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Competitive photocatalytic removal of aqueous phase selenocyanate (SeCN-) in the presence of some critical co-pollutants: adsorption modelling, process kinetics, and reaction mechanisms

l Competitive photocatalytic removal of aqueous phase selenocyanate (SeCN-) in the presence of some critical co-pollutants: adsorption modelling, process kinetics, and reaction mechanisms. PhD thesis, King Fahd University of Petroleum and Minerals.

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Arabic Abstract

إن هذ الدراسة تهدف الي معالجه ميا ة الصرف من مركبات SeCN- باستخدام كلا من طريقة Adsorption و Photocatalysis. بداية, تم اختبار معالجة مياه الصرف الملوثه بمركب SeCN- في وجود بعض مصادر السيلينيوم الاخري مثل SeO32- و SeO42- عن طريق امتزازها علي سطح TiO2 متبوعة بكسر الرابطة في مركب SeCN- باستخدام طريقة Photocatalysis معتمدة علي TiO2 في وجود بعض الملوثات الاخري مثلammonia, cyanide, thiocyanate, and phenol. لوحظ انه في حالة معالجة نظام مكون من SeO32- و SeO42-, ازالة مكون SeO32- لا تتاثر بوجود مركب SeO42- في حالة استخدام عملية Adsorption والعكس بالعكس. ايضا, تم نمذجة عملية Adsorption للثلاث مركبات مجتمعة (SeCN-, SeO32-, SeO42-) عبارة عن حبيبات كروية من TiO2 متصلة من الخارج في حالة SeCN- و SeO42- او الداخل في حالة SeO32-. لذا فإنه يمكننا القول ان عملية Adsorption للمركبات الثلاث تكون فعالة جدا في معالجة مياة الصرف تحت ظروف معينة. ان هذا البحث تطرق الي امكانية استخدام الاشعة الفوق بنسفجية (UV-light) من مصادر مختلفة مثل ضوء الشمس (Solar Photocatalysis) او من مصدر اصطناعي (UV-lamp) لمعالجة مياة الصرف الملوثة بمركبا السلينيوم (SeCN-). اولا بالنسبة الي استخدام solar photocatalysis, فانه تم ملاحظة اكبر نسبة معالجة عند الاوساط الحامضية وتزداد بزيادة تركيز مركبات السيلينوم الابتدائية وكذلك تركيز EDTA. ايضا في وجود بعض المركبات الاخري السابق ذكرها, فانه استخدام طريقة solar photocatalysis تعالج مياة الصرف من مركبات SeO32- بصوره اكبر من مركب SeCN- والتي بدورها اكبر من معالجة مركبات SeO42-. ايضا شهدت عملية المعالجة عدم ازالة مركب ammonia ولكن في حالة مركب phenol فانه يتحول الي مركب النزين وبعض المركبات الاخري. ايضا تم استخدم النمذجة الرياضية (RSM) لتوقع نسبة معالجة مياة الصرف من مركبات SeCN- في وجود مركبات phenol باستخدام طريقة Photocatalysis. استخدام هذه الطريقة ف النمذجة اثبتت انها اقتصاديه وسريعة بشكل كافي حيث كانت اوضحت ان انسب الظروف لازالة مركب SeCN- و phenol هي عند درجة حامضية 4 و 10 وتركيز 450 مجم/لتر من EDTA.

