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The existence of heavy metals on the earth’s surface and disposal of industrial effluents to the environment causes severe health issues that need to be dealt with. Adsorption of Cr(VI) was carefully studied onto the furfural residue in a batch system. Furfural residue was treated in the microwaveassisted HTC with water as an effective medium, and the subsequent solid material was further treated by a low concentration of potassium hydroxide. Under optimum conditions (pH 2, 25oC, and 2.5 g/L adsorbent dosage), 91.72% Cr(VI) was removed at the initial concentration of 100 mg/L to achieve equilibrium condition. The effects directed that a higher adsorption capacity of Cr(VI) (36.91 mg/g) was reached by integrating microwave-assisted treatment at 200oC and 0.05 N KOH concentration. Important parameters like effect of pH, contact time, temperature and solution concentration were optimized to investigate their effectiveness. The experimental adsorption data were best fit for the Freundlich model, which instantly followed the pseudo second-order kinetics model. Investigation of thermodynamic studies demonstrated negative values. The findings of the study suggested the modified hydrochar generated from furfural residue could be considered as an alternative to high-cost adsorbents. keywords: adsorption, Cr(VI), furfural residue, kinetics isotherms, microwave-assisted HTC *e-mail: zhanglei78@dlut.edu.cn DOI: 10.15244/pjoes/109307 ONLINE PUBLICATION DATE: 2020-01-08 Khushk S., et al. 1672 However, these techniques have revealed limitations due to high economic rates, high-energy consumption and non-suitability to environmental conditions. Conversely, the adsorption method has gained high attention because of its suitable economic rates, high efficiency, an easily designed process and stable performance [7, 8]. For adsorption, the most important element is to prepare an adsorbent, which addresses the key aspects to eliminate a defined pollutant’s removal efficiency, process and its cost management. Commercial activated carbon is widely used as an adsorbent, but its cost management and redevelopment is challenging. Hence, several materials have been broadly studied such as animal wastes, agricultural wastes, and clays to prepare adsorbent [1]. The material which we had prepared for removal of Cr(VI) had never been used by any researcher for an adsorption process. This adsorbent can help the purification industry to remove Cr(VI) through a microwave-assisted treatment method by improving its efficiency with minimum resources. Microwave-assisted hydrothermal carbonization has made it more modest than manual heating (or conventional) machine setup. Therefore, this technique is considered the best technology for economic stability because of its inherent advantages such as numerous end products [9]. Compared with the conventional HTC method, microwave-assisted HTC is likely the more efficient, faster, and appropriate method. This method has the potential to effective depolymerize and decrystallize cellulose during microwave-assisted reaction under low temperatures, which has been reported in studies [10]. Furthermore, Elaigwu and Greenway studied the comparison between conventional and microwave-assisted HTC of lignocellulosic waste material, where microwave-assisted HTC was shown to rapidly decompose the waste material in terms of time compared with the conventional method [11]. The temperature range for the microwave-assisted heating is suggested as being 180-250oC at 0.5 hours [9]. Characteristically, biomass thermal degradation is a source of getting activated carbon that is a carbonaceous material, where water is used as a medium, followed by two steps of activation. In addition, high porosity and surface area of activated carbon must be considered. The greater the pore structure the higher the adsorption capacity [12]. Corncob residues, cottonseed hulls, oat hulls, and sugarcane processing are the sources of preparation for furfural residue after the bio-refinery process, where furfural residue is considered waste. These agriculture materials are waste lignocellulose substances, adding a certain volume of acid (5-8% dilute H2SO4) tends to hydrolysis reaction at a particular temperature (175-185oC) and pressure. Later, the extracted chemical substance having furfural form and the remaining amount is furfural residue, which is rich in lignin and cellulose [13]. It has been calculated that about 23 million tons/year of furfural residue is generated in China only. Approximately 12 to 15 tons of furfural residues are required to prepare 1 ton of furfural chemical [14]. Almost 40% of carbon is found in furfural residue, so its approach as an adsorbent for the wastewater industry is of great importance. Unluckily, a huge amount of furfural residue stayed either unused or fired in an open atmosphere, which causes several health and environmental problems. Furfural residue possesses a great potential to be utilized for adsorption of heavy metals from wastewater. Hence, the synthesis of furfural residue was carried out using the microwaveassisted HTC process for adsorption of Cr(VI). The key objective of this research is to introduce furfural residue as a highly efficient adsorbent (hydrochar) after its microwave-assisted HTC and alkali treatment. In addition, we demonstrated the adsorption capacity of the enhanced hydrochar towards Cr(VI) from wastewater by determining the adsorption capacity, kinetics study, adsorption isotherms, and thermodynamics. The process that we selected is very simple and low cost. We examined the effects of key parameters, for instance solution pH, contact time, temperature, concentration of chromium ions and alkali treatment for the capacity level of the hydrochar adsorption. Furthermore, the furfural residue was characterized by Fourier transform infra-red (FTIR), scanning electron microscopy (SEM), and BuranerEmmett-Teller (BET). Materials and Methods

Microwave-Assisted Hydrothermal Carbonization of Furfural Residue for Adsorption of Cr(VI): Adsorption and Kinetic Study