احيا بيوالكتروشيميايي نيترات از فاضلاب با استفاده از گرافيت پوشش داده شده با نانولوله‌هاي كربن چند ديواره

Bioelectrochemical reduction of nitrate from wastewater using graphite-coated multi-walled carbon nanotubes

هدف از اين مطالعه بررسي احيا نيترات از فاضلاب به روش دنيتريفيكاسيون بيوالكتروشيميايي با استفاده از گرافيت پوشش داده شده با نانولوله هاي كربن بعنوان كاتد است. اين مطالعه در مقياس آزمايشگاهي و در يك راكتور بيوالكتروشيميايي دو محفظه‌اي انجام شد. از الكترود استيـل به عنوان آند و از گرافيت به عنوان كاتد استـفاده شد. براي افزايش كارايي احيا نيترات، از پوشش نانولوله‌هاي كربن در كاتد استفاده شد. در اين پژوهش، اثر pH، دانسيته جريان و زمان در احيا نيترات در راكتور بيوالكتروشيميايي مورد ارزيابي قرار گرفت. بيشترين ميزان احيا نيترات در محدوده pH خنثي و دانسيته جريان حدود 15 ميلي آمپر بر سانتي‌متر مربع حاصل شد. به علاوه در دانسيته جريان 15 ميلي آمپر بر سانتي‌متر مربع و مدت زمان 8 ساعت، اين سيستم بيوالكتروشيميايي، ميزان نيترات را از 150 ميلي‌گرم بر ليتر به كمتراز حد استاندارد رساند. بر اساس يافته‌هاي به دست آمده، مي‌توان نتيجه‌گيري كرد كه استفاده از نانولوله‌هاي كربن چند ديواره به عنـوان پوشش روي كاتد مي‌توان ميزان احيا نيترات را افزايش داد. افزايش كارايي در شرايط بهينه اين پژوهش حدود 14 درصد تعيين شد. بنابر اين، استفاده از نانولوله‌هاي كربن در فرآيند احيا بيوالكتروشيميايي نيترات مي‌تواند با كمك به افزايش ميزان بيوفيلم توليد شده و سرعت بخشيدن به احيا نيترات در كاتد، موجب كاهش مدت زمان دنيترفيكاسيون و اخذ استانداردهاي زيست محيطي شود.

Increasing of nitrate load in drinking water has resulted in a decrease in water quality and has even let to seriously affect human health problems such as methemoglobinemia in infants and gastric cancer in adults. Various techniques have been presented for nitrate treatment, such as ion exchange, reverse osmosis, adsorption, electrocoagulation, electromagnetic, and biological process. Biological denitrification is an efficient method for nitrate removal from environment. The aim of this study is bioelectrochemical removal of nitrate from wastewater using carbon nanotubes immobilized in cathode. Bioelectrochemical processes is a novel environmental friendly method for wastewater treatment. In the bioelectrochemical denitrification, autotrophic denitrifying microorganisms utilized hydrogen generated at the cathode by the process as electron donor to reduce nitrate into nitrogen gas. Autohyrogenotrophic denitrifying bacteria generally adhere to the cathode surface and make a biocathode. Therefore, cathode electrode material is one of the main factors that affecting in the bioelectrochemistry process efficiency. It has been reported that carbon nanotubes could transfer of electrons between bacteria and electrode in the process. In this study, the graphite- coated multi-walled carbon nanotube was utilized as biocathode in the process for nitrate reduction. This study has been done in a laboratory scale bioelectrochemical rector. A plexiglass reactor consists of two chambers separated by a cation exchange membrane was utilized for the experiments. It was consisted of an anode and a cathode chamber with the working volume of 2000 ml. The chambers of bioelectrochemical process were fed by the same synthetic wastewater and isolated autotrophic denitrifying bacteria. The electrodes were placed on reactor and connected to DC power supply. The biofilm of denitrifier bacteria were formed on the cathode electrodes. The effect of various operating parameters such as current density, pH and retention time using the constructed biocathode were evaluated for nitrate reduction efficiency. The nitrate concentrations were determined using UV/Vis spectrophotometer. Increasing the current densities up to 15 mA/cm2, nitrate reduction efficiency was increased from 51.82% to 87.96%. Maximum nitrate reduction in bioelectrochemistry reactor used in this study was carried out at 15 mA/cm2current density and neutral pH values. Hydrogen production at cathode influences the current density. Therefore, increasing the current density causes higher production of hydrogen, which enhances reduction of nitrate at the cathode. Furthermore, at current density of 15 mA/cm2 and retention time of 8 hours, the bioelecterochmical system can reduce the nitrate levels to below the WHO standard. According to the obtained results, at higher current density, nitrate reduction rate was decreased. It seems that decrease in denitrification efficiency by increasing the current density is due to excess H2 production in cathode. At a constant condition, and initial pH values of 6.0, 7.0, 8.0 and 9.0, nitrate removal efficiency were 55.26, 85.52, 84.18 and 43.38, respectively. Decomposition of carbonate ions takes place when pH value is lower than 7.0. While, nitrite accumulation occurred when pH value is higher than 8.6. Therefore, Nitrite accumulation and decomposition of carbonate ions decreased denitrification efficiency at pH values of 6.0 and 9.0. At retention time of 2 to 6 h nitrate reduction efficiency was increased from 43.18% to 92.7%. While at retention time of 6 to 24 h denitrification rate was limited due to increasing accumulation of H2 and nitrate byproducts such as nitrite and ammonia in the surface of cathode. Increasing the retention time in the reactor makes the denitrifying bacteria consume greater extent of nitrate The results showed that utilization of MWCNT as cathode modifier increase the nitrate reduction efficiency about 14 present. The use of MWCNT can increased biofilm formation and therefore the reduction time for achieving to WHO nitrate standard was reduced .