Kırmızı Çamur İle Sulardan Arseniğin Giderilmesi

Abstract

Kırmızı çamur, alüminyum üretiminde, boksit cevherinden alümina üretmek içinkullanılan Bayer prosesi sonucunda oluşan bir atıktır. Nüfus ve şehirleşmedeki artışa paralel olarak alüminyum talebi de artmakta olup, alüminyum üreten tesisler için kırmızı çamurun depolanması ve bertaraf edilmesi en önemli sorunlardan biri haline gelmektedir. Arsenik ise, metal ile ametal arasında özelliğe sahip olup, gerek doğal olarak gerekse antropojenik kaynaklardan ötürü yerkabuğunda yaygın halde bulunan bir elementtir. Doğada inorganik ve organik halde bulunan arsenik, toksik ve kanserojen sınıfına girebildiğinden, son yıllarda yer altı ve yer üstü sularında karşılaşılan en önemli sorunlardan biridir. Bu çalışmada, kırmızı çamurun arseniğin uzaklaştırılması üzerindeki etkisi araştırılmıştır. Öncelikle adsorbent olan kırmızı çamur ile arseniğin uzaklaştırılması için optimum pH değeri, karıştırma süresi vekırmızı çamur miktarı saptanmıştır. pH 2,5 ile pH 10 arasında farklı değerlerde çalışılmış ve sentetik olarak hazırlanan çözeltilerden en iyi adsorblanmanın pH 5,5 ta olduğu gözlenmiştir. Kırmızı çamur miktarının incelendiği deneylerde, 0,5 gr/lt kırmızı çamurun az miktarda olmasına rağmen etkin olduğu anlaşılmıştır. Farklı arsenik konsantrasyonlarında da sürdürülen çalışmalarda,100 ppm e kadar oldukça yüksek bir verimle elde edilmiştir. 100 ppm üzerindeki yüksek konsantrasyonlarda arsenik etkinliğini arttırmak için kırmızı çamurun ısı ve asit ile aktifleştirilmesi yoluna gidilmiştir. Asit aktivasyonu ile istenilen sonuç elde edilemezken, 600 °C de ısı ile aktivasyon sonucu arsenik gideriminde yüksek konsantrasyonlarda verim artışı sağlanmıştır. Arsenik giderimi için bir diğer adsorban olan piritin, kırmızı çamur ile birlikte kullanımının da etkisi araştırılmış; ancak kırmızı çamurun tek başına daha etkin bir adsorban olduğu görülmüştür. Sentetik arsenik konsantrasyonundan elde edilen veriler bir de gerçek atık suya uygulanmak istenmiş ve Eti Maden Emet Bor tesislerindenalınan doğal pH sı 9 olan 38,5 ppm arsenik içeren atık su numunesi ile de çalışmalar yapılmıştır. Bunun neticesinde doğal pH da 4gr/lt kırmızı çamurun dahi yetersiz olduğu, ancak pH 5 değerinde arsenik gideriminde oldukça başarılı sonuçlar elde edilmiştir. Son olarak bu çalışmaya bir atık değerlendirme olarak baktığımızda, kırmızı çamur atığının kullanıma elverişliliğinin de göz önünde bulundurulması gerektiğinden, nakliyesinde kolaylık olması açısından pelet haline getirilmesi düşünülmüş ve elde edilen veriler ışığında iri ve ince boyutta peletler hazırlanıp bir de bunların arsenik adsorbsiyonu üzerindeki etkisi incelenmiştir.Bunun neticesinde ince peletlerle daha iyi sonuçlar elde edilmiş; ancak numune alma süreleri arttıkça arsenik giderme veriminin azaldığı gözlenmiştir.

