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Title: Removal of Pesticides using Bacteria and Hydrogel-Silver Nanocomposite
Authors: Kaur, Ravneet
Supervisor: Goyal, Dinesh
Keywords: Biodegradation;Nnanocomposite;Hydrogel;Butachlor;Monocrotophos;Chitosan-PVA
Issue Date: 13-Aug-2021
Abstract: To combat crop loss due to pest infestation, pesticide application has become integral part of agriculture. Despite of benefits like improved crop yield and pest removal, frequent use and lack of timely degradation of pesticides has led to their persistence in the environment, worsening the soil quality, contaminating ground water, entering the food chain and affecting all living systems. The current study was aimed at screening of bacterial isolates capable of degrading butachlor and monocrotophos and synthesis of silver immobilized chitosan-PVA hydrogel nanocomposite (Ch/PVA-Ag) for their remediation under in-vitro conditions. Degradation of monocrotophos and butachlor was carried out using 15 bacterial isolates, where maximum monocrotophos degradation was carried out by Bacillus licheniformis BPRIST030 (NA14), whereas, isolate A16 identified as B. altitudinis A16 by 16S rRNA sequencing showed maximum butachlor degradation. Monocrotophos spiked in Bushnell Hass broth was solely used as nutrient source by NA14, with increase in viable cell count from 0.8 × 106 to 54 × 1012 cfu ml-1 in 120 h. Monocrotophos degradation took longer in comparison to butachlor degradation by bacteria, as about 70% monocrotophos was degraded in 5 days. GC-MS analysis showed generation of five major metabolites including, dimethyl phosphate, phosphoric acid, malonic acid, dihydrazide, N-3-dimethylbut-2-enamide and N-methyl acetoacetamide. Based on the produced metabolites, a metabolic pathway was proposed. Butachlor was utilised as a sole source of carbon by B. altitudinis A16 degrading 90% of 50 mg L-1 butachlor in 5 days at a rate constant and half-life (t1/2) of 0.02 h-1 and 34.65 h, respectively, following the first-order reaction kinetics. Five metabolites (N-(butoxymethyl)-N-(2-chloroethyl)-2,6-diethylaniline, (N-(butoxymethyl)-2-chloro-N-(2-ethylphenyl) acetamide, N-(butoxymethyl)-2,6-diethyl-N-propylaniline, 2-chloro-N-(2,6-diethylphenyl) acetamide and 2,6-diethylaniline) were produced during the breakdown of butachlor by B. altitudinis A16 as identified by GC-MS analysis, which are further mineralized to carbon dioxide and water. A metabolic pathway was proposed and compared with other bacteria. Silver nanoparticle (AgNP) incorporated chitosan-PVA (Ch/PVA) hydrogel nanocomposite was fabricated by repeated freeze-thaw treatment using glutaraldehyde as crosslinker for removal of herbicide butachlor from aqueous solution. Ch/PVA hydrogel acted as a nanoreactor in providing a matrix for in-situ immobilization of AgNPs and was characterized by FTIR, SEM-EDS, TEM, ICP-AES, XRD, XPS, TDA-DSC and BET analysis. Incorporation of AgNPs led to increase in thermal stability and surface area and possessed crystalline structure with a size of about 5-20 nm. Effect of adsorption parameters such as pH, butachlor concentration, sorbent dosage, temperature and contact time on removal of butachlor was investigated. Comparative adsorption capacity of Ch/PVA hydrogel showed a maximum adsorption of 86.55%, while Ch/PVA-Ag showed an increased adsorption of about 96.87%. Linear and non-linear forms of Langmuir and Freundlich models with different error functions (R2, R2adj, RSME, ꭓ2 and RSS) confirmed monolayer adsorption of butachlor, favouring linear Langmuir model. Ch/PVA-Ag nanocomposite employed for adsorption-coupled-photocatalytic removal of butachlor proved to be a promising photocatalyst, degrading 85% of 10 mg L-1 butachlor in the presence of UV-light following first order kinetic model with a half-life of 114.54 min. Activity of nanocomposite hydrogel was due to synergism effect taking place between biopolymer nanocomposite hydrogel part acting as adsorbent and AgNPs as photocatalyst. Based on eight major metabolites ((1-(2-butoxy-7-ethyl-3-methylindolin-1-yl)propan-1-one, 1-(2-butoxy-7-ethyl-3-methylindolin-1-yl)ethenone, 3-(2,6-diethylphenyl)-1,3-oxazonan-4-one, 3-(2,6-diethylphenyl)-1,3-oxazonane, (((2,6-diethylphenyl)amino)methoxy)methanol, 2,6-diethyl-N-(hydroperoxymethyl)cyclohexa-2,4-dienamine, (E)-2,4,5-trihydroxypent-2-enoic acid and (Z)-penta-2,4-dienoic acid) obtained from LC-ESI-Q-TOF-MS analysis, a plausible degradation pathway was proposed. Reusability of nanocomposite and break-down of butachlor with no release of Ag ions in the solution makes it highly suitable for photocatalytic degradation of pesticides.
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