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Kinetics and Thermodynamics of Chromium Iii Removal from Aqueous Solutions by the Organic Ion Exchangers

Thesis Info

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Author

Shah, Khizar Hussain

Program

PhD

Institute

University of Peshawar

City

Peshawar

Province

KPK

Country

Pakistan

Thesis Completing Year

2013

Thesis Completion Status

Completed

Subject

Chemistry

Language

English

Link

http://prr.hec.gov.pk/jspui/handle/123456789/1092

Added

2021-02-17 19:49:13

Modified

2024-03-24 20:25:49

ARI ID

1676726569336

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Chromium is a toxic element and exists in two stable oxidation states, Cr(III) and Cr(VI) where the later is very toxic to human beings. The presence of strong oxidants in soil and water can change Cr(III) into harmful Cr(VI). Therefore, it is necessary to remove both the chromium species from aqueous solutions. Thus, the present study pertains to the use of commercially available different organic ion exchangers for the removal of Cr(III) ions from aqueous solutions. The exchangers used are macroporous weak acid exchanger Amberlite.IRC-50 and strong acid exchangers, microporous Amberlite.IR-120 and macroporous Amberlyst-15. The sorption studies are conducted employing the concentration in the range of 0.962-19.231 mmol/L at different temperatures of 293, 313, 323 and 333K. It is observed that Cr(III) sorption increases with increasing concentration, time and temperature of the solution. The selectivity of exchangers towards Cr(III) ions is found to follow the order Amberlyst-15 > Amberlite.IR-120 > Amberlite.IRC-50 which is controlled by the surface morphology, functionality and porosity of the resin matrix and mobility of the exchanging ions. The maximum exchange capacity observed for macroporous Amberlyst-15(H+) is 1.20 mmol/g which increases to 1.31 mmol/g at 333K. All the Na+ forms of the exchangers particularly the weak acid exchanger Amberlite.IRC-50 are found to co-sorb H+ along with Cr(OH)2+ ions. This H+ co-sorption is observed to increase with the increase in temperature and is thus endothermic in nature. The equilibrium data is subjected to the Langmuir equation to determine the maximum exchange capacities (Xm) and binding energy constants (Kb). The Amberlyst- 15 has greatest exchange capacity among the all exchangers due to its porous structure and largest contact area, while the weak acid exchanger Amberlite.IRC-50 has the greatest binding energy constants due to stronger interaction of Cr(III) with the carboxylic groups as compared to sulphonic groups in strong acid exchangers. The thermodynamic parameters (ΔH, ΔS and ΔG) for Cr(III) sorption are also evaluated. The values of both ΔH and ΔS are positive showing that process is endothermic and is accompanied by the dehydration of Cr(III) ions. Further, these values are found to be lower for macroporous Amberlyst-15(Na+) due to the presence of abundant water molecules in the resin matrix. The ΔH and ΔS are linearly related showing the process to be entropy driven ion exchange. The kinetics data and the interruption tests suggest the pre-dominance of particle diffusion mechanism. The macropore diffusion rates are higher than micropore diffusion rates in Amberlyst-15. The activation parameters are calculated by Arrhenius and Eyring equations. The lower activation energy of weak acid exchanger is due to the increased co-sorption of H+ ions at higher temperature which facilitates the dissociation of carboxylic group for Cr(III) binding. The IR and XPS studies confirmed the electrostatic interaction is the mechanism of chromium binding with the ionogenic sites of the exchangers. Both the co-ions and counter-ions are observed to have a profound effect on the removal of Cr(III) ions by the Amberlyst-15(H+). To find out the co-ions effect, Cr(III) sorption is undertaken as a function of time and temperature using CrCl3.6H2O and [Cr4(SO4)5(OH)2] solutions, while for counter ions effects, the sorption on H+, Li+, Na+, Ca2+ and Al3+ forms is investigated. The rate is found to be governed by the particle diffusion for both the co-ions chloride and sulphate and is faster for Cl- solution than SO42-. The exchange capacities are, however, found to be higher for SO42- system than Cl- . It is suggested that in case of Cl- solutions, the metal is exchanged as Cr3+, while in case of SO42- solutions, the metal exchanging specie is CrSO4+. The selectivity of Amberlyst- 15 is observed to follow the order univalent > divalent > trivalent forms which is associated to the electrostatic interaction of ions with the fixed group of the exchanger. The thermodynamic and activation parameters reveal that the mechanism of Cr(III) sorption for all the counter ions is the entropy driven ion exchange. The rate of sorption of three metal ions Cr(III), Ca(II) and Al(III) on Amberlyst- 15(H+) at different temperatures (293, 313 and 333K) is also studied from equimolar mixed system. The selectivity of metal ions is observed to be in the order: Ca(II) > Cr(III) > Al(III). The hydration energy changes of metal ions are playing the dominant role in determining the selectivity of the resin. The kinetic and thermodynamic parameters like activation energy, enthalpy and entropy of activation have also been evaluated and their significance is discussed.
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ڈینگی مکائو مہم میں معاشرے کا کردار

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ڈینگی بخار کے خاتمے کے لیے معاشرتی طور پر ایک اہم رول ادا کرنے کی ضرورت ہے۔ واعظ ممبر رسول صلی...

