Temas havalandırmalı sistemler ile demir giderilmesi

Abstract

Yeraltı ve yüzeysel sularda yüksek konsantrasyonlarda demir bulunması, evsel ve endüstriyel amaçlı kullanımları olumsuz yönde etkilediğinden bu tür sulardan demirin giderilmesi gerekmektedir. Bu amaçla bu çalışmada, Fe(2)'nin oksijenle oksidasyonunda reaksiyon ürünü olan Fe(3)'ün katalitik etkisi ve bu katalitik etkinin kinetiği incelenmiştir. Birinci bölümde, yapılan çalışmanın amaç ve kapsamı açıklanmış ve önemi vurgulanmıştır. İkinci bölümde, demir giderilmesinin önemi, sulu ortamlarda demirin kimyası ve demirin çökelmesinin esasları detaylı bir şekilde verilmiştir. Üçüncü bölümde, havalanma ile Fe(2) oksidasyonunun kinetiği ve Fe(3)'ün katalitik etkisi incelenmiştir. Bu konuda mevcut bilgi düzeyi bilimsel çerçevede sunulmuştur. Dördüncü bölümde, deneysel çalışmada kullanılan yöntem ve düzeneklerle ilgili ayrıntılı bilgi verilmiştir. Deneysel çalışma iki safhada gerçeMeştirilmiştir. Birinci safhada, Fe(2)'nin oksijen ile oksidasyonunun kinetiği incelenmiştir. Fe(3) konsantrasyonunun 0-1000 mg/1 arasındaki konsantrasyon değerlerinde Fe(3)'ün katalitik etkisi kontrollü olarak doldur-boşalt tipi 2 litrelik reaktörlerde incelenmiştir. Bu deneylerde, oksijenin kısmi basıncı, pH ve sıcaklık sabit tutularak ve sadece Fe(3) konsantrasyonları değiştirilerek deneyler tekrar edilmiştir. Fe(3)'ün katalitik etkisi, çamur yaşma bağlı olarak incelenmiştir. Çamur yaşının etkisini incelemek için belirli bir Fe(3) konsantrasyonunda 1-10 gün gibi değişik yaşlarda hazırlanmış Fe(3) çamurları ile oksidasyon deneyleri yapılmıştır. İkinci safhada, sürekli akımlı geri devirsiz ve sürekli akımlı geri devirli laboratuvar ölçeğindeki bir sistemde Fe(2)'nin giderilmesi çalışılmıştır. Sistem esas olarak PVC malzemeden yapılmış 4 l'lik havalandırma havuzu ve 30 l'lik çöktürme havuzundan oluşmuştur. Besleme suyu sentetik olarak hazırlanarak sabit bir debi ile sisteme verilmiştir. Beşinci bölümde, deney sonuçlan değerlendirilmiştir. pH= 6.7, sıcaklığın 25°C, alkalinitenin 2.1 0"2 eq/l, p02 de 0.85 atm.^'de sabit iken Fe(2) başlangıç konsantrasyonunun yüksek değerlerinde (25 mg/1) otokatalitik etki nedeniyle oksidasyon hızı önemli ölçüde artmıştır. Fe(2) başlangıç konsantrasyonunun 3 mg/1 ve Fe(3)'ün 0-1000 mg/1 arasında alındığı oksidasyon deneyleri sonucunda Fe(3)'ün katalitik etkisinin 600 mg/Tye kadar devam ettiği, bu değerin ötesinde Fe(2)'nin oksidasyonuna, Fe(3)'ün önemli bir katalitik etkisi olmadığı gözlenmiştir. Çamurun yaşlanmasıyla ilgili yapılan deneylerin sonucunda, çamurun yaşlanmasıyla katalitik etkinin arttığı gözlenmiştir. Sürekli sistemde Fe(2) giderim verimi %57'den, %99.6 değerine ulaşmıştır. Fe(3) konsantrasyonunun 300 mg/1 değerine kadar Fe(2) giderim verimi artmış, bu değerin ötesinde önemli bir verim artışı gözlenmemiştir.

