Abatement of heavy metals from aqueous solution using modified bentonite and biochar derived from lepironia articulata

Abstract

Heavy metal once released from industries have adverse effects on environment as a whole. Adsorption technique through cost effective adsorbents was said to be more effective comparing with commonly used conventional wastewater treatment methods. Current study initially aimed to produce organic and inorganic clay adsorbents and further to modify them using surfactant and alkali to get higher surface area and more developed surface functional groups. Bentonite clay adsorbent was modified using benzylhexadecyldimethyl ammonium chloride (BCDMACI) surfactant to increase its surface functional groups and cation exchange capacity (CEC) value. Lepironia articulata was taken as biomass to prepare hydrochar (HC) using hydrothermal carbonization (HTC) method. The HTC temperature was kept at (180, 200, 220, and 240 °C) and the residence time was varied at (2, 4, 6, 12, and 24 h), respectively. HC produced was used as precursor for the preparation of modified biochar (LABC). Modification of LAHC to LABC was performed in a tubular reactor under slow pyrolysis conditions at 700 °C. KOH was used as a chemical activating agent by keeping the HC to chemical ratio of (1:3). All the adsorbent materials before and after modification were characterized using Scanning electron microscopy )SEM(and energy dispersive X-ray (EDX), Fourier transform infrared spectroscopy )FTIR(, and Brunauer-Emmett-Teller)BET(, Cation exchange capacity (CEC), proximate analysis, thermogravimetric analysis (TGA), higher heating value (HHV), Wettability were evaluated by contact angle, to determine the elemental composition of materials X-ray fluorescence (XRF) was used. The surface area of modified biochar (LABC) was remarkably increased to 820.54 m2 g1 for LABC from 69.23 m2 g1 due to chemical activation. To investigate the performance of biochar after modification, batch experiments were performed with aqueous solutions of chromium )VI(, Nickel (II), and Zinc (II). The maximum adsorption capacity and the removal of Cr(VI), Ni(II), and Zn(II) were calculated to be 27.24 mg g1 and 98.86% at pH 2.0, respectively. The Cr)VI(, Ni(II), and Zn(II) adsorption isotherm on the Lepironia articulata biochar )LABC( was tested with Langmuir, Freundlich, Dubinin-Radushkevich and Temkin isotherm models, and the Langmuir isotherm showed the best fit. From thermodynamic parameters AH, AG°, and AS°, calculate at variable temperatures (20, 40, and 60 °C), the adsorption of Cr)VI( was concluded to be endothermic, spontaneous, and increased disorder. In addition, HTC was revealed to be effective technique for the preparation of carbon materials with variable physico-chemical properties and consequently HCs could be applied in numerous different applications e.g. as alternative low-cost adsorbent for pollutant removal from water, pre-cursor for energy generation, fertilizer and soil amendments etc. Further its use to prepare modified biochar has increased its efficiency for the removal of Cr (VI), Ni (II), and Zn (II) from aqueous solution and laboratory wastewater. Preparation of monolith by binding of modified organic (biochar) and inorganic (bentonite) has successfully proved to be efficient adsorbents, particularly when used in continuous operation. Almost 99% of the heavy metals were removed from aqueous solution through continuous operation in a fixed bed glass column with random packing. Variation in flow rate and bed height suggested the highest removal with flow rate 5 mL/min with a bed height of 4 cm. It was also observed that when using methylcellulose in combination with PVDF binder can not only enhance the strength and durability it can also retain its porous structure, surface area and restrict the excess use of PVDF which make it much cost effective as well. Sustainable development towards obtaining such adsorbents from inexpensive product LA can bring new material for various applications. The liquid and gas products produced from the HTC process are still to be considered for variable by-products from the same species.