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Application of Biochar in Reducing the Levels of Pesticide Residues in Contaminated Soils and Food Crops: A Strategy to Minimize the Health Risk

Organochloride pesticides (OCPs) also known as ―endocrine disrupting‖ chemicals are one of the persistent group of contaminants in soil that has gained worldwide concerns over the past few decades. They can transferred into the vegetables grown on the contaminated soils and eventually entered the food chain. The associated risk to human health makes the remediation of pesticide-contaminated soil a crucial undertaking. This research work is divided into different parts and several experiments were conducted to achieve the proposed objectives. The first study was carried out to evaluate the concentrations of OCPs in soils and vegetables grown in selected sites of Khyber Pakhtunkhwa (KP), Pakistan. The soil and selected vegetable species including Spinacia oleracea, Raphanus sativus, Brassica rapa, Lactuca sativa, Allium cepa and Allium sativum were collected from six districts (Peshawar, Charsadda, Nowshera, Mardan, Swabi and Swat), and were analysed for selected OCPs. Results showed that the concentrations of the selected OCPs exceeded their respective maximum residue limits (MRLs) in most of the soil samples and showed great variation from site to site. In case of the studied vegetables, the most contaminated observed was Lactuca sativa (28.9 μg kg-1), followed by Raphanus sativus (26.6 μg kg-1), Spinacia oleracea (25.8 μg kg-1), Allium cepa (16.3 μg kg-1), Brassica rapa (15.6 μg kg-1), and Allium sativum (14.7 μg kg-1). Analysis of health risk revealed that incremental lifetime cancer risks (ILCR) associated with dietary exposure of Σ30OCP through vegetables consumption by adults and children were below the acceptable risk level (10-6), indicating no cancer risk to the consumers. Moreover, exposure to the endocrine disruptor and probable carcinogen heptachlor epoxide (in children only) contributes toward non-cancer risk (HQ>1) from ingestion of selected vegetables. The presence of the banned OCPs in soils and vegetables in xvi the study area indicates that these legacy chemicals are still being used illegally for agricultural purposes which may be a cause for concern. Similarly, the second study was focused on the remediation of OCP-contaminated soils, as most of the OCPs are known to be possible/suspected endocrine disruptors and carcinogenic in nature and may severely affect humans or other living biota. Therefore, a 65-d incubation experiment was conducted to investigate the efficacy of biochars on the accessibility of OCPs, and also to evaluate their influence on soil microbial community. Results indicated that despite greater persistence of OCPs in soil, the application of selected biochars significantly (p<0.01) reduced the accessibility of åOCPs in contaminated soil as compared to the control soil during 65-days incubation period. Moreover, the findings from total phospholipid acid (PLFA) and Illumina next-generation sequencing revealed that the incorporation of biochar have altered the soil microbial community structure over time. Higher abundances of Proteobacteria, Firmicutes, Gemmatimonadetes, and Actinobacteria, were found in the biochar amendments. This recommends that biochar amendment (3%) could be advantageous for sustaining soil microbial communities and may thus likely influence the eco-toxicological processes and potential accumulation of OCPs. The third study was conducted to evaluate the effects of biochar (BC), biochar-based organic fertilizer (OF) and their combination (BCOF) on the bioaccessibility and accumulation of endocrine disruptive pesticides (EDPs) into leafy vegetables (Lactuca sativa and Spinacia oleracea) grown in a contaminated soil. The results revealed that application of BCOF and OF significantly (p<0.05) decreased bioaccessible concentrations of åEDPs in leafy vegetables by 23 and 44%, respectively, BC was particularly effective and decreased åEDPs bio-accessible concentrations by 60%, as compared to control. Biomass production, however, xvii increased the most for the BCOF amendments, (with a 70% increase in crop production compared to the control soil). Moreover, the incremental lifetime cancer risk (ILCR) values for selected EDPs were significantly (p<0.01) reduced in the amended soil as compared to control. These findings suggest that OF, BCOF and BC could be useful soil amendments to minimize the EDPs bioaccessibility in a degraded soil and their subsequent exposure to humans through vegetable consumption. Likewise, to investigate the effects of different biochars on the bioaccessible fractions of OCPs in a contaminated soil and their subsequent bioaccumulation into vegetable crops a fourth study was conducted. Four different biochars, derived from sewage sludge, soybean straw, rice straw and peanut shells were applied to a pesticide-contaminated soil at 2%, and the resulting impacts were measured. The results indicated that following biochar additions, the bioaccessible fractions of OCPs in soil and their accumulation in vegetable crops significantly (p<0.01) reduced, as compared to control. Risk assessment showed that biochar amendments markedly (p<0.01) decreased the hazard quotient (HQ) indices and the incremental lifetime cancer (ILCR) values for OCPs associated with the consumption of vegetable crops. From the results of the present studies it is concluded that the application of biochar could play a promising role in enhancing microbial abundance, reducing accessibility, restricting bioaccumulation of organochlorine pesticides in vegetables and decreasing their associated human health risk.