Functionalized Walnut Shell Based Adsorbents For Removal Of Typical Pollutants From Water

Water that is free of contaminants is essential to the survival of humans and other organisms.Now water pollution is a concern worldwide.The adsorption technique is often considered to remove refractory pollutants.So it is important to select the proper adsorbent.Commercial adsorbents,such as activated carbon(AC)and alumina(Al2O3)with higher uptake capacity are employed;however,they are expensive and difficult to regenerate.Therefore,using agricultural waste as a cheap,alternative adsorbent for wastewater treatment provides significant environmental,societal,and economic benefits.Nonetheless,most AWs in their raw state seem to have limited adsorption capacity;however,owing to their low cost,availability in nature,and ability to be modified,they have been widely employed in the adsorption process.Herein,different chemical modification approaches have been adopted to fabricate adsorbents based on Walnut shells(WNS).In the design process,the Walnut shell was functionalised with Cetyltrimethylammonium bromide(CTAB)to obtain an adsorbent labelled as WNS-CTAB.Additionally,walnut shells were functionalised with diethylenetriamine(DETA)and triethylamine(TEA),producing an adsorbent named ACWNS.ACWNS@Zr and ACWNS-Fe were also engineered by loading Zr(Ⅳ)and Fe(Ⅲ)onto ACWNS,respectively,in the bid to promote adsorption.The modification procedure introduced additional functional sites,which improved the surface morphological structure to overcome the drawbacks,such as the low removal capacity associated with pristine walnut shells.The synthesised adsorbents were applied to decontaminate harmful pollutants such as dyes(congo red,CR),alizarin red,AR),micropollutant(bisphenol A,BPA),herbicide(2,4Dichlorophenoxyacetic acid,2,4-D),aquatic nutrient sources(phosphate,PO43-),nitrate,NO3-)and(heavy metal,hexavalent chromium(Ⅵ))in a batch and column modes.To affirm the morphological changes endowed to the as-synthesised adsorbents,analytical techniques such as X-ray photoelectron spectroscopy(XPS),X-ray diffractometer(XRD),Fourier transform infrared spectroscopy(FTIR),scanning electron microscopy(SEM),Bruaneur-Emmet-Teller(BET)method were employed.The findings reveal that the produced adsorbents were crystalline with me soporous structural features and various functional groups that play roles in the adsorption processes.(1)For the WNS-CTAB adsorbent,the Langmuir isotherm model best described the adsorption equilibrium results,confirming the monolayer adsorption of the pollutants’ molecules onto the surface of the adsorbent.The maximum monolayer adsorption quantity of WNS-CTAB via the batch experiment was 38.5 mg g-1 for BPA and 104.4 mg g-1 for CR at 303 K.The pH of the solution highly influenced the uptake.For CR adsorption,pH=6 was optimum,while pH=9 favoured the adsorption of BPA.Pseudo-second-order kinetic models described the adsorption kinetics of both BPA and CR.Again,it was noticed that the adsorption capacity of CR onto WNS-CTAB reduced as the salt concentration was raised from 0.01-0.2 mol L-1 but did not greatly affect BPA uptake.The CR adsorption process was spontaneous and exothermic,while the BPA adsorption was spontaneous and endothermic.The column studies show that the maximum breakthrough durations were 113 min for CR and 23 min for BPA at flow rates of 6 mL min-1,respectively.Yan’s model best fitted the column’s breakthrough curve.The mechanisms associated with BPA and CR adsorption onto the WNS-CTAB may include van der Waals interaction,hydrophobic interaction,and electrostatic force.WNS-CTAB demonstrated good reusability potential with desorption through successive adsorption-desorption cycles performed in batch and column mode.Moreover,the Langmuir and Freundlich isotherm models predicted the column adsorption of CR and BPA,respectively.