online June 2021 0013-9351/© 2021 Elsevier Inc All rights reserved D.L.T Nguyen et al Environmental Research 200 (2021) 111492 2018), pine bark (Litefti et al., 2019), and wet-torrefied microalgal biochar (Yu et al., 2021) have been studied Advanced biochar-based methods have also investigated to enhance the CR removal such as hybrid treatment system of biochar adsorption and ozonation (Goswami et al., 2020), nano-zerovalent manganese-biochar composite (Iqbal et al., 2021a), ZnO/cotton stalks biochar nanocomposite (Iqbal et al., 2021b), and microwave activated biochar (Yek et al., 2020) However, to date, metal salt-MBs have not been studied in relation to CR adsorption Besides metal salt-MB derived from agro-waste is a novel for treating CR in water, it is also a low-cost and feasible adsorbent due to simple technology Consequently, we have selected metal oxide-biochar for CR removal for the following reasons Firstly, the wattle bark of Acacia auriculiformis is an agro-waste that is potentially harmful to the environment Sec ondly, biochar derived from wattle bark has the point of zero charge (pHpzc) of 5.48, which means that the net surface charge of its solution is negative under normal conditions, and its electrostatic adsorption is weak owing to the negatively charged CR Additionally, wattle bark biochar has a molecular diameter (1.98 nm) smaller than that of CR (2.3 nm), which limits the pore-filling mechanism in the adsorption Finally, the precipitation of metal oxides onto the biochar is thought to increase the surface area available (Premarathna et al., 2019; Wang et al., 2018) by depositing more heterogeneous particles on the surface and have more functional groups (Du et al., 2019) It is believed that the presence of metal oxides of Fe, Ca, and Al onto the biochars could accelerate the degradation of dye (Chaukura et al., 2017; Pang et al., 2019) Here we tested the hypothesis that metal oxide-MBs result in higher adsorption efficacy than the BC In this study, biochars were produced by impregnation-pyrolysis of wattle bark at 500 ◦ C with metal salts of AlCl3.6H2O, FeCl3.6H2O, and CaCl2 The main objective of this study was to compare the potential of MBs, and BC derived from wattle bark to remove CR, thereby suggesting the most feasible biochar for real application Also, this work was to clarify the adsorption mechanism and kinetics of three MBs for CR adsorption The characteristics of biochars were investigated by Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), and Brunauer–Emmett–Teller (BET) analysis Abbreviation definition CR BC MBs BAl BFe BCa pHpzc FTIR SEM BET PFO PSO RSS qt qe qm IDP R2 Congo red Normal biochar Modified biochars AlCl3-biochar FeCl3-biochar CaCl2-biochar The point of zero charge Fourier transform infrared Scanning electron microscopy Brunauer–Emmett–Teller Pseudo-first-order kinetic model Pseudo-second-order kinetic model Residual sum of squares Adsorption capacity at time Adsorption capacity at equilibrium Max adsorption capacity Intra-particle diffusion The coefficient of determination complicated operation (Katheresan et al., 2018; Zheng et al., 2021), and the use of waste materials (Gupta et al., 2013; Mittal et al., 2010) or bioadsorbents (Gupta et al., 2015) In recent years, biochar has become one of the most promising adsorbents for dye treatment (Saleh and Gupta, 2014); it is an economical and eco-friendly alternative material Biochar can be produced by various methods such as pyrolysis, gasifi cation, and hydrothermal carbonization (Yaashikaa et al., 2020), in which pyrolysis is most common Pyrolysis converting biomass into biochar is a thermal conversion process at high temperatures (~300–900 ◦ C) in low or absence oxygen availability The temperatures of pyrolysis are optimal covering from 400 to 550 ◦ C to maximize bio-oil and solid product yields (Xia et al., 2021) Pyrolysis includes fast (high heating rates: ∼ 1000 ◦ C/min, elevated temperatures: 400–600 ◦ C), and short vapor residence time: 0.05) The mean qt of BFe is higher than BAl, and BCa could be due to the presence of Fe–O bonds on BFe and surface complexation in general magnetic biochar (Li et al., 2019; Zhang et al., 2018) 32.6%, 68.2%, and 22.4% for BAl, BFe, and BCa, respectively