English Abstract

Present study investigated removal of selenocyanate species from aqueous phase using TiO2 based adsorption and photocatalysis. Initially, competitive adsorption of selenite, selenate and selenocyanate onto TiO2 was studied under varying mixed scenarios that was followed by a detailed investigation on the efficiency of TiO2 based photocatalytic degradation (PCD) process for the competitive destruction of selenocyanate complex along with simultaneous removal of co-pollutants ammonia, cyanide, thiocyanate, and phenol from simulated mixed wastewater streams. For mixed selenite & selenate binary systems we noted high selenite adsorption with no noticeable selenate effect onto selenite adsorption even up to 10 ppm selenate. However, selenate adsorption was markedly affected by the selenite species. Nevertheless, results from the binary selenite & selenocyanate mixed systems indicated no such competitive adsorption trend and similar was also noted for the binary selenate & selenocyanate mixed adsorption studies. Findings from the tertiary mixed selenite, selenate, and selenocyanate studies showed similar adsorption trends. In general, the adsorption results above pH 4 showed selenite > selenocyanate > selenate. For adsorption modelling an inner sphere type complex i.e., Ti-SeO3- reasonably predicted selenite adsorption whereas selenate & selenocyanate adsorptions were well predicted considering outer sphere complexes, i.e., Ti-H2O-SeO4- and Ti-H2O-SeCN respectively. The adsorption results from the present work indicate that selenite, selenate, and selenocyanate species can be effectively removed from respective aqueous streams under varying mixed competitive conditions using the TiO2 based adsorption process with a careful control of process parameters. The present study also investigated the efficiency of both UV and solar light assisted photocatalysis for the removal of selenocyanate from aqueous phase under a varying set of mixed conditions. Higher solar photocatalytic removal of selenium species transpired at pH 4 and it increased with an increase in both initial selenium and initial EDTA concentrations. The mechanism for selenocyanate removal is via SeCN complex destruction followed by stepwise oxidation to selenite and then to selenate, which in turn are reduced to elemental selenium using hole scavenger EDA. Solar photocatalysis in the presence of co-pollutants follows the following order: SeO32- > SCN- > CN- > CN-/SCN-/NH4+. It is worth noting that while selenium and thiocyanate species were converted to elemental selenium and sulfate respectively, no significant removal of ammonia and cyanide was noted for other experimental conditions. The mechanism of phenol removal in the presence of selenocyanate involves the formation of benzeneseleninic acid, hydroquinone, resorcinol, and pyrocatechol as primary intermediates. Secondary intermediates include maleic acid, formic acid and fumaric acid among others. Linear and two factor interaction mathematical models were developed for photocatalytic removal of selenocyanate and phenol under competitive environment using face-centered central composite design. Higher R2, adjusted R2, predicted R2 were obtained from regression analysis establishing the prediction ability of the models. Natural log transformation was applied to selenocyanate model as suggested by Box-Cox plot while perturbation plot was utilized for identifying relative significance of the factors. After successful model diagnosis and validation using various statistical approaches, the models were also validated experimentally. Optimal process conditions for photocatalytic removal of selenocyanate and phenol form aqueous phase were pH 4, 10 ppm selenocyanate, 5 ppm phenol and 450 ppm EDTA concentrations. RSM modelling prove to be an economical way for the removal of selenocyanate and phenol from aqueous phase using the optimization techniques. Lastly, kinetic models for photocatalytic removal of selenocyanate and phenol under varying experimental conditions were developed by considering degradation of reactants and the formation and disappearance of reaction intermediates. Considering steady state assumption, ordinary differential equations representing the rate of change of each reactant were setup and solved using Mathematica to obtain reaction rate constants. ParametricNDSolve and NonlinearModelfit command were utilized for solving the differential equation and fitting the experimental data to the model respectively. Excellent to very good model fits were achieved as suggested by adjusted R2 values as high as 0.999. Modeling results suggest selenocyanate removal via oxidation to selenite and then selenate and reaction of selenocyanate and phenol to form benzeneseleninic acid especially at low pH values. Formation of benzenediols and benzeneseleninic acid were the dominant mechanisms for phenol removal at high and low pH values respectively.



Item Type:Thesis (PhD)
Subjects:Environmental
Civil Engineering
Civil Engineering > Water and Environmental Engineering
Engineering
Research > Environment
Divisions:College Of Engineering Sciences > Civil Engineering Dept
Committee Advisor:Vohra, M. S.
Committee Co-Advisor:Al-Suwaiyan, M. S.
Committee Members:Al-Malack, M. H. and Bukhari, A. A. and Shaikh, Abdur Razzak
ID Code:140399
Deposited By:LABARAN BASHIR ALHAJI (g201004700)
Deposited On:06 Aug 2017 15:45
Last Modified:02 Nov 2017 10:26

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