Laboratory study of cadmium removal from drinking water

Master thesis in the field of chemical engineering

Abstract

Laboratory study of cadmium removal from drinking water

In this research, the process of removing cadmium from Drinking water was studied by co-precipitation method of calcium carbonate. Today, the pollution of natural waters with heavy metals has caused concern in many countries of the world due to the destructive effects it has on living organisms. Cadmium enters drinking water through sewage and erosion of mineral rocks and effluents of industrial units. In this research, the removal of cadmium from a synthetic water sample containing 0.968 ppm of cadmium was investigated in a stirring crystallizer through co-precipitation process with calcium carbonate. Finally, the amount of cadmium removal by co-precipitation method of calcium carbonate was used from a real sample and using the effluent of an industrial unit as a reagent. Experiments showed that 95.8% cadmium removal was achieved at pH=4, injection time of 40 minutes, stirring speed of 200 rpm, reagent concentration of 1 M and temperature of 20 degrees Celsius. The removal rate for the real sample with cadmium concentration of 0.05 ppm was 96%. The amount of cadmium removal using industrial unit effluent as a reagent was 96.1%. The results of the experiments showed that about 15% of cadmium removal was due to absorption and 85% of it was mechanical confinement in the calcium carbonate crystal network, and co-precipitation with calcium carbonate is a suitable and economical method to remove cadmium below the permissible limit in Iran (0.01 ppm).

Key words: cadmium removal, co-precipitation, calcium carbonate, water Drinking

- Preface

Protection of the environment in which today's generation and future generations should have a growing social life is considered a public duty. There are various types of pollution in the water coming out of factories, power plants and refineries. For this reason, there should be enough information available to the society about water systems, types of polluting sources, pollution effects, pollution disposal methods, and pollution prevention methods (Manahan, 2004).

Today, pollution of natural waters with heavy metals has potentially toxic effects on living organisms. Urbanization and the development of agriculture and industry lead to water pollution with heavy metals (Ghaedi et al., 2010; Silva et al., 2009). For example, the density of cadmium is 8.7 grams per cubic centimeter, while the density of calcium and barium is 1.55 and 2.6 grams per cubic centimeter, respectively. The limitation of water resources, lack of rainfall, the risk of dehydration crisis and the importance of water recovery on the one hand and the increase of surface and underground water pollution by heavy metals on the other hand, make it necessary to find acceptable environmental solutions to remove these substances from water sources (Mohammad and Shashi, 2001). Ghanemi, 2010). Industrial effluents are the most important sources of environmental pollution, for example, batteries, pigments, metal coating, plastic, chemical fertilizers and purification of zinc, lead or copper ore are among environmental pollutants (Chamjangali et al., 2010). Introduction of cadmium

Cadmium is a metal with atomic number 48, mass number 112/41, oxidation state 2+ and chemically similar to zinc (Zn) metal (WHO, 2011). Cadmium usually does not exist in the environment in its pure form, but as a mineral in combination with other elements such as oxygen (cadmium oxide), chlorine (cadmium chloride) and sulfur (cadmium sulfide and cadmium sulfate). Cadmium often exists in nature in the form of oxide, sulfide, and carbonate complexes in copper, lead, and zinc ores. Cadmium chloride and sulfide compounds are easily soluble in water. Identifying the type of cadmium compound is important to prevent the risk of adverse health effects (Taylor et al., 1999).

1-2-1- Physical properties of cadmium

Cadmium is a white and soft metal with a density of 8.64 grams per cubic centimeter, melting point 320.9 degrees Celsius, the boiling point is 765 degrees Celsius at a pressure of 100 kPa and is soluble in dilute nitric acid and concentrated sulfuric acid. (WHO, 2011) Rust and steel plating are used. Cadmium sulfide (CdS) and cadmium salamide (CdSe) are used as pigments in plastic. Cadmium compounds are also used in electric batteries, electronic compounds and nuclear reactors (WHO, 2011). It is water whose physical, chemical, biological and radioactive properties are such that its consumption for drinking does not cause adverse effects to human health in the short or long term (Iran Institute of Standards and Industrial Research, 1388). Water pollution is the change of dissolved or suspended substances, temperature change and other physical, chemical and biological properties to the extent prescribed for water consumption, making it useless (Article 1 of the Water Pollution Prevention Regulations, 1373). The direct sources of cadmium in water include sewage, suspended particles in the air, and indirect sources include the washing of minerals caused by weathering, sewage sludge, waste disposal sites with underground and surface water (Merian, 1991).

Chemical fertilizers produced from phosphate rocks are an important source of spreading cadmium pollution. The solubility of cadmium in water largely depends on the acidity of the solution, and with the increase in the acidity of water, cadmium compounds are easily dissolved in the form of precipitates and suspensions in water. (WHO, 2011) style="direction: rtl;">

1-2-4-1- Air

Cadmium exists in the surrounding air in the form of suspended particles, cadmium oxide is one of the most important components of suspended particles. Daily, naturally more than 0.8 micrograms of cadmium enters the human body through breathing. Smoking increases the concentration of cadmium in the body. The average daily intake of cadmium into the body, in case of smoking (20 cigarettes per day), is 2-4 micrograms (WHO, 2011). The concentration of cadmium in unpolluted natural water is usually less than 1 ?g/liter. Cadmium concentrations measured in 110 different regions of the world were less than 1 ?g/L, the maximum reported value was 100 ?g/L in Rimau, Peru [1]. The average value in the Rhine and Danube rivers (of European rivers) in 1988 was 0.1 microgram/liter (from 0.02 to 0.3 microgram/liter) and 0.025 microgram/liter, respectively. The amount of cadmium present in the sediments near the port of Rotterdam was 1-10 mg/kg sediment in 1985-1986 (WHO, 2011).

Drinking water contamination is the result of the presence of cadmium as an impurity in galvanized pipes, cadmium-containing solders in fittings, heaters, water coolers and water taps. The concentration of cadmium in drinking water from shallow springs in areas with acidic soil in Sweden was about 5 ?g/L (WHO, 2011). Thornton reported concentrations of 1 mg/L in waters near cadmium-bearing ores. The concentration of cadmium in a groundwater in New Jersey has been reported to be about 450 µg/L. In a survey of groundwater near areas where industrial effluents are discharged, a cadmium concentration of 6 mg/L was reported (Taylor et al., 1999).