Iron is the fourth most abundant element by weight in the earth's crust. The chemistry of aqueous iron primarily involves the ferrous(2) and ferric(3) oxidation states, and is of engineering interest in water supply and wastewater treatment. Iron cycling in nature is also of interest due to its nutrient status and the active surface chemistry of iron oxyhydroxide. Occurrence of iron levels in water supplies exceeding the 0.3 mg/1 limit is common. The oxidation of the ferrous iron is affected by several factors such as Fe(2) and oxygen concentration, pH, temperature, organic matter and other ions present in the solution. Ferrous iron oxidation is also affected and is accelerated in the presence of the ferric hydroxide. The recent studies have demonstrated the catalytic effect of the ferric hydroxide but the effect becomes noticeable at Fe(3) concentrations exceeding 5~10 mg/1 (Sankaya,1980; Tamura et.al.,1976; Robinson et.al.,1981). It has been reported that the oxidation rate increases linearly with Fe(3) concentrations up to 100 mg/l (Tamura et.al.,1976). In the presence of Fe(3), the oxidation of ferrous iron by the atmospheric oxygen occurs by two simultaneous processes. One of these is the homogeneous reaction taking place in solution, and the other one is the heterogeneous reaction occurring on the surface of the ferric hydroxide precipitates. Thus: Fe(2) + 02 -^->Fe(3) + 0~ (homogeneous) (1) Fe(2) + 02 -*?->. Fe(3) + O" (heterogeneous) (2) The reaction rate under the constant pH and O2 concentration is given as (Sankaya,1980; Tamura et.al.,1976; Sung et.al.,1980): -d[Fe(2)]/dt = (k + k'[Fe(3)]) [Fe(2)] (3) in which the first term indicates the homogeneous and the second one heterogeneous reaction rates. The explicit form of the constants are(Tamura et.al.,1976; Sung et.al.,1980): XIV k = k0[OH-f[O2] (4) k' = ks,0[O2]K/[H+] (5) k' = ksK/[H+] (6) ks=ks,o[02] (7) where ko and ks,0 are the real rate constants, ks is the surface rate constant and K is the equilibrium constant for the adsorption of Fe(2) on Fe(3) hydroxide. The numerical values of the constants are (Tamura et.al.,1976); k =2.3xlOI4M"Y1 o k =73M_1s"1 S,0 K=10'96Mmg1 One of the aims of this study is to determine the catalytic effect of Fe(3) on the ferrous iron oxidation by atmospheric oxygen at Fe(3) concentrations beyond 100 mg/1 (Tamura et.al.,1976). In the experimental study in which a large amount of Fe(3) hydroxide was added at the beginning of the oxygenation of a small amount of Fe(2), the concentration of Fe(3) hydroxide is almost constant throughout the experiment. So equation (3) can be written as -d[Fe(2)]/dt = k0jFe(2)l (8) where kcat is a constant for a constant pH and 02 concentration. In the first chapter, the importance and the general objectives of the study are defined in detail. In the second chapter, the chemistry of aqueous iron and character of iron precipitates are given. In the third chapter, the kinetics of ferrous iron oxidation and its oxygenation products in aqueous systems are explained. In the fourth chapter, the kinetics of the ferrous iron oxidation in contact aeration systems with recycling is investigated. The effects of the ferric iron concentrations; especially the upper limits for the catalytic effect and the effects of the sludge age on autocatalytic oxidation of ferrous iron were examined. The study was carried out in two stages: In the first stage, the kinetics of ferrous iron oxidation by atmospheric oxygen has been studied in batch systems of 2 1 volume. Fe(3) concentrations were varied within the range of 0-1000 mg/1, keeping partial pressure, pH, and temperature constant. A modified 1.10 phenanthroline method which enables the analysis of ferrous iron in the presence of the high concentrations of Fe(3) has been applied in the study. The effect of the age of the ferric hydroxide sludge on its catalytic effect has been investigated. Oxidation experiments have been xv simultaneously carried out for the sludge ages of 1 to 10 days for the same ferric iron concentration. In the second stage, ferrous iron oxidation was studied in continuous flow lab scale system with and without ferric sludge recirculation. The system consists of a Plexiglas aeration tank of 4 1 and a sedimentation tank of 30 1 volume. The synthetic feed water was pumped into the system at constant flow rates. Fe(2) solution was injected into the feed line in order to obtain the desired influent concentration. Na2C03 buffer was used to control the pH. A mixture of air and CO2 was given into the aeration tank through diffusers. The desired pH was reached by varying the CO2 flow. The temperature was kept at 25 °C and the pH within 6.5-6.7. After establishment of the steady state conditions, influent and effluent ferrous concentrations were measured. In order to evaluate the catalytic effect of the ferric iron, the system was operated both with and without sludge recirculation. In the fifth chapter, the results of the experiment are discussed. The experimental results obtained for low (~3 mg/1) and high (25 mg/l) Fe(2) concentrations are given. No Fe(3) was initially added in these experiments. The linear relationship between the Fe(2) concentration and time on a semi logarithmic plot shows a first order kinetics. This is in agreement with the results given in the literature. Reproducibility of the results is good in the homogeneous systems. When initial Fe(2) concentration was increased keeping all other experimental conditions the same, the reaction is accelerated due to the Fe(3) formed as a result of oxidation. Average homogeneous reaction rate constant, k for initial Fe(2) concentration of 3 mg/1 is found as 0.035068 min"1. The corresponding k value for [Fe(2)]0 = 25 mg/1 is found as 0.22327 min"1. The