When the initial CR concentration fixed a constant at 50 mg.L− 1, the adsorption efficiency of CR was influenced by the rapid change of biochar dosage in the range 0.5–2.0 g.L− (qi from 100 to 12.5 mg g− 1) This behavior was due to the increase in the number of surface adsorption sites, thereby resulted in a higher adsorption capacity CR removal was stable at approximately 96% (24 mg g− 1) for BAl and BFe at a qi of 25 mg g− (2 g L− 1), while only 65.4% reduction of CR achieved for BCa These results of MBs are higher than those for biochar derived from vermicompost with approximately 5–12 mg g− (Zhang et al., 2016) The increase in CR removal capacity for BCa was maintained at 91% with a dosage of g.L− (qi of 12.5 mg g− 1) Although the mass removal of CR was the same, approximately 12 mg g− at g.L− of biochar dosage, the adsorption efficiencies were significantly different For further adsorp tion investigation, an optimal initial loading of 25 mg g− (2 g.L− 1) has been chosen 3.3 Adsorption capacity and influential factors 3.3.1 Effect of initial loading and pH As an important factor impacting the adsorption process, pH of the solution was tested with the range 2–12 at 50 mg.L− of CR initial concentration Fig 4a shows that the CR removal capacity decreased with a rise in pH, with a considerable drop at pH range 10–12 This trend was also found in most cases of CR adsorption (Zheng et al., 2021) All MBs achieved the highest CR reduction at pH of 2, suggesting that electrostatic attractions did not contribute to the adsorption This is because, at that pH, MBs are positively charged (pH of the solution is lower than pHpzc) and the molecular structures of CR are predominantly positive (the isoelectric point of CR is 3) Moreover, the decrease in CR removal capacity with higher pH, i.e., the gradual reduction of adsorption capacity of BCa and BAl, did not depend on pHpzc (pHpzc of BCa = 8.01 and BAl = 5.57); this confirms that electrostatic interaction was negligible In acid conditions, the adsorption mechanism could be partly controlled by the hydrophobic force, van der Waals interactions, and H-bonding The highest CR reduction was found to be at a pH of and reduced as the pH increased, but the maximum equilibrium adsorption capacity (of pine bark) only attained 0.47 mg g− 1, which was also presented by Litefti et al (2019) In addition, by using porous NiCo2O4 nanosheets, Bao et al (2019) reported pH at was the optimal point reaching the largest CR adsorption For normal conditions, pH values of CR and textile wastewater were and in the range 7–9, respectively (Patel and Vashi, 2015) Therefore, in a practical applica tion, BAl and BFe can achieve a CR removal of more than 99%, while BCa was 75.6% The operational parameter regarding dosage is considered misleading and therefore difficult to compare across various studies If the authors reported the dose level but did not declare the initial con centration, the resulting data would be meaningless Therefore, in this study, we reported both the initial mass loading and the dosage used The influence of the initial loading on CR removal was investigated in the batch tests under normal conditions that is presented in Fig 4b With a qi of 100 mg g− (dosage of 0.5 g.L− 1), the reduction of CR achieved 3.3.2 Effect of contact time The correlation between the contact time and CR adsorption capacity of the MBs is plotted in Fig In general, an increase in reaction time led to the extent of CR adsorption efficiency but with a different magnitude for each biochar and the lower initial concentration of CR also resulted in a shorter equilibrium time for all biochars For example, with 40 mg L− initial CR concentration the equilibrium times were 10, 20, and 180 for BFe, BAl, and BCa, respectively Similarly, the equilibrium times were found to be 20, 120, and 360 min, for BFe, BAl, and BCa, respec tively, with an initial concentration of 60 mg.L− These values of BFe and BAl are lower than those of biochars derived from vermicompost, with 80–300 of equilibrium time (Zhang et al., 2016) The results for reaction time also confirm that each type of biochar produced from different materials exhibited different equilibrium times; this does not agree with the