Heavy metals represent an interesting group of naturally existing inorganic micro-pollutants that are released in the environment by various phenomenon. Heavy metals are important both biologically and industrially but at high concentration in environment they have toxic effects. Metallic elements which have a density more than 5gm/cm 3 are termed as heavy metals. Heavy metals like Fe, Cd, Mn ,Hg Cr and many more are the indicators of human progress. Kennish (1992) 1 classified heavy metals as the metallic elements having atomic weights usually greater than 50. Hence in periodic table the transition elements from Vanadium (Except Sc and Ti) to the metalloid As, from Zirconium (except Y) to Antimony from Lanthanum to Polonium, the lanthanides and the actinides can be defined as heavy metals 2 .In All the major civilization past or present these heavy metals were used in one form or other. we can not imagine of any industry without these metals .Most of the toxic organic pollutants can be destroyed by combustion and can be converted into easily controllable pollutants such as CO, CO 2, SO x and NO x, but combustion has no effect on metals .Metals can not be destroyed. These heavy metals exists normally in the earth environment because of the continuous chemical and physical changes occurring due to metamorphic and igneous rocks. Similarly naturally occurring decay of plant and animal waste matter, precipitation or atmospheric accumulation of airborne particles from volcanic eruption, forest fire smoke, wind erosion and oceanic spray also contribute exposure of heavy metals in the environment 1. It has been found that in the past few years rapid industrialisation throughout the world has increased the concentration of these heavy metals in the environment. Toxic metals mobilize from Industrial activities and Fossil fuel combustion and eventually they are passed into the food chain leading to ecological and severe health problems. Table 1:-Industrial sources of some common heavy metals. Metals Industry Chromium (Cr) Lead (Pb) Mercury (Hg) Arsenic (As) Copper (Cu) Nickel(Ni) Cadmium (Cd) Zinc (Zn) Mining , chrome plating, coolants in industry ,As catalyst in tanning of leather , road runoff and making alloys. Mining, smelting, Lead batteries, electronic-waste, ceramics, bangle industