This earth which we have borrowed from our ancestors is highly polluted and to an extent that it is harmful for the environment and the human species to survive. As the world’s population continues to grow, so does the amount of potentially toxic substances that are released into the ecosystem.These elements which pollute our environment are called pollutants they can be found in various forms in different places on this earth polluting every known ecosystem.These include heavy metals, carbon monoxide,sufur dioxide, radioactive substance and many other metals realesed through different sources into the environment.
Scientist research and scholars have developed ways to eliminate these pollutant from different sources like soil water and air. The methods employed in these research can include microorganisms,water treatment using variable devices and using filtration methods.
In this report we would be studying the removal of heavy metals using membrane technology. This technology is under a lot of research to make it cost effective and available to a larger public so that the application of the technology is exploited to a greater level.
Heavy metals are naturally occuring elemnents that have a atomic weight and density at least 5 times greater than that or water .Their multiple industrial, domestic, agricultural, medical and technological applications have led to their wide distribution in the environment; raising concerns over their potential effects on human health and the environment. Their toxicity depends on several factors including the dose, route of exposure, and chemical species, as well as the age, gender, genetics, and nutritional status of exposed individuals The heavy meatls the are removed using these membrene technology are mostly found in the air water and soil. These are mostly released out from industires like: dye colouring industry, tanning industry, e waste, paint industry and petroluem industry. These are released into the air as emission from the transport used on road. Because of their high degree of toxicity, arsenic, cadmium, chromium, lead, and mercury rank among the priority heavy metals.These heavy metas caus ea health risk to the human species as these cannot be eliminated from our body by normal metablolism and increasing level of toxicity is a harmful health risk.
HEAVY METAL : CADMIUM
Cadmium is a chemical element with symbol Cd and atomic number 48. This soft, bluish-white metal it demonstrates oxidation state +2 in most of its compounds, it has a lower melting point than the transition metals in groups 3 through 11. The average concentration of cadmium in Earth’s crust is between 0.1 and 0.5 parts per million (ppm). It was discovered in 1817 simultaneously by Stromeyer and Hermann, both in Germany, as an impurity in zinc carbonate.
Cadmium occurs as a minor component in most zinc ores and is a byproduct of zinc production. Cadmium was used for a long time as a corrosion-resistant plating on steel, and cadmium compounds are used as red, orange and yellow pigments, to colour glass, and to stabilize plastic. Cadmium use is generally decreasing because it is toxic and nickel-cadmium batteries have been replaced with nickel-metal hydride and lithium-ion batteries. One of its few new uses is cadmium telluride solar panels.
Although cadmium has no known biological function in higher organisms, a cadmium-dependent carbonic anhydrase has been found in marine diatoms.
Cadmium is a soft, malleable, ductile, bluish-white divalent metal. It is forms complex compounds. cadmium is resistant to corrosion and is used as a protective plate on other metals. As a bulk metal, cadmium is insoluble in water and is not flammable; however, in its powdered form it may burn and release toxic fumes.
Naturally occurring cadmium is composed of 8 isotopes. Two of them are radioactive, and three are expected to decay but have not done so under laboratory conditions. The two natural radioactive isotopes are 113Cd (beta decay, half-life is 7.7 × 1015 years) and 116Cd (two-neutrino double beta decay, half-life is 2.9 × 1019 years). The other three are 106Cd, 108Cd, and 114Cd only lower limits on these half-lives have been determined. At least three isotopes – 110Cd, 111Cd, and 112Cd – are stable. Among the isotopes that do not occur naturally, the most long-lived are 109Cd with a half-life of 462.6 days, and 115Cd with a half-life of 53.46 hours. All of the remaining radioactive isotopes have half-lives of less than 2.5 hours, and the majority have half-lives of less than 5 minutes.
Another type of battery based on cadmium is the silver-cadmium battery.
Cadmium electroplating, consuming 6% of the global production, is used in the aircraft industry to reduce corrosion of steel components.
Cadmium is used in the control rods of nuclear reactors, acting as a very effective “neutron poison” to control neutron flux in nuclear fission.36 When cadmium rods are inserted in the core of a nuclear reactor, cadmium absorbs neutrons, preventing them from creating additional fission events, thus controlling the amount of reactivity.
QLED TVs have been starting to include cadmium in construction. Some companies have been looking to reduce the environmental impact of human exposure and pollution of the material in televisions during production.
Cadmium oxide was used in black and white television phosphors and in the blue and green phosphors of color television cathode ray tubes. Cadmium sulfide (CdS) is used as a photoconductive surface coating for photocopier drums.
Various cadmium salts are used in paint pigments, with CdS as a yellow pigment being the most common. Cadmium selenide is a red pigment, commonly called cadmium red. To painters who work with the pigment, cadmium provides the most brilliant and durable yellows, oranges, and reds so much so that during production, these colors are significantly toned down before they are ground with oils and binders or blended into watercolors, gouaches, acrylics, and other paint and pigment formulations.
Helium–cadmium lasers are a common source of blue-ultraviolet laser light.
Cadmium is a component of some compound semiconductors, such as cadmium sulfide, cadmium selenide, and cadmium telluride, used for light detection and solar cells. HgCdTe is sensitive to infrared light and can be used as an infrared detector, motion detector, or switch in remote control devices.
TOXICTIY OF CADMIUM
Resorption into human body :Basically there are three possible ways of cadmium resorption: Gastrointestinal, pulmonary and dermal.
The uptake through the human gastrointestinal is approximately 5% of an ingested amount of cadmium, depending on the exact dose and nutritional composition. An average smoker has an additional intake of 30 ?g per day. Several factors can increase this amount, such as low intakes of vitamin D, calcium, and trace elements like zinc and copper.
