Soy Flour Based N-doped Porous Carbons for Industrial Waste Water Treatment

Abstract

With increasing population, the requirement for pure water is increasing day by day. Moreover, to fulfill the demand of society, industrial growth is also essential. These industries also require pure water for developing their products. In turn, they discharge highly polluted water. Water containing pollutants when dumped into water bodies causes harm to aquatic animals and humans. Thus, the treatment of wastewater is the need of the hour. Among the available water treatment techniques, adsorption and photocatalysis are fast, efficient, easy to implement and cost effective. The most crucial factors in the adsorption of contaminants are porosity and surface chemistry. Out of different adsorbents, porous activated carbons are considered as the best for adsorption due to their porosity and surface area. Nitrogen doping in porous carbons has shown the improved adsorption capacity of carbons by increasing their basicity and selectivity. However, to dope nitrogen from green and clean synthesis methods using easily available precursors is a great challenge. Proteins in bio-based materials act as the main source for both nitrogen and carbons. Among different protein-containing agro-waste precursors, soy flour is cheap, abundant and has> 40% of protein content. The hydrothermal carbonization (HTC) approach, which is economical and environment friendly, can be used to synthesize N-doped porous carbons from protein-based precursors. This makes it feasible to clean the environment in two ways. Along the HTC sample, biochar is also considered to be a green alternate for energy and environment applications. In this N doped carbons were synthesized using HTC and pyrolysis method by soy flour as precursor. The entire work is presented in nine chapters which are as follows: Chapter 1 introduces briefly the problem of wastewater containing dyes, phenols, pharmaceuticals, and phthalate esters wastes along with the discussion about different wastewater treatment methods. The role of adsorption in pollutant removal has been discussed in detail. For N-doped carbons, different synthesis methods, the significance of HTC, and the pyrolysis approach have been explained. The importance of protein-based agro- waste precursors for both carbon and nitrogen source has also been described along with their use in the adsorption of various pollutants. Chapter 2 describes the details of literature related to wastewater treatment for dyes, phenols, pharmaceutical and phthalate esters wastes using agrowaste synthesized porous activated carbons by adsorption and photodegradation. Different synthesis routes for the synthesis of carbons reported in the literature has been discussed briefly. The role of hydrothermal treatment, pyrolysis method and other methods in the synthesis of N-doped carbons has been discussed. The importance of protein based precurosrs for the synthesis of N doped carbons has been described in details. Literature available on the use of soy flour as source of N and C to synthesize N doped carbons has been discussed in detail. Chapter 3 presents the details of precursors, approach for sample synthesis and their characterization. All the characterization techniques used in the current work have been discussed in this chapter. This include electron microscopy, X-ray diffraction, surface area measurement, X-ray photoelectron spectroscopy, UV visible, FTIR, Raman spectroscopy, thermogravimetric analysis, adsorption studies, and photodegradation studies. The fundamental aspects of synthesis/characterizations, as well as the required test conditions, have been described. Chapter 4 presents the results of the optimized conditions to get high yield of N doped carbons using hydrothermal method (HTC) where glucose as carbon source and protein-rich defatted soy flour as nitrogen and carbon source were used. The synthesized carbons (GS1) were found to be spherical in shape and show better adsorption efficiency than both glucose derived HTC spheres and HTC synthesized soy flour hydrochar. To understand the properties of synthesized samples, SEM, TEM, XRD, FTIR, UV-visible spectroscopy, N 2 adsorption- desorption isotherm, XPS were carried out. The molten salt technique was used to determine the point of zero charge (pH pzc ) of the samples. The as-synthesized N-doped carbons have been used for the treatment of wastewater containing cationic dyes MB and CV, anionic dye EBT, pharmaceutical waste CIP, phenol PNP and phthalate ester DEP by adsorption. For the best adsorption results, kinetic modeling and isotherm analysis has also been undertaken to determine the adsorption rate and adsorption mechanism. Chapter 5 reports the N-doped carbon spheres synthesized using glucose and soy flour via the HTC method (GS1) and were heat-treated at 900 ⁰C in Ar and N 2 atmospheres separately to activate/modify the surface without additives. The effect of heating atmospheres on the sample properties has been studied using SEM, XRD, FTIR, UV–Visible, Raman, XPS, BET, and TG-DSC. The difference in surface chemistry was confirmed by XPS and the point of zero charge values of synthesized samples. Variation in results was explained on the basis of the diffusivity and density of the gases used and the corresponding mechanism has been proposed. The role of heat treatment atmosphere has been compared for the adsorption efficiency of synthesized samples for the chosen model pollutants: MB, CV, EBT, PNP, CIP and DEP. The adsorption rate and mechanism have been determined using kinetic model and isotherm analysis based on the best adsorption results. Chapter 6 describes the chemical activation of N-doped carbon spheres synthesized using glucose and soy flour via the HTC method (GS1) explained in chapter 3 using KOH and ZnCl 2 in different impregnation ratios, separately. The effect of the chemical activating agent on the sample properties has been studied using FE-SEM, XRD, FTIR, UV–Visible, Raman, XPS, and BET. The difference in surface area and surface chemistry with impregnation ration as well as with activating agent was confirmed by BET, XPS and pH pzc of samples. Variation in the results was explained on the basis of the reactivity of activating agents with the sample at low temperature and the corresponding activation mechanism has been proposed. The adsorption efficiency of the synthesized samples was carried out for cationic dyes MB and CV, anionic dye EBT, pharmaceutical waste ciprofloxacin, PNP and phthalate easter DEP. Based on the best adsorption data, the adsorption rate and the corresponding mechanism were determined using kinetic model and isotherm analysis. Chapter 7 describes single-step synthesis of biochar by pyrolysis of defatted soy flour in argon atmospheres at various temperatures: 450, 650, and 750 ᵒC. The pyrolysis temperatures were determined by TG/DTG/DTA analysis. The pyrolyzed sample was treated with HNO 3 for complete demineralization. Further, the variation in the samples characteristics with pyrolysis temperature and acid treatment was explained by FE-SEM, XRD, FTIR, UV- visible, FTIR, Raman, BET and XRD results. pH pzc of the samples were also determined using the molten salt method. The synthesized samples were used for the adsorption of pollutants viz MB, CV, EBT, PNP, CIP, and DEP. The adsorption rate and mechanism were calculated using kinetic model and isotherm analysis based on the best adsorption data as done in the previous chapters. Chapter 8 presents the the photo-induced activity studies for the samples which had the best adsorption chracteristics as observed and discussed in the previous chapters. These studies were done under UV irradiation and sunlight. The % decolorization efficiency of the chosen samples was tested for the pollutants viz. MB, EBT, PNP, CIP, and DEP. To determine the mechanism for decolorization enhancement, scavenger tests were carried out and explained with the help of valence band spectra and bandgap of the samples. Chapter 9 summarizes the work done presented in this thesis. It also discusses the scope for future work.