Removal of Cr(VI) from aqueous solution by biochar derived from rice husk

In this research, biochar was produced from local paddy rice husk in Sumatera and used as a bio-sorbent of Cr(VI) in an aqueous solution. The rice husk biochar (BC) was characterized using X-ray diffraction (XRD), Fourier Transform Infra-Red (FTIR), surface area-specific, and SEM morphology analysis. This XRD characterization showed a 002 reflection at 23 with broad intense diffraction, as well as the appearance of silicate oxide, and carbon vibrations. The surface area and SEM morphologies confirm that after pyrolysis the rice husk surface contained various particle sizes with a seven-fold increase in surface area. Furthermore, the Fourier Transform Infra-Red spectra verify the presence of functional groups, such as carboxylic C=O and aromatic C=C stretching. The result of the adsorption indicated the BC follows 2-order reaction with a 0.004 mg/min rate constant and Langmuir isotherm models with a coefficient correlation close to unity. Also, the maximum adsorption capacity of this substance increased from 68.996 mg/g to 161.290 mg/g. The thermodynamic analysis showed a positive enthalpy and entropy value, indicating Cr(VI) adsorption is endothermic. In addition, high amounts of heat favor the adsorption process due to the decrease in Gibbs energies caused by increasing temperature. The reusability adsorption study of Cr(VI) showed BC was a good potential adsorbent in the removal of the heavy metal from an aqueous solution.


Introduction
Chromium (Cr) is a great electroplating source and a heavy metal pollutant obtained from several industrial processes that can affect the environment [1,2]. This compound is also released from mines, pottery, and photography [3,4], with the ion structure usually formed as 3+ and 6+ in an aqueous environment. Cr Hexavalent is a rare metal ion usually obtained as chromate (CrO4 -) and di-chromate (Cr2O7), with extreme toxicity dependent on its valance [4,5]. Furthermore, to protect the environment during the extraction of Cr (VI), certain treatments have been used to remove the wastewater dye, including reduction, ionic exchange, extraction, and adsorption [5][6][7]. Adsorption is a popular treatment that has low preparation cost, high efficiency, and is easy to operate depending on the adsorbent ability [8][9][10]. Research on the potential adsorbent which is inexpensive, non-toxic, and easy to prepare have been the focus of the last decade. The most appealing forms are agricultural wastes-based biomass, such as wheat straw, mangifera indica, bagasse, rose waste, and maize corncob [11][12][13][14].
Several research on the use of agricultural wastes as adsorbent such as the adsorption processes of succinylated cellulosic biomass using Cu(II) and Zn(II) in wastewater have been reported by Singha et al [11]. These processes obtained an adsorption capacity of 7.27 and 5.711 mg/g for these compounds, respectively. The research by Nadeem et al [12] used mangifera indica to adsorb these compounds from an aqueous solution, where equilibrium was reached after 120 min with adsorption capacity of 8.65 and 9.45 mg/g, respectively. Also, Iqbal et al [13] studied a rose waste biomass (Rosa damascena) to remove Cr(III) and Cr(VI) and discovered the adsorption capacity of these compounds to be 15 and 7.6 mg/g, respectively. Garg et al [15] also reported Cr(VI) adsorption using sugarcane bagasse, maize corncob, and Jatropha oil cake. The results showed that the adsorption capacities were 5.75, 3.00, and 11.75 mg/g, respectively.
According to previous research, the adsorption capacity obtained from agricultural waste biomass is still low and has limitations. Therefore, there is a growing need to find new, economical, easily available, and effective adsorbents for practical application. The natural-based and carbon-rich materials such as biochar (BC) [16,17] are used because they are effective and renewable. Present research explore BC adsorbents such as wheat [18], coconut shell [19], wheat straw [20], peat husk [21], sawdust [22], and rice husk. Furthermore, rice husk has difficulty in bio-decomposition due to its hard surface and high silicon content and mainly contains cellulose, lignin, and silicon [23]. This substance is one of the wide agricultural by-products seen in Indonesia [24] which is thrown as a non-used waste hence polluting the environment and irrigation water [25]. Therefore, to enhance the utilization of this substance as an adsorbent, some further treatments were required [26]. BC from rice husk can be applied as an economical-potential adsorbent for the removal of heavy metal cations and organic contaminants from wastewater because it is a porous material with a large surface area that contains a rich functional group on the active sites [27,28].
Several research have used BC to adsorb heavy metals [29] including the use of rice husk biochar to adsorb Pb(II) from wastewater with an adsorption capacity of 143 mg/g as reported by Sun et al. Cuong et al [30] reported the absorbance of Cu(II) using this material at pH 5 and obtained an adsorption capacity 165 mg/g. The use of chestnut shell biochar was reported by Özcimen and Ersoy-Mericboyu [31], where the adsorption of Cu(II) in an aqueous solution reached equilibrium after 120 min and obtained an adsorption capacity of 98 mg/g. The main objective of this research was to investigate and evaluate the effectiveness of BC derived from rice husk for heavy metal removal in an aqueous solution. Also, the physical, chemical, morphological, and adsorption ability of RH and BC were investigated. The adsorption was conducted with kinetic, thermodynamic, isotherm adsorption parameters and reusability of adsorbent.