Furthermore a high fiber diet increases the dietary cadmium intake. The most important metabolic parameter for cadmium uptake is a person’s possible lack of iron. People with low iron supplies showed a 6% higher uptake of cadmium than those with a balanced iron stock . This is the main reason for the higher cadmium resorption in people with anaemia and habitual iron deficit, such as children or menstruating women. Low iron blood levels stimulate the expression of DCT-1, a metal ion transporter in the GI tract, serving as a gate for cadmium resorption.
The major source of inhalative cadmium intoxication is cigarette smoke. The human lung resorbes 40–60% of the cadmium in tobacco smoke. A 50 year-old average non-smoker has a cadmium body burden of 15 mg. While a comparable life-long smoker shows a value of 30 mg. Smokers generally have cadmium blood levels 4–5 times those of non-smokers.
Two mechanisms facilitate cadmium absorption by the skin: binding of a free cadmium ion to sulfhydryl radicals of cysteine in epidermal keratins, or an induction and complexing with metallothionein.
Harmful effects of toxic level of cadmium
The main organ for long-term cadmium accumulation is the kidney . Here the half-life period for cadmium is approx. 10 years. A life-long intake can therefore lead to a cadmium accumulation in the kidney, consequently resulting in tubulus cell necrosis.
The blood concentration of cadmium serves as a reliable indicator for a recent exposition, while the urinary concentration reflects past exposure, body burden and renal accumulation. Excretion of Cadmium takes place via faeces and urine.
The respiratory system is affected severely by the inhalation of cadmium-contaminated air: Shortness of breath, lung edema and destruction of mucous membranes as part of cadmium-induced pneumonitis are described.
Kidney damage has long since been described to be the main problem for patients chronically exposed to cadmium . As mentioned above, cadmium reaches the kidney in form of cadmium-metallothionein (Cd-MT). Cd-MT is filtrated in the glomerulus, and subsequently reabsorbed in the proximal tubulus. It then remains in the tubulus cells and makes up for the major part of the cadmium body burden. The amount of cadmium in the kidney tubulus cells increases during every person’s life span. An increasing cadmium load in the kidney is also discussed to result in a higher calcium excretion, thus leading to a higher risk of kidney stones.
REMOVAL OF CADMIUM USING MEMBRANE TECHNOLOGY
Membrane technology is a generic term for a number of different, very characteristic separation processes. Membranes are used more and more often for the creation of process water from groundwater, surface water or wastewater. Membranes are now competitive for conventional techniques.The membrane separation process is based on the presence of semi permeable membranes.
Membrane filtration which is classified into microfiltration (MF), ultrafiltration (UF),nanofiltration (NF) and reverse osmosis (RO) is a pressure driven process in which the membrane acts as a selective barrier to restrict the passage of pollutants such as organics, nutrients, turbidity, microorganisms, inorganic metal ions and other oxygen depleting pollutants, and allows relatively clear water to pass through. The processes operate without heating and therefore use less energy than conventional thermal separation processes such as distillation, sublimation or crystallization. The separation process is purely physical and both fractions (permeate and retentate) can be used. Cold separation using membrane technology is widely used in the food technology, biotechnology and pharmaceutical industries. Depending on the type of membrane, the selective separation of certain individual substances or substance mixtures is possible. Important technical applications include the production of drinking water by reverse osmosis filtrations in the food industry, the recovery of organic vapours such as petro-chemical vapour recovery and the electrolysis for chlorine production.
In waste water treatment, membrane technology is becoming increasingly important. With the help of ultra/microfiltration it is possible to remove particles, colloids and macromolecules, so that waste-water can be disinfected in this way.
Reverse osmosis (RO) and nanofiltration (NF) are relatively new processes, which were initially developed for the production of potable water from saline and brackish water .
Membrane techniques like RO, NF, UF and ED are more often applied to remove of heavy metals from water solutions in the industrial scaleThe results showed that the removal efficiency of individual heavy metals by RO was high and amounted to 98% for Cu and 99% for Cd, while for NF it was above 90%.the application of ED for the recovery of metals for electroplating with such metals as Au, Pt, Ni, Ag, Pd, Cd, Zn and Sn/Pb from diluted electroplating wastewaters has been gaining attention. The principal disadvantage of ED is the inability to remove simultaneously the non-ionic substances (e.g. organic compounds) from the dilute stream, what can be done with the use of RO.Electrodialytic removal of Cd(II) from wastewater sludge, was studied. During the remediation a stirred suspension of wastewater sludge was exposed to an electric dc field. The liquid/solid (mL/g fresh sludge) ratio was between 1.4 and 2. Three experiments were performed where the sludge was suspended in distilled water, citric acid or HNO3 (Fig. 13). The Cd(II) removal in the three experiments was 69%, 70% and 67%, respectively.Montmorillonite, kaolin, tobermorite, magnetite, silica gel and alumina that removed more than 80% from a solution of initial concentration range 1–100 ppm for cadmium For the nanofiltration (NF) membrane, research showed removal efficiencies around 97% for cadmium (initial concentration C0 = 500 ppm), UF membranes to remove Cd(II) and Cr(III) ions from synthetic solution. The highest capacity for Cd(II) was 313mg/g, using sugar- cane bagasse treated with triethylenediamine. This high capacity was caused by a greater number of nucleophilic sites (amide groups) as a result of chemical modification by the triethylenetetramine. Another modified plant waste adsorbent used for removal of Cd(II) is Eucalyptus bark. In this study the influence of temperature on the sorption isotherms of cadmium was also been studied. When the temperature was raised from 20 to 50 C, the sorption capacity increased.
Aluminium oxide is also beneficial in terms of efficiency and economy. This study illustrated that the amount of adsorbed cadmium ion increased with initial metal concentration, contact time and with the solution pH but decreased with adsorbent dosages or masses.