(2)The efficiency of ACWNS was investigated using CR,Cr(Ⅵ),PO43-and NO3-as pollutants using the batch and column methods.In both systems,the adsorbent illustrated its effectiveness with high adsorption capacity,and the maximum adsorption for all the studied pollutants occurred in the acidic pH range.The maximum monolayer adsorption capacity in the batch mode was 224.4,226.8,78.2 and 24.8 mg g-1 for CR,Cr(Ⅵ),PO43-(according to P)and NO3-at 3o3 K,respectively.However,salinity negatively influenced all pollutants’ uptake capacity as salt concentration increased from 0.02-0.1 mol L-1.The pseudo-second-order and the Thomas kinetic mode best describe the adsorption in batch and column modes,respectively.The experiment’s outcome suggested that electrostatic interaction mediated the adsorption of these pollutants onto ACWNS.Again,the adsorbent depicted higher selectivity towards targeted pollutants,among other molecules.The adsorbent demonstrated improved regeneration capacity and therefore showed potential for the uptake of studied pollutants ions from wastewater.The mass transfer factor determined in the column studies revealed a stronger adsorbent-adsorbate affinity between ACWNS and the pollutants.Based on the isotherm fitted results,the Langmuir model better represented the obtained data for PO43-,NO3-,CR,and Cr(Ⅵ)adsorption during the column studies.(3)Further adsorption studies indicated that ACWNS@Zr had a maximum monolayer capacity of 415.5,73.9,and 227.4 mg g-1 for AR,PO43-,and 2,4-D(303 K),respectively,which was significantly dependent on factors such as reaction time,solution pH,temperature,and the presence of some common anions.The adsorption capacity for the pollutants exhibited a declining trend in the presence of salt ions.The uptake of AR dye was more favorable when the temperature increased from 298-313K,and the reverse occurred for the adsorption of PO43-and 2,4-D.Pseudo-second-order kinetic model was more favorable for describing the uptake of all the pollutants.Moreover,the Langmuir model was observed to describe better both adsorption processes of AR and 2,4-D,while the Freundlich model best-fitted PO43-adsorption with chemisorption as the principal underlying mechanism.The column studies show that a low flow rate and higher mass of adsorbent favours higher removal efficiency.Comparatively,the order of the affinity of the studied pollutants towards ACWNS@Zr is PO43->2,4-D>AR based on the β1 obtained from the column’s mass transfer analysis.The Thomas model described AR and 2,4-D uptake,while the Clark model described phosphate.Moreover,the column isotherm studies revealed that the Langmuir model predicted the adsorption data of AR,PO43-,and 2,4-D,which was consistent with batch adsorption of AR and 2,4-D.(4)The adsorptive capacity of ACWNS-Fe towards 2,4-D and TC followed the Langmuir model,whiles the Temkin model was suitable to describe the adsorption of phosphate.The maximum adsorption capacity as reviewed by Langmuir was 75.4,408.1,and 58.1 mg g-1 at 303 K for phosphate,2,4-D,and TC,respectively.Thermodynamic studies suggested that the adsorption process could have occurred via a chemical reaction(chemisorption),with complexation,electrostatic attraction,and ion exchange as the underlying mechanisms for the adsorbate uptake onto ACWNS-Fe.The uptake process of TC followed the endothermic route while the exothermic reaction mediated the uptake of 2,4-D,and Po43-.During the column studies,a low flow rate of 6 mL min-1 and bed height of 9 cm proved to exhibit higher removal efficiency.The results suggest that the AWNS-Fe may still be used for pollutant adsorption after successive adsorption and desorption cycles.For the uptake of 2,4-D and PO43-in the column system,the Clark model best fitted the experimental data,while Yan’s model described the uptake of TC.On the other hand,the column isotherm studies reveal that the Langmuir model best described the uptake of 2,4-D and PO43-.However,both Langmuir and Freundlich models appeared to predict the uptake of TC onto ACWNS-Fe based on R2 values.Finally,it was observed that all the synthesized adsorbents(WNS-CTAB,ACWNS,ACWNS@Zr,and ACWNS-Fe)demonstrated some antibacterial potential,with WNS-CTAB showing the highest inhibition(>99%)against S.aureus and E.coli.The adsorbent demonstrated good adsorption properties and reuse of spent adsorbents through regeneration.Consequently,these modified walnut shells could be a promising adsorbent with significant potential for pollutant decontamination.The findings of this work are expected to expand knowledge on the development of adsorbents based on agricultural waste materials with great properties for wastewater remediation.