results of the experiments with initial Fe(2) concentration of 3 mg/1 and with varying initial Fe(3) concentrations are given. Catalytic rate constant, kcat is calculated from the slopes of the lines on semi logarithmic plots of Fe(2) versus time. Time needed for the completion of the reaction is about 130 minutes when no Fe(3) is present in the reactor. The reaction time is reduced to about 12 minutes when 600 mg/1 of Fe(3) is added initially into the reactor. It has been found that k^ increases linearly with increasing Fe(3) concentration up to about 50 mg/1 and the rate of the increase decreases beyond this value, k^ reaches its maximum value at an Fe(3) concentration of about 600 mg/1. This means that there is no additional catalytic effect of Fe(3) on the ferrous iron oxidation at Fe(3) concentrations beyond 600 mg/1. In the first stage of this study, the oxidation of Fe(2) was studied in batch reactors in which the concentration of Fe(3) was in the range 0-600 mg/1. A quadratic equation has been found to represent the catalytic reaction rate constant as a function of Fe(3). It has been experimentally demonstrated that there is no significant effect of Fe(3) on the ferrous iron oxidation at Fe(3) concentrations beyond 600 mg/1. The kinetics of the catalytic reaction can be represented by linear expression in the concentration range of Fe(3) 0-50 mg/1 and the second order polynomial in the range 50-600 mg/1. Applying the curve fitting techniques to the data, the following equations are obtained between the k^ and Fe(3) concentration. xvi In the linear range, lc^ is given as. kcat = k + k'[Fe(3)]0 (9) kcat= 0.035+0.0039 [Fe(3)] where [Fe(3)]0 indicates the initial Fe(3) concentration. Oxygenation rate in terms of k^ andFe(2)is. ^ = -k"[Fe(2)] (.0) Assuming Fe(3) is constant, integration of equation 10 yields [Fe(2)] = [Fe(2)]oe-k«t (11) in which [Fe(2)]Q is the initial Fe(2) concentration. Substituting the value of k^ obtained from equation 9 into equation 1 1 yields: [Fe(2)] = [Fe(2)]0 e*0035^0039^3»» In the second order polynomial range, k^ is found as: kcat = -IxlO-6 [Fe(3)p + 0.0012 [Fe(3)] +0.1575 [Fe(2)] = [Fe(2)]0 e*-1*10^3»2 + 0.0012 [Fe(3)]+o.o35}t where [Fe(3)] and [Fe(2)]0 indicate the concentration of Fe(3) and initial concentration of Fe(2), respectively. A quadratic equation has been obtained to determine the catalytic reaction rate constant, k^ as a function of Fe(3). For low Fe(3) concentrations the oxidation rate is dominated by the homogeneous reaction whose rate constant, k has been determined as k=0.035068 min"1 for pH=6.70, T=25 °C, p02= 0.85 atm and alkalinity=2xl0"2eq/l. As a result of the experiments on the ageing of the sludge, it was found that the catalytic effect increased by ageing of the sludge. A decrease in the catalytic effect of Fe(3) is expected because of the increase in the active surface area of Fe(OH)3 by ageing of the ferric sludge. However, the experimental data obtained are in contrast with these expectations. In the case of pH=6.7, the rate of the reaction is accelerated up to the 3rd day, and almost no increase is observed beyond this day. At pH=6.2, the rate of the reaction is accelerated up to 6th day, and almost no increase is observed beyond this day. Thus it is found that the catalytic effect is increased by ageing the sludge. Although the acceleration of the catalytic effect by ageing of the sludge could not be explained clearly, it is supposed that the increase in the rate of the reaction is a xvu result of each or both two factors: The OH* radicals which are exposed as a reaction product in the oxygenation of Fe(2) and each of the different structural forms of Fe(OH)3. When the continuous system was run without recycle, the efficiency of the treatment of Fe(2) changed between 57-79%. When the system was run with recycle, the recycle ratio were chosen as 50 %, 80% and 100%. The efficiencies of the Fe(2) removals obtained from the experiments, are 80-89%, 90-97% and 98- 99.6%, respectively. Concentration value of Fe(3) in the aeration tank was 43.7 mg/1 at the beginning of the experiments and this value reached 462 mg/1 at the end of the experiments. In the continuous system, until Fe(3) concentration reached 300 mg/1, Fe(2) removal rate has increased. Beyond this level, no increase in removal rate was observed. When the reaction kinetics derived from the data of the batch experiments was applied to the continuous system, the following expression for the aeration tank volume and Fe(2) concentration were found. In the linear range, the volume of the aeration tank is given as; Q([Fe(2)]o-[Fe(2)]) (0.035 + 0.0039[Fe(3)]) [Fe(2)] In the second order polynomial range, the volume of the aeration tank is given as; y= Q([Fe(24-[Fe(2)]) (0.1575 + 0.0012[Fe(3)]- 1 x 10_6[Fe(3)] ) [Fe(2)] In the linear range, Fe(2) concentration in the effluent of the aeration tank is given as; Co 1 + 0(0.035 + 0.0039x) In the second order polynomial range, Fe(2) concentration in the effluent of the aeration tank is given as; C* 1 + 9(0.1575 + 0.0012x - 1.10_(V) In the absence of Fe(3), aeration tank volume was 25.7 1. With Fe(3) concentration of 600 mg/1, the volume decreased to 1.73 1. When the Fe(3) xvui concentration is increased from 0 to 600 mg/1, aeration tank volume is decreased by 15 times. It has been experimentally demonstrated that the volume of reactors for ferrous iron oxygenation can be significantly reduced by keeping high concentrations of ferric iron in the reactor. It has been suggested that high ferric iron concentrations can be maintained by recycling the ferric precipitates. Cost comparison is essential in the final decision among the various strategies such as use of stronger oxidants, recycle and non-recycle treatment systems.