Kimyasal indirgeme ile ağır metal arıtımı

Abstract

Bu çalışmada, metal içeren atıksuların konvansiyonel olmayan, yeni geliştirilmiş yöntemlerle arıtılması konusunda bir literatür araştırması ve değerlendirmesini taki ben; sodyum borhidrür ile indirgeme yönteminin serbest ve kompleks olarak bağlı metal gideriminde uygulanması konusu incelenmiştir. Birinci bölümde, yapılan çalışmanın önemi vurgulanarak amaç ve kapsamı açıklan mıştır. İkinci bölümde, metal endüstrisi altkategorilerine göre atıksu kaynakları ve özellikleri belirlenerek, bu atıksuların arıtılmasında uygulanmakta ve yeni geliştirilmekte olan yöntemler belirtilmiştir. Üçüncü bölümde metali geri kazanmaya yönelik bir proses olan sementasyon ve uygulanmasında proses verimine etki eden faktörler literatür araştırmasına dayanı larak incelenmiştir. Dördüncü bölümde, kimyasal çöktürmede konvansiyonel yöntemler olan hidroksit ve sülfür çöktürmesinin avantaj ve dezavantajları değerlendirilerek, özellikle deza vantajların giderilmesi ve daha yüksek verim sağlanması için geliştirilen sülfeks pro sesi, karbamat çöktürmesi ve oksalat çöktürmesi yöntemleri literatür araştırmasına dayanılarak incelenmiştir. Beşinci bölümde, metallerin element hale ve altı değerlikli kromun üç değerlikli krom halinde indirgenmeleri için geliştirilen elektrokimyasal ve kimyasal indirgeme yön temleri hakkında yapılmış olan çalışmalar incelenmiştir. Kimyasal indirgeyici olarak sodyum borhidrür kullanımıyla serbest veya kompleks bağlı metalin indirgenmesi konusunda yapılmış olan çalışmaların değerlendirilmesi yapılmıştır. Altıncı bölümde, daha önce yapılmış olan çalışmaların sentezinden yola çıkılarak; serbest veya kompleks halde bağlı nikelin borhidrür ile indirgenmesinin araştırılaca ğı deneysel çalışma için sistem koşulları tayin edilerek teorik çözümler yapılmıştır. Yedinci bölümde hem teorik çözümler çerçevesinde incelenen faktörler dikkate alı narak hem de daha önce yapılmış olan bir çalışmanın tekrarıyla ilgili gerçekleştirilen deneyler ve sonuçlar verilmiştir. Sekizinci bölümde, deney sonuçları ile elde edilen teorik çözümlerin değerlendiril mesi yapılmıştır. Yapılan bu değerlendirme sonucunda kompleks metal halinde lite ratürde yer alan deney sonuçlarının aksine kuvvetli kompleks yapıcıların bulunduğu koşullarda sodyum borhidrür ile indirgemenin etkili olmadığı görülmüştür. Sonuç olarak borhidrür ile indirgemenin serbest metalin geri kazanılması düşünüldüğünde, ortamda bulunan oksidan türlerin (çözünmüş oksijen gibi) giderilmesi sağlandıktan sonra uygulanması gerektiği belirlenmiştir.

Heavy metal ions are pollutant parameters having significant harmful effects such as toxicity and accumulation in the environment. Major heavy metal ions carrying importance from viewpoint of pollution are cadmium, copper lead, tin and mercury. Besides these ions, sometimes arsenic, cobalt and antimon can gain importance. In addition, precious metals such as gold, silver, palladium, platinum, iridium are pollutant parameters requiring treatment. Major source of heavy metals are industrial wastewaters. Treatment of such waste water is a subject-matter required to be assesed very carefully both for reasons of many varieties of industrial sources and importance carried on pollution. Hydroxide precipitation and sulphur precipitation are major methods applied as chemical precipitation which are widely used treatment method of heavy metals Nevertheless, it is impossible to remove metals bound by strong complexing agents by application of hydroxide precipitation around pH 9-10. However complexing agents cannot be destructed at this pH, and will retain the metal in the solution. For this reason, wastewater containing complexed metal, are being treated through various pre-tratment methods or through hydroxide precipitation at high pH as a conventional method. Hydroxide precipitation has a more wide spread application from viewpoints of operation facility and being economical. However, disadvantages such as formation of high sludge volume and different optimum precipitation pH for each metal will constitute problems in the operation. Another significant aspect from viewpoint of treatment method is that chemical precipitation cannot make recovery of heavy metal possible. Instead of disposal of metals existent in precipitated sludge in the form of mud, recovery of metals in wastewater by conversion into elemental form is very important especially for recovery of precious metals. For above-explained reasons, assessment of existing and newly developed treatment techniques which are used for recovery and removal of complexed metals gains importance. The purpose of this study is to investigate new methods which are applied to limited extent or which are at the research and development stage and to research the effects of sodium borhydride reduction on removal of free and complexed metals by using and benefiting from a theoretical assessment by an experimental study. For this purpose following investigation and evaluation of literature about the subject of treatment by Tiewly developed methods of wastewater containing metals and reduction method with sodium borohydride in removal of metal asJree and bound in the complexed form have been examined. In the first chapter, purpose and scope of the study carried out has been explained by emphasizing its significance. In the second chapter, wastewater sources and properties have been specified in accordance with subcategories of metal finishing industry and consequently methods, being applied in treatment of such wastewater and newly developed, have been indicated. We can point out in sequence methods developed for the purpose of metal removal or recovery from wastewater as follows: 1- In reduction of Cr6+ions to CrHons;. Reduction with ferrous sulfate. Reduction by sulfex process,. Cementation,. Electrochemical reduction 2- In metal treatment by precipitation;. Tiocarbamate precipitation,. Sulfex process (for complexed metals),. Oxalate precipitation (for complexed metals). 