Arsenic exhibits properties of metal and non metal. Due to both natural and anthropogenic sources, arsenic is the 20th the most abundant elements in the earth s crust and is a component of morethan 245 minerals. These are mostly ores containing sulfide, along with copper, nickel, lead, cobalt, or other metals. Arsenic exists in nature in the oxidation states +V (arsenate), +III (arsenite), 0 (arsenic) and -III (arsine). Commonly inorganic arsenic appears in the aqueous environment in the states +V and +III as arsenous acid (As(III)), arsenic acid (As(V)), and their salts. It is usually found in the environment combined with the elements such as oxygen, chlorine, and sulfur. Arsenic combined with these elements is called inorganic arsenic. On the other hand if arsenic combined with carbon and hydrogen is called organic arsenic. Inorganic arsenic compounds can be methylated by bacteria, fungi, and yeasts to the organic compounds such as monomethylarsenic acid (MMA), dimethylarsinic acid (DMA), and arsine. Arsenic compounds are used in wood preservation and insecticides, as an additive to lead and copper for hardening, glass manufacturing, in small quantities in semi-conductor manufacturing. Arsenic is carcinogenic and may cause lung cancer, bladder cancer, liver cancer, renal cancer, and skin cancer. It is also harmful to the nervous system. Other health effects may include vessel damage, high blood pressure, anemia, stomach sickness, and diabetes. The toxicity of arsenic depends on its binding form. Organic arsenic compounds are less toxic than inorganic arsenic compounds. It has been estimated that tens of millions of people are at risk exposing to excessive levels of arsenic from both contaminated water and arsenic-bearing coal from natural sources. Humans are exposed to this toxic by breathing, eating, or drinking the substance, or by skin contact. The current World Health Organization recommended value for arsenic in drinking water is 0.010 mg/Land in wastewater is 0,5 mg/L. Arsenic cannot be destroyed in the environment. It can only change its form, or become attached to or separated from particles. Therefore it is one of the main problems considering underground and ground source waters in recent years, because of its toxicity and carcinogenic. On the other side, aluminium is the second-most used metal after steel, largely because of its use in a wide variety of products. It is estimated that aluminium composes about 8% of the earth s crust. Aluminium is a light silvery-white metallic element and it is resistant to corrosion. it is very reactive but forms a tough layer of oxide when exposed to air, preventing further corrosion and it is ductile and easily malleable.It is an excellent conductor of heat and electricity. Aluminium is used electrical equipments such as car, ship, aircraft construction; metallurgical and chemical processes; domestic and industrial construction; packaging and kitchen utensils. Aluminium is non-toxic (as the metal) nonmagnetic and non-sparking. It occurs naturally in many foods in very concentrations and is also present in many pharmaceuticals and drinking water. High levels in the body can be toxic. Production of aluminium from ore is dependent upon alumina, which is extracted from bauxite ore which contains 30-60% alumina. The production of alumina consumes more than 90% of the world s production of bauxite. The main minerals in bauxite are gibbsite (Al203.3H20), boehmite (Al203.H20), and diaspore. Bauxite ore refers to bauxite that contains sufficiently high levels of Al203 and suitably low levels of Fe203 and silica to be economically mineable. Red Mud is a waste left after alumina has been extracted from bauxite in Bayer process for refining bauxites into alumina via digestion with sodium hydroxide.This process can be separated into two parts; firstly the extraction of alumina from bauxite, and secondly the smelting of aluminum metal from alumina. Sodium hydroxide is used to dissolve the aluminum oxide. This produces a sodium-aluminium solutionfrom precipitated 〖Al(OH)〗_3.Afterwards it is calcined to produce oxide, from which the metal is recovered. The remaining waste product of bayer process is called red mud due to the colour of the original bauxite ore and the iron oxide it contains.In addition to iron, it contains other particles include silica, unleached residual aluminium, and titanium oxide. For each tonne of alumina produced, the process can leave behind about two tonnes of red mud. However, This ratio is dependent on the type of bauxite used in the refining process. The red mud is a complex chemical soup, a watery slurry of fine rock particles and salts, containing toxic heavy metals. The red mud also has a high pH because of the sodium hydroxide solution used in the refining process. Therefore it is strong enough to kill plant and animal life, and to cause burns and damage to airways if it is breathed. Parallel to the increase in population and urbanization, the demand of aluminum is also increasing and as a consequence, storage and removing red mud becomes more and more important for the facilities produce aluminum. Therefore, cosidering the facts of both arsenic and red mud issues, in this study, the influence of red mud on removing arsenic from waters is investigated. The aim was, by using red mud as a low cost adsorbent, having a good efficiency of arsenic removal from waters and waste waters. Primarily, optimal pH for arsenic removal, stiring duration, and amount of red mud are determined. In the course of this study in a range from pH 2.5 to pH 10 have been studied in many different values and the best in synthetic absorbable s concentration of arsenic is observed at pH 5.5. Also it has been concluded that even 0,5 gr/L red mud is enough effective for arsenic removal at pH 5,5 with a concentration of 10 ppm arsenic. In other experiment, instead of synthetic arsenic, 38,5 ppm arsenic contaminated water taken from Eti Mine Emet Boron facilityis used. The experiment performed in the original pH 9 and despite incresing the amount of red mud, good results couldn t be achieved which shows the importance of pH arrangement in the arsenic removal process. This study also investigates different arsenic concentrations, enough high yields is obtained up to 100 ppm. In order to increase the arsenic efficiency the heat, acid, and red mud are activated at concentrations as high as 100 ppm. While desired results could not be obtained by acid activation, with heat activation at 600°C the yield increase is achieved with high concentrations. Pyrite is another adsorbant to remove arsenic. Although pyrite s removal effect together with red mud has been elaborated as well, it has been found that red mud alone is more effective to remove arsenic. In order to apply the data obtained by sentetic arsenic concentration to the waste water, some experiments has been conducted with natural pH 9 and 38,5 ppm arsenic concentration obtained by Eti Maden Emet boron facility. As a result, it has been determined that at natural pH value, even 4 gr/L of red mud is not enough, but the yield quite rises at pH 5. It has been also worked with another waste sample from Eti Maden Emet boron facility which has a 81,66 ppm arsenicconcentration. With this sample, it has been tried to see the effects of the amount of red mud in arsenic removal with a value of pH 5 in wastewater. As a result, with 4 gr/L red mud, it has been able to reduce the arsenic concentration to a level below 0,5 ppm, which is the limit for permitted levels of arsenic in waste waters. For the application and the transportation with ease, red mud transformed to the red mud pellets by using starch. The adsorption experiments in a colunm system with two different size of these pellets show that with the fine pellets, better results are obtained. While the course sized pellet has a 42% of arsenic removal efficiency in the samples taken after 30 minutes, the fine sized pellet has about 60% of arsenic removal efficieny. On the other hand, these efficiencies are reduced with increasing sampling time. When the fine sized pellets are worked with the real arsenic contaminated water taken from Emet boron facility, about 80% of arsenic removal efficiency is achieved. As a result, red mud which is the biggest waste problem of aluminum production either with the storage necessity or the disposal difficulties, is used for removing arsenic from the waters within the scope of waste utilization.