Resource Recovery and Value- Added Products From Agricultural Waste

Abstract

Lignocellulosic biomass resources such as agri-wastes are utilized as suitable feedstock to produce bioenergy and value-added products. In the present study, the Arecanut husk (AH) (Areca catechu) was selected as the feedstock to recover and produce value-added products. The study was carried out in three phases, (i) evaluation and ranking of various pretreatments methods using multiple attribute decision-making (MADM) approach, (ii) pretreatment and co-digestion of AH for biogas yield, and (iii) synthesis of AH derived lignin-carbon material for oil-water separation. In phase I, the objective is to evaluate and rank different pretreatment methods and select the best pretreatment method using MADM approaches to facilitate the increased biogas yield. The evaluation was done using Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) and integrated Design of Experiments (DoE) - TOPSIS. Seven alternatives with five relevant attributes were adopted for this study. Based on the above decision-making framework, the alkaline pretreatment (NaOH (8%)) option was ranked first, followed by Ca(OH)2 and NH3.H2O (10%) pretreatment as second and third options. The integrated DoE - TOPSIS method has reduced the uncertainty in results by considering different weights and replications. The model portrayed the best pretreatment method employed in anaerobic digestion; thus, minimizing the experiments done during the downstream pretreatment process aided anaerobic digestion. Phase II of the study aims to evaluate the reduction of the recalcitrance of the AH by two approaches, (i) applying various pretreatment methods to access the cellulosic content and (ii) co-digesting it with food waste for the biogas production. The study evaluates the feasibility of utilizing chemically pretreated AH for biogas production. The effects of various pretreatment methods on the material solubilization to enhance biogas production from AH were checked. The AH was pretreated by four methods viz., acidic (H2SO4), alkaline (NaOH), oxidative (H2O2), and organosolv (ethanol in 1% H2SO4). The dosing of chemicals in acidic, alkaline, and oxidative pretreatments were 2, 4, 6, 8, and 10% (w/v), whereas, in organosolv, the dosage was varied from 25%, 50%, 75%, and 100% (v) for the batch hydrolysis. The batch hydrolysis trials were conducted at two different temperatures, i.e., 25⁰C and 90⁰C, and solids/liquid ratio of 1:10 ratio for 24 hours. The obtained experimental data from the ii solubilization study were analyzed using the TOPSIS technique, which showed that alkaline pretreatment at a temperature of 90⁰C had favoured the material solubilization among the four pretreatment methods. The pretreated AH was carried further for anaerobic digestion maintained at mesophilic condition. A maximum biogas yield of 683.89mL/gVS was obtained with 2.3 times more when compared with raw AH. Four kinetic models viz., First-order exponential, Logistic, Transference function, and Modified Gompertz model, were used to fit the experimental cumulative biogas production data. The Modified Gompertz model and logistic model (correlation coefficient > 0.99) were obtained as best fit to the cumulative biogas curve. The overall process performance is represented by the kinetic parameters obtained from these models. Furthermore, a multiple linear regression equation for the biodegradability index (BI) is formulated as a technical tool to predetermine biomethane production. It is depicted as a function of biomass compositions (cellulose, hemicellulose, and lignin) with a high correlation (> 0.95). The suitability of AH as the co-substrate with food waste (FW) for biogas production was examined in this research. The substrate mix ratio (AH: FW) was varied as 0:1, 1:3, 1:1, 3:1 and 1:0 in terms of volatile solids (VS) for a batch mode enclosed reactor (1L) at mesophilic (35⁰C) condition for 34 days. The 1:1 mix ratio, which yielded the highest biogas (321.12mL/gVS), is fixed for further experiment for optimizing the S/I ratio. The phase III lignin extracted from AH was used as an additive in lignin-carbon foam synthesis as a potential adsorbent for the oil-water separation. The lignin yield from the AH increased as the husk fibre size reduced. The extracted lignin and lignin-carbon foam were characterized by morphological, structural, compositional, and thermal degradation examinations. The synthesized carbon foam exhibited ultralight weight (density=0.0294 g/cm3), excellent hydrophobicity (water contact angle from 110°~ 132°), mesoporous structure (3D cell-like), good fire-retarding capacity and thermally stability due to lignin addition. The foam showed an excellent sorption capacity for different oils, and the highest sorption was observed for diesel oil (7842.71mg/g). The optimization of contact time, carbon foam dosage, and initial oil concentration were done for the diesel oil sorption. The isotherm study and kinetic model evaluation were done for the diesel adsorption on the lignin-carbon foam. Temkin model was found the best fit for the adsorption isotherm. The adsorption kinetics of the lignin-carbon foam iii for diesel oil was best described by pseudo-second-order kinetics. The thermodynamic parameters showed that the adsorption was endothermic and spontaneous (ΔH°=+4926.46 J/mol and ΔS°= 25.249 J/mol/K). The proposed mechanism depicts that the adsorption primarily influenced H-bonding and n-π interactions. The enduring adsorption of oil into the lignin-carbon foam within few seconds shows the material oleophilicity and confirms their application prospect in oil spill clean-ups.