3- For the purpose of recovery;. Reduction to elemental form by cementation,. Reduction to elemental form by electrolysis,. Reduction to elemental form by using sodium borohydride as a chemical reducing agent. These methods are generally at research and development stages and it appears that precipitation by sulfex process, cementation and chemical reduction processes will be more potential method. In the third chapter, cementation which is a process directed to metal recovery and factors effecting the efficiency of the process has been examined as based on literature research. In conclusion of conducted studies advantages of metal recovery by cementation process can be summarized as follows;. No chemical addition is required for wastewaters with a pH in the range of 7 to 9. No sludge is generated, eliminating the costs associated with sludge handling. Spent iron fillings can be reprocessed or can be sold to steel foundries.. Operator attention is minimal. Energy requirements are moderate. When advantages are taken into consideration, it can be said that cementation - xi - process which is applied in limited extent only in copper treatment is an effective method for the removal of free and complexed metals and assures recovery of metal. For these reasons, it will be a very good alternative method. In the fourth chapter, advantages and disadvantages of hydroxide and sulfide precipitations which are conventional methods in chemical precipitation have been evaluated. Then, sulfex process, carbamate precipitation and oxalate precipitation which have been developed to achieve elimination of disadvantages and higher efficiency have been examined and studied as based on literature research. In conclusion of this research, the following assesments have been made respectively about newly developed methods as follows.. Basic compound of sulfex process is FeS. Ferrous sulfide is a salt not having high solubility. For this reason, odor problem caused by releasing of H2S can not be seen. Sulfex process is not confined and limited only to precipitate metal sulfides, but also achieves emergence as Cr(lll) hydroxides form at pH 8-9 without any need for reduction of Cr6+ ions. When chelates causing complexes with metal are in waslewater, sulfex process gives much better results than hydroxide precipitation at high pH.. In spite of the fact that metal carbonate precipitation is not used widely, it is more attractive method in comparison with hydroxide precipitation. Some of the reasons suggested for using carbonate precipitation are: - Precipitation occurs at a lower pH than hydroxide precipitation. - Metal carbonate precipitate is more dense than hydroxide precipitate which results in better settleability. And carbonate sludges have better dewatering characteristics than hydroxide sludges.. Oxalate precipitation is a fairly new method for the removal of metals in vastewater containing complexed metal at higher concentrations. If necessary, oxidation of organic matter is made in the system prior to precipitation. In metal oxalate precipitate, metals such as copper, cobalt, nickel in higher purity can be reused as by product. In the fifth chapter, studies made about electrochemical and chemical reduction methods developed for both reduction of hexavalent chromium to threevalent chromium and reduction of metal ions to elemental form have been examined. In conversion of metal ions into elemental form by reduction method, reduction is realized by electron exchange. So reactions of oxidation-reduction are taken place together. Reduction of metal ions is realized by electrochemical and chemical methods. In electrochemical reduction based on electrical potential application method, metal Xli ion is reduced by oxidation of another metal being more active in the activity sequ ence. Cementation process is also an electrochemical reduction method and reduc tion occurs by itself. In chemical reduction method, metal ions are reduced by chemical reducing agents such as sodium bisulphite, sodium metabisulphite, hydrazine, sodium borohydride, iron salts. Among these, chemical reducing agents especially the application of so dium borohydride subject to a wide research. Sodium borohydride shows properties that make it fairly interesting as a chemical reductant. One of the most important property is that although it has a standard redox potential of-1.23 V. capable of reacting with water at any pH value with H2 release, it can remain