Preparation of BC
RH was obtained from local field paddy after washing and drying in the sun. According to Poo et al, the BC was prepared from RH by pyrolysis at 600 o C for 3 hours [32]. Subsequently, the reactor was cooled down and the substance was washed several times with water and dried at 110 o C for 48 hours.

Chemicals and instrumentations
Water was purified by Purite® ion exchange method supplied by Research Center of Inorganic Materials and Complexes, in Sriwijaya University. These chemicals were purchased from Merck and Sigma-Aldich with >99% purity and directly used without further purification. The analysis of these materials was performed by XRD powder, FTIR spectroscopy, BET surface area, thermal, and SEM image analyses. The powder XRD analysis was performed by Rigaku Miniflex-600 and the sample scanned at 1º/min. Furthermore, identification of functional groups was conducted by FTIR Shimadzu Prestige-21 using KBr pellet while the BET surface area analysis was conducted using Quantachrome Micromeritic 2020 instrument and the sample degassed by liquid N2 before the process. Also, thermal and SEM analyses were conducted by TG-DTA Shimadzu analyzer using N2 flow and the SEM Quanta 650 Oxford instrument, respectively. The analysis of chromium was performed by UV-Visible spectrophotometer Bio-Base BK-UV 1800 PC after complexation by diphenylcarbazide as a ligand. Furthermore, the resulting complex molecule was analyzed at 543 nm.

Reusability of BC
The adsorbent was desorbed after Cr(VI) adsorption by an ultrasonic system at 30 minutes and was dried at 110 o C for 5 hours. This process was conducted by a reused adsorbent, where the reusability process was performed after five cycles of the re-adsorption process. The adsorbent stability was evaluated at the end of the re-adsorption process and the reusability procedures were repeated three times.

Adsorption of Cr(VI) by BC and RH
The effect of initial Cr(VI) concentration, temperature, and time on the Cr(VI) adsorption by BC and RH was studied. These experiments were conducted in an initial pH range of 2 to 4 because the precipitation of heavy metal ions could occur and interfere with the adsorption process measurements at pH levels higher than 4 [33]. The data obtained from the initial Cr(VI) concentration of 10, 20, 30, 40, and 50 mg/L at 303-333 K was then calculated to obtain the thermodynamic and kinetic parameters. In addition, adsorption time was conducted at 10, 20, 30, 40, 50, 60, 70, 90, 120, 150, 180, and 210 minutes. Furthermore, the thermodynamic data were obtained using Langmuir and Freundlich isotherms and also G, H, and S [34][35][36] while the kinetic data were obtained by1 st and 2 nd order as well as Elovich kinetic models [37][38][39].

