Abatement of 2-chlorophenol from the Aqueous Environment by Implementing the Adsorbents Prepared from Lignocellulosic Biomasses

Abstract

The scarcity of clean and consumable water is a serious concern in the current era. Gradually diminishing water bodies are further getting contaminated by hazardous contaminants. 2-Chlorophenol (2-CP) is a hazardous pollutant capable of causing dreadful diseases. Despite toxicity 2-CP is widely used in several industries. The solubility of 2-CP in water is significantly higher i.e. 28 g L-1 at 20 oC. The United States environmental pollution agency has issued permissible levels for 2-CP in consumable water and effluents as high as 0.1 and 2 μg L-1. Most of the wastewater (polluted by 2-CP) treatment technologies have limitations in the form of operation cost, operation duration, and complicated technology. On the other hand, the adsorption method is an economical, versatile, and robust technique with the capability to effectively remove 2-CP from the aqueous environment. The present study aims to investigate the 2-CP adsorption potential of laboratoryprepared adsorbents neem seed activated carbon (NAC) and rice straw activated carbon (RSAC) from aqueous environments. Furthermore, commercially available powdered activated carbon (PAC) was used for 2-CP adsorption for performance comparison. The NAC and RSAC were prepared from their respective precursors via thermochemical activation routes. The yield of NAC and RSAC was found to be 80.49 and 78.17%, respectively. Characterization studies have revealed that all the adsorbents are composed of more than 55% carbon. The pHpzc values of NAC, RSAC, and PAC were 7.12, 7.5, and 9.12, respectively. The BET analysis has confirmed the presence of both mesopores and micropores. The BET surface areas were 248, 239, and 547 m2 g-1 for NAC, RSAC, and PAC, respectively. All three of the adsorbents were confirmed to be amorphous by XRD analysis. The FTIR analysis has revealed the presence of carbonyl, nitrile, hydroxyl, and silanol groups in abundance on the surface of all the adsorbents. The Raman spectroscopy analysis has provided evidence for the graphitic structural arrangement for NAC and RSAC, whereas the defective structural arrangement for PAC. The univariate optimization experimentations were performed to optimize process parameters. The optimal NAC, RSAC, and PAC doses were found to be 1.2, 1.25, and 0.72 g L-1. The optimal pH values and equilibrium contact times were found to be 8:120 min, 8: 125 min, and 9: 90 min for NAC, RSAC, and PAC, respectively, in the order of their appearance. The temperature increment has a negative impact on the adsorption capacity. The kinetics studies revealed that the 2-CP-NAC adsorption system followed pseudo-second-order kinetics. In contrast, both RSAC and PAC were found to be in good agreement with pseudo-firstorder kinetics. The adsorption kinetics studies have confirmed the existence of both film and pore diffusions for all three adsorbents. The experimental data for 2-CP adsorption for isotherm modeling was found to be fitted to Langmuir, Redlich-Peterson, and Freundlich models for NAC, RSAC, and PAC, respectively. From the Langmuir isotherm model, the maximum adsorption capacities for 2-CP by NAC, RSAC, and PAC were found to be 43.5, 45.34, and 54.87 mg g-1, respectively. Adsorption thermodynamics studies have revealed that the 2-CP adsorption was exothermic by nature. The negativity of Gibb's free energy values has suggested the spontaneity of the 2-CP adsorption onto the adsorbents. Statistical model for adsorption thermodynamics studies has satisfactorily explained the thermodynamic behavior at micromolecular levels and was also found to be in accordance with the outcomes of the classical model. The isosteric heat of adsorption estimation for 2-CP adsorption systems revealed that the surface of NAC and RSAC were energetically homogeneous, whereas the surface was energetically heterogeneous for PAC. Taguchi's experimental design was employed to statistically optimize the process parameters for 2-CP adsorption onto NAC and RSAC. The results have revealed that Taguchi's design has successfully optimized the process parameters with the help of S/N ratio estimation and analysis of variance. The most influential factors were the initial concentration of 2-CP and pH for NAC and RSAC, respectively. The continuous adsorption of 2-CP was investigated with an up-flow pattern fixed bed adsorption column. The column adsorption studies have revealed that the breakthrough time was reduced with an increment in 2-CP concentration and flow rate. The Thomas and Yoon-Nelson kinetics models were found to be fitted well with the experimental data. The reusability studies of the adsorbents were carried out by employing desorption techniques. Among the physical desorption techniques, the microwave irradiation-based desorption technique was the bestperforming desorption method rendering as high as 85.49% desorption efficiency for RSAC. Among the chemical-mediated desorption methods, the ethanol-mediated elution technique was the best, with the highest desorption percentage obtained at 93.45% for PAC. The investigation of the 2-CP adsorption mechanism onto three adsorbents was predominantly governed by ππ electron donor-acceptor complexation, hydrogen bonding, electrostatic interaction, etc., for 2-CP adsorption onto NAC, RSAC, and PAC, respectively.