Alüminyum yüzey işleme endüstrisi proses atıksularından elektrokoagülasyon yöntemiyle florür giderimi

Abstract

Fluoride concentration higher than 4 mg/L creates several dental problems and decompositions in the skeletal structure. Daily intake of 6 mg fluoride may cause articulation diseases, weight losses and bone breakages. Therefore, fluoride concentration in drinking water has been strictly regulated as a maximum value of 1.5 mg/L by Turkish Standards Institute (TSE), World Health Organization (WHO) and European Commission (EC). Owing to its high toxicity, legal authorities have established the discharges standards for fluoride from wastewater treatment plants. Therefore, the increasing demand for the control of fluoride being released from industrial activities has led to the search of more effective treatment methods for fluoride bearing effluents. Although, several physico-chemical treatment methods such as adsorption using different types of adsorbents such as activated alumina, activated carbon, fly ash etc., coagulation with alum, chemical precipitation with lime, and electrocoagulation using aluminum electrodes have been addressed for removal of fluoride from the samples bearing moderately low amount of fluoride in the related literature. Among these treatment processes, electrocoagulation (EC) is deemed a promising one in removing fluoride from industrial effluents. The target of the present study is to examine the treatability of an aluminum surface treatment effluent bearing high amount of fluoride by EC with aluminum electrodes and to explore the effect of varying operating parameters such as the applied current density, initial pH, and electrolyte (NaCl) concentration on fluoride abatement as well as organic matter removal and aluminum reduction. In order to assess the effects of the operation parameters on process performance and to determine the optimum operation conditions in terms of removal efficiencies, a detailed experimental study was performed using composite samples taken from an aluminum surface treatment process. An aluminum surface treatment effluent bearing high amount of fluoride (3180-6490 mg/L) and aluminum (2695-4690 mg/L) together with organic matter (485-750 mg/L) was used in the experimental study. NaCl was selected as an electrolyte because of high efficiency on EC. The electrocoagulator consisted of a 10 L-capacity polyethylene reactor equipped with 4 pairs of anodes and cathodes made of aluminum. The process performence was followed by COD, fluoride, and aluminum. In order to determine the effect of the applied current density on EC process performance a wide range of the applied current density varying between 3.7 and 18.6 mA/cm2 was applied at an electrolyte concentration of 500 mg NaCl/L and at original pH of sample (2.52). The results indicated that increasing the applied current density improved fluoride removal efficiencies. Nevertheless, this improvement in fluoride removal was observed after 30 minutes of operation time. In these experiments commenced at an initial pH of 2.52, solution pH gradually increased and reached to ~6.0. The lowest remaining aluminum concentration was obtained at the end of EC process performed at the highest applied current density. The results obtained at EC applications run at an initial pH of 2.54, a high NaCl concentration of 1500 mg/L, and at a wide range of applied current density varying between 3.7 and 29.8 mA/cm2 indicated that increasing the applied current density increased fluoride and aluminum abatement efficiencies and shortened the reaction time. Similar to EC runs conducted at 500 mg NaCl/L solution pHs gradually increased up to 6.0 during these EC applications. Up to 40% COD removal efficiencies were obtained at the end of these EC applications. The effect of electrolyte concentration and initial pH on process performance was investigated at an applied current density of 18.6 mA/cm2 for an NaCl concentrations of 1500 and  3000 mg/L  and at initial pHs of 2.52 and 3.8. EC performed at initial pH of 3.8 yielded better fluoride abatements and aluminum reductions than EC run at original pH of 2.52.While an increase in electrolyte (NaCl) concentration from 1500 to 3000 mg/L slightly enhanced removal efficiencies at the beginning of EC operation, almost the same fluoride abatements as well as COD and aluminum reductions were obtained at the end of 180 minute EC operation. In the present study, the electrocoagulation using aluminum electrodes proved to be a promising treatment method for fluoride removal as well as aluminum and organic matter reductions from an aluminum surface treatment effluent bearing high amount of fluoride and aluminum. Keywords: Aluminum, aluminum surface process, aluminum electrode, electrocoagulation, fluoride.Bu çalışmada, yüksek florür içeriğine sahip atıksuları temsil etmek üzere, alüminyum yüzey işleme endüstrisinden alınan numuneler üzerinde, elektrokoagülasyon (E.K.) prosesi ile florür ve diğer kirleticilerin giderimi ile ilgili deneysel çalışmalar yürütülmüştür. Alüminyum yüzey işleme endüstrisi proseslerinden alüminyum yüzey temizleme ve durulama banyosundan alınan atıksu numuneleri hacimsel olarak 1:1 oranında homojen bir şekilde karıştırılarak kompozit numuneler hazırlanmıştır. Alüminyum yüzey temizleme banyosunda sırasıyla %19, %8.5 oranında hidroflorik asit ve fosforik asit ile birlikte tensit (noniyonik) kullanımından dolayı, pH’sı düşük, florür, alüminyum ve KOİ konsantrasyonu yüksek atıksular meydana gelmektedir. Bu atıksuların arıtımı gerek florür gerekse alüminyum ve KOİ değerlerinin alıcı ortam deşarj standartlarını sağlayabilmesi açısından önem kazanmaktadır. Deneysel çalışmalarda E.K. uygulamalarında işletme parametrelerinin proses performansı üzerine etkileri incelenmiştir. İşletme parametreleri olarak, akım yoğunluğu, elektrolit olarak kullanılan NaCl konsantrasyonu ve pH ele alınmıştır. Ham kompozit numuneler üzerinde uygulanan E.K. denemelerinde artan akım yoğunluğu ve zamana bağlı olarak florür için yaklaşık %97 giderim verimi elde edildiği, artan akım yoğunluğu ve NaCl konsantrasyonuna bağlı olarak alüminyum gideriminde yüksek giderim verimlerine ulaşılabildiği ancak organik maddeyi temsil eden KOİ gideriminin ise %40 civarında kaldığı tespit edilmiştir. Deneysel çalışmaların sonucu olarak ham numunelerin E.K. prosesi ile gideriminde florür, alüminyum ve KOİ parametreleri için alıcı ortam deşarj standartlarının sağlanamadığı tespit edilmiştir. Anahtar Kelimeler: Alüminyum, alüminyum yüzey işleme, alüminyum elektrod, elektrokoagülasyon florür