conclusion reported by Yu et al (2018), which indicated the same saturation time of 60 for both normal and TiO2-nano particle biochars 3.4 Adsorption kinetics The fitting results of CR adsorption data by kinetic models are pre sented in Table and Figs and The values of R2 indicate that two kinetic models described adsorption experiment data of BAl and BFe better than BCa This difference may link to the presence of Fe–O bonds on the surface of BFe (Fig 2), the higher pHpzc, and the disappearance of the adsorption peak of 2.974 cm− after adsorption (symmetric stretching of –CH2) of BCa compared BAl and BFe The RSS values indicate that experimental result of BFe was fitted the most to FSO and PFO models This means that kinetic models are favorable to describe CR adsorption mechanism The rate-determining step of CR adsorption by MBs may include chemisorption (Ho, 2006; Huang et al., 2018) In addition, a quick increase of R2 from single stage to multiple stages (IDP Fig The variation of CR removal efficiency in term of the percentage under the change of pH (a) and dosage of the modified biochars (b) D.L.T Nguyen et al Environmental Research 200 (2021) 111492 Fig Effect of reaction time on CR removal in terms of percentage and adsorption capacity at initial concentration of 40–60 mg.L− biochar (b,c), and FeCl3-biochar (c,f) of AlCl3-biochar (a, d), CaCl2- Table The results of variable parameters of pseudo-first-order (PFO), pseudo-second-order (PSO), and Intra-particle diffusion – multilinear (IDP) kinetic models fitted by experimental data of CR adsorption onto modified biochars Type BAl BCa BFe PFO kp1 0.15 0.07 0.25 qe 24.70 20.86 24.64 RSS 1.28 41.55 0.88 R2 0.99 0.91 0.99 PSO kp2 0.02 4.4e-03 0.04 qe 25.36 22.38 24.96 R2 0.99 0.96 0.99 RSS 1.86 16.17 0.22 IDP kW1 6.16 4.16 1.07 B1 1.5e-13 − 3.8e-14 19.27 kW2 0.11 0.59 0.07 B1 23 12.5 23.83 R2 (Single) 0.34 0.75 0.67 R2 (Multi) 0.99 0.99 0.99 Fig The experimental results of CR adsorption capacity by time and non-linear kinetic models of pseudo-first-order (PFO), and pseudo-second-order (PSO) of modified biochars including AlCl3-biochar (a), CaCl2-biochar (b), and FeCl3-biochar (c) The CR initial concentrations of 50 mg.L− and 2.0 g.L− of biochar dosage D.L.T Nguyen et al Environmental Research 200 (2021) 111492 Fig The results modeled by intra-particle diffusion-multilinear kinetics of AlCl3-biochar (a), CaCl2-biochar (b), and FeCl3-biochar (c) model) was thought to have consisted of multiple adsorption mecha nisms (Table & Fig 7) The qe derived from both models (24.64–25.36 mg g− 1) was found to be close to the qe derived from the experimental results for BAl and BFe (Table 3), which is further clarified by the curves plotted in Fig 6a and c In addition to non-linear approaches, a multiple linear kinetic model, namely IDP, was used to describe the adsorption data and the results are shown in Fig The considerable difference in R2 for single and multiple models indicates that the nature of the CR adsorption was not linear or single linear The R2 of the IDP simple linear regression ranged from 0.34 to 0.75, while those for IDP multiple models were from 0.96 to 0.99 As the underlying assumption of IDP, single lines in Fig not pass through the origin, meaning that the rate-limiting process is not due to intra-particle diffusion alone; this observation is previously reported (Cardoso et al., 2011; Li et al., 2018) The CR adsorption consisted of multiple stages and this was proved with power fitting with IDP multiple lines The CR adsorption process by MBs was modeled by two stages The first phase of adsorption, which was the fastest phase, was controlled by the diffusion process, while the second phase was of intra-particle diffusion and was the delayed phase (Cardoso et al., 2011; Vaghetti et al., 2009) equilibrium time for three MBs The detailed results are presented in Table and Fig The experimental data of BFe and BCa fitted the two models more closely than those of BAl The R2 value of BAl accounted for 91% (Langmuir) and 95% (Freundlich) compared to the range of 98–99% for BFe and BCa These results indicate that adsorption (i.e monolayer adsorption and chemisorption) may occur on a heteroge neous surface (Gerente et al., 2007; Litefti et al., 