Results and Discussion
The morphologies of RH and BC were presented in figure  1. Typically, RH has a globular structure in nature formed by the lemma or palea, tied to each other [29,40]. The crinkle form at the epidermis is highly spiked, with conceived papillae and long-small shag on its surface [41]. Meanwhile, the morphology of the identified BC had many holes with various particle sizes, presented by the porous network and large specific surface area as listed in table 1. Table 1 showed a specific surface area of rice husk was increased after pyrolysis treatment. This finding indicates the volatilization of organics created a porous structure, resulting in a much higher surface area after further heating. The average pore size was increased too because the temperature formed a larger pore during the heating treatment process [42].  (110) and (002), which were assigned as carbon and inorganic contents and used as raw starting materials. According to the results by Park et al [41], the (002) intense counts taken on the surface of lemma, show a dominant silica peak, which appears on the entire lemma outer surface and internal tissue of RH. The second peak indicated the formation of carbon and oxygen and their disappearance after pyrolysis. Furthermore, the pyrolysis of RH to BC, which is a carbon-rich material known as biochar remained on one peak at (002), after 3 hours. The board pattern of this material was due to the presence of amorphous silica [43]. Also, these findings are supported by increasing the surface area, pore size, and volume size as presented in table 1.  Figure 3 exhibits the FTIR images for the adsorption of RH and BC with some cases of RH peaks characteristic at 754,1103, 1620, 1800, 2924, and 3448 cm −1 [44,45]. The region at 3448 cm −1 is related to the free hydroxyl groups on the RH surface and an intensive peak at 1103 cm −1 denotes Si-O-Si. Meanwhile, the IR spectra indicated weak and broad peaks about 1620 cm −1 corresponding to the -C=O and -C-OH groups correlated with hydroxyl bonds [46]. Therefore, BC showed similar peaks with RH. Figure 4 shows the thermograms of RH and BC with the three decomposition stages. The first was the presence of water molecule decomposition, where DTA showed the small endotherm as seen in the first peak indicating that the oxidation and this endotherm is followed by an exothermic shift. The second was the decomposition of organic compounds following the weight loss of these materials' composition. As similarly reported by Vlaev et al [47], the major decomposition of RH such as cellulose, lignin, and silica occurred at the temperature ranging from 500 to 650 K. Therefore, the last exothermic peaks indicated the combustion of gases product, as shown in TG which is the major weight loss of cellulose. Furthermore, RH and BC were applied as adsorbents to remove Cr (VI) in an aqueous solution. Figure 5 showed the effect of initial Cr (VI) concentration vs. adsorption capacity at various temperatures. The effect set up from 10-50 mg/g obtained a higher adsorption capacity of 16.22 mg/g and 23.01 mg/g for RH and BC, respectively at 333 K. These figures showed the increasing temperature and adsorption capacity, where equilibrium was attained in the initial concentration of 30 mg/L for RH but not for BC. However, this has not been attained at 50 mg/L, hence, the determination of the (b) (a) equilibrium state from the adsorption process was described using Langmuir and Freundlich isotherm models. The formula of these models has been widely reported by other researchers [34][35][36]48]. Table 2 contains a list of the calculated data results.  According to Alagha et al [49], Langmuir is a monolayer adsorption process, belonging to chemical sorption while Freundlich is a multilayer process that occurs with heterostack [19]. Based on the results in table 2, RH and BC followed Langmuir adsorption with R 2 closed to 1. The better fitting of Langmuir isotherm data means that Cr(VI) adsorption occurred in the homogeneous surface of materials. Table 2. shows BC has a higher adsorption capacity than RH at 333 K. This finding assumed that the adsorption is favored by high temperature and occupied an inactive site onto adsorbents when the saturation value was reached. Furthermore, the adsorption capacity of BC was higher than RH, due to the greater surface area and pore size of BC, as seen in table 1. Rosa damascene 76 [13] sugarcane bagasse 5.75 [15] maize corncob 3.00 [15] Jatropha oil cake 11.75 [15] Coconut tree sawdust-AC 3.46 [52] Biochar derived rice husk According to previous literature, the comparison with values as listed in table 3 shows that BC is the highest in relation to the other adsorbents. These findings show BC has a high surface area and a great potential for the removal Cr (VI) from an aqueous solution.
The adsorption process was determined from the effect of time and calculated using kinetic adsorption models such as Elovich kinetic model, pseudo-1 st , and -2 nd orders. Also, the formula of these kinetic models has been reported by previous researchers. Figure 6 showed the correlation between the effects of adsorption time vs. capacity. The capacity of BC was higher than RH, due to the greater surface area and pore size of BC, with both substances reaching equilibrium after 2 h. Furthermore, there was a quick increase in Cr (VI) removal initially until equilibrium reached. Table 4 shows the adsorption kinetic model was tested by Elovich, 1 st , and 2 nd orders. Based on the 2 nd order and Elovich data results, the adsorption data for RH and BC has R 2 all close to one, while the 1 st order had R 2 >0.8. The 2 ndorder model was a better fit for both RH and BC adsorbents as presented in table 4. Also, the fittings of the three models to BC were slightly better than RH, indicating the adsorption of Cr(VI) on RH was more unfavorable. These models probably described the difference between the adsorption mechanisms [55]. The reusability of adsorbent was a very important study to evaluate its effectiveness. This experiment conducted five cycle adsorption-desorption treatments on RH and BC. Fig 7  shows the results of the reusability data were dramatically decreased in the fifth cycle. Also, the adsorption percentage was decreased from 69.8% to 13.4% for BC and 50.9% to 6.0% for RH. However, the efficiency of BC was better than RH assuming it has good potential as a low-cost, environment-friendly, and effective adsorbent for reuse, although BC had a low adsorption percentage after the fifth cycle.

Conclusion
This research showed that BC derived from Indonesian RH using thermal treatment has good proficiency in the removal of Cr(VI) from aqueous solutions. The adsorption process of this compound obtained from the adsorption capacity of biochar with a value of 161.290 mg/g at 333 K in a 2 h interaction time is comparable to RH and the other adsorbents. Furthermore, BC showed good potential as a reusable adsorbent for the removal of the heavy metal in an aqueous solution.