dissolved without reaction for large periods of time in strongly caustic solutions. It has a very low equivalent weight since eight electrons may be released. BH"4 + 3H20 -> H3B03+7H+ + 8e" The general equation for the reduction of divalent cations to the metallic state can be expressed by BH"4 + 4Me2+ + 3H20 -> H3B03+4Me°+7H+ However, when relevant literature are studied, it is defended by some reserachers that divalent nickel gives the followng reactions with borohydride depending on pH. At pH>12, BH"4 + 4Ni2+ + «DH- - -^ 4 Ni0+BCr2+6H20 At pH<12, 2BH"4 + 4 Ni2+ + 60H^±2 Ni2B+6H20+H2 The removal weight ratios theoretically achievable for different metals are given below. Table 1. Theoretical Weight Ratios of Reduced Metal Obtainable from Ionic Species With Sodium Borohydride * Weight ratio = maximum grams of metal reduced/gram of NaBH4 XIII The other studies which had been carried out by some researchers on the subject of reduction of complexed metals or free metals by using sodium borohydride as chemical reductant are evaluated. In accordance with this evaluation, it can be seen that treatment by addition of chemical reductants achieves recovery of metal and less sludge is formed in comparison with conventional precipitation methods. Removal of water from the sludge is easier. And depending on these, costs of sludge handling and disposal are lowered. It is possible to summarize attained results as follows:. Under optimum operation conditions, reduction of copper can be completed within 20-30 minutes and decreases under 1 mg/l.. In studies related to copper and cobalt optimum pH for reduction is found to be 8.5. For nickel removal, the adjustment of pH to 7.6 or 9.2 before NaBH4 addition is reduced stochiometric borohydride requirement.. In case of complexed metals, adequate studies have not been made how the system is operated. In one of the studies made for nickel, it can be seen that reduction by borohydride provides high efficiency in case of metal bound to weak complexing agents.. Due to dissolved oxygen and other oxidants in wastewater, borohydride consumption increases above the stochiometric NaBH4 dosage. Thus, it is observed that NaHS03 is being used for removal of dissolved oxygen. In the sixth chapter, in order to start to sodium borohydride reduction experiments, various binding ratios for EDTA, NTA, acetic acid with nickel have been calculated theoretically by using stability constants of complexing agents and hydroxocomplexes obtained from the literature as a single ligand with nickel system. In the seventh chapter, experimental studies have been carried out for the system for of which theoretical examinations and studies had been made. In preparation of experimental study program two approaches are taken into consideration. First approach is the execution of experiments in which reduction function is researched. For this purpose, execution of the following studies has been planned.. Comparison of the results obtained in previous studies in the literature by repeating experiments for the same conditions.. In case of absence of complexing agent in the medium, investigation of reducing effect of borohydride.. In case of presence of complexing agent in the medium, investigation of reducing effect of borohydride in detail under different conditions (different pH - xiv - values, different precipitation times etc.). The second approach is the execution of control experiments. For this purpose following studies have been planned:. When acetic acid is used as weak compexing agent, execution of acetic acid measurement to control whether acetic acid is destructed or not.. Execution of COD measurements in order to investigate effects of borohydride upon destruction of complexing agents.. Execution of solid phase analyses in order to determine structure of precipitate formed as a result of reduction. In direction of above-explained approaches, experimental studies were carried out. In the eight chapter, results of experiments which were carried out have been evaluated and reccommendations have been presented. At the conclusion of evaluations, it has been indicated that complexing agent cannot be destructed in acidic and alkaline medium. In opposition and contrary to results contained in the literature, reduction with borohydride under conditions where strong complexing agents exists, has not been effective. Although complexing agents used in the experiments had not been destructed, observation of reduction to metallic form was explained by ionization as dictated by equilibrium reactions and reaction stability constants. As Ni2+ ions passed into free form by ionization of complexed nikel, these ions were reduced to metallic form by borohydride until the ionization reaction of complex reached to equilibrium. However, since dissolved oxygen, had not been removed from medium before reduction process, nickel which had been reduced into metallic form precipitated as Ni(OH)2 form by sustaining oxidation during the precipitation time. In conclusion, it is advised that the reduction with borohydride should be applied by taking necessary measures (removal of dissolved oxygen or other oxidants in the medium) and when recovery of free metal ions is taken into consideration.