2019) The KF values represented as affinity coefficient, obtained the highest with 36.01 for BFe, while BCa had the lowest KF with only 4.41 (Table 4) Moreover, high values of intensity parameter (n) of the Freundlich model (1.18–1.63), suggesting that the adsorption conditions were favorable Previous studies reported considerably different results of RSS, such as 0.27 and 0.40, for the Langmuir and Freundlich models, respectively, (Chowdhury et al., 2011), 193 at 25 ◦ C for Langmuir and 70.07 at 20 ◦ C for Freundlich (Babaei et al., 2016) Wang et al (2018) reported n value of Freundlich model ranged 0.04–1.30 for two types of adsorbents of BC and Ca-BC The highest value for maximum adsorption capacity (qm), according to the Langmuir model, achieved for BFe being 130.0 mg g− 1, followed by 44.86 mg/g and 30.80 mg g− for BCa and BAl, respectively The qm for adsorbing CR by BFe in this study is comparable to results reported by Iqbal et al (2021a) with 25.32 mg g− 1, Yek et al (2020) with 91.0 mg g− 1, and Li et al (2012) with 105.0 mg g− and lower than by Du et al (2019) with 436.68 mg g− 1, Zheng et al (2019b) with 714.0 mg g− 1, Huang et al (2018) with 1916 mg g− 1, and Dai et al (2018) with 20317 mg g− The difference of biochar production methods resulted in different qm, which was also confirmed by Yek et al (2020) Also, the qm for CR adsorption reported from literature with various adsorbents is significantly different (Table 5) 3.5 Isotherm study Two isotherm models, namely Langmuir and Freundlich, that were used to model the experimental data at 35 ◦ C corresponded to the Table The results of fitting parameters derived from two isotherm models for three modified biochars KL and KF denote interaction energies and affinity coefficient, respectively RSS is residual sum of squares of non-linear isotherm models and n is the linearity constant or intensity parameter Models Langmuir Freundlich Parameters − kL (L.mg ) qm (mg.g− 1) RSS R2 kF ((mg.g− 1)/(mg.L− 1)-n) N RSS R2 BAl BCa BFe 1.91 30.80 2.87 0.91 18.26 2.63 1.83 0.95 0.08 44.86 1.06 0.98 4.41 1.53 1.12 0.98 0.37 130.0 1.22 0.98 36.01 1.18 1.13 0.99 3.6 Possible adsorption mechanisms Many factors of adsorption contribute to and control dye removal by biochar such as porous structure, charged surface, and surface func tional groups In this study, MBs produced by combination with metal salts presented different features The difference in the physical prop erties of MBs had a clear influence on the CR adsorption capacity Furthermore, the interaction between the adsorbate and adsorbent, experimental conditions, hydrophobic effect, electrostatic attraction, hydrogen bonding, and π-π interaction, among others, significantly D.L.T Nguyen et al Environmental Research 200 (2021) 111492 Fig Adsorption isotherm graphs of CR by three modified biochars: CaCl2 biochar - BCa (a), AlCl3 biochar – BAl (b), and FeCl3 biochar – Bfe The qe is adsorption capacity at equilibrium and Ce is the remaining CR concentration interaction may be not strong because although BCa had a high BET, it obtained lower CR adsorption capacity compared to BAl (195.64 m2 g− 1) and BFe (137.25 m2 g− 1) The results of the effect of pH on adsorption efficiency suggesting adsorption were not significantly gov erned by electrostatic attraction; for example, the gradual reduction of adsorption capacity of BCa and BAl did not depend on pHpzc In addition, all MBs had micropores and mesopores with a size range of 1.98–3.91 nm; these are larger than the diameter of CR molecules, thus revealing the high potential for absorption of CR into the pores of biochars In addition to the common functional groups for BC derived from – C– and –C– – N triple bonds, carboxyl groups, C– –C plants, such as –C– – – – O and C– – C functional groups, the MBs double bonds, and aromatic C– exhibited new peaks which can be assigned to Fe–O bonds and OH stretching vibrations These functional groups led to a significant in crease in CR adsorption onto the MBs through chemisorption The benzene rings (aromatic ring) and –NH2 groups presence in the CR molecules may interrelate with π-π interactions and –OH groups (hydrogen bonding) on the MBs (Zheng et al., 2019a) The occupying before the adsorption and change after adsorption of these functional – C and C– – O in groups, as observed through FTIR (Fig 2), including C– the aromatic bonds, π-π interactions, metal oxides, –OH, C–C, and – C–O illustrate that they have contributed significantly to carboxyl O– the CR removal Related to the inherent characteristics of CR (Huang et al., 2018), concluded that a high reaction between chromium of biochar and –NH2 groups enhanced the CR adsorption The CR adsorp tion process could also consist of multiple mechanisms as the assump tions of PFO and PSO models Additionally, the fitting well with the IDP model indicates that the rate-limiting process was not only due to the intra-particle diffusion but involves two stages - the first was the fastest phase and controlled by the diffusion process, and the second was of intra-particle diffusion and was the delayed phase Table The comparison of maximum adsorption capacity (qm) derived from Langmuir isotherm model of different adsorbents from literature Adsorbents qm (mg g− 1) References Fe3O4-graphene-biochar composite Calcium-rich biochar Leather shavings biochar 436.68 20317 1916.00 Nano-zerovalent manganese-biochar composite 25.32 Commercial MgO powders Hierarchical MgO structures Microwave activated biochar by steam Microwave activated biochar by CO2 Chlorella sp microalgal biochar Al2O3@ZnO core-shell microfibres 105.00 197.00 136.00 91.00 164.35 714.00 Du et al (2019) Dai et al (2018) Huang et al (2018) Iqbal et al (2021a) Li et al (2012) Li et al (2016) Yek et al (2020) 3D hierarchical graphene oxide-NiFe layered double hydroxide composite FeCl-Biochar 489.00 130.00 Yu et al (2021) Zheng et al (2019b) Zheng et al (2019a) This study affected the adsorption efficiency of CR from aqueous solutions The BET area of BCa achieved almost double compared to BFe and BAl; however, BCa showed the lowest adsorption capacity, which means the surface area not controlling or significantly affect the CR removal The presence of a considerable proportion of Fe and Al oxides (SEM image in Fig 1) could support the reduction of CR by BAl and BFe Related to this, Chaukura et al (2017) stated that a combination of biochar matrix and nanocrystals of metal oxides in MBs contributed to adsorption of contaminants The high BET of BCa (380.78.64 m2 g− 1) is likely to provide favorable active sites for adsorption Adsorbate-surface D.L.T Nguyen et al Environmental Research 200 (2021) 111492 Conclusions Chaukura, N., Murimba, E.C., Gwenzi, W., 2017 Synthesis, characterisation and methyl orange adsorption capacity of 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BAl and BFe (96.8%) The metal oxides were successfully integrated onto the surface of the biochar matrix The CR removal efficiency decreased with an increase in pH of the solution and increased with a rise in biochar dosage, which had an optimal loading of 25 mg g− (2 g.L− 1) Maximum adsorption capacity achieved 130.0, 44.86, and 30.80 mg g− for BFe, BCa and BAl, respectively BFe is easily separated from the solution and achieves a high adsorption capacity, therefore it is the preferred biochar for CR treatment application Author contribution Dang Le Tri Nguyen: Conceptualization, Methodology, Data cura tion, Writing – original draft Quach An Binh, Thi Thanh Huyen Nguyen, Quang Nha Vo, Trung Duong Nguyen, Thi Cuc Phuong Tran, Thi An Hang Nguyen, Thang Phan Nguyen, and Jeahan Bae: Visualization, Investigation Xuan Cuong Nguyen, Soo Young Kim, Il Tae Kim and Quyet Van Le: Supervision, Writing- Reviewing and Editing Declaration of competing interest The authors declare no conflict of 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(Yek et al., 2020) However, to date, metal salt-MBs have not been studied in relation to CR adsorption Besides metal salt-MB derived from agro-waste is a novel for treating CR in water, it is also... Saravanan et al., 2016) were investigated to remove by adsorption methods For Congo red (CR) dye, several biochars, which are produced from vermicompost (Zhang et al., 2016), cabbage waste (Sewu et al.,... 96% (24 mg g− 1) for BAl and BFe at a qi of 25 mg g− (2 g L− 1), while only 65.4% reduction of CR achieved for BCa These results of MBs are higher than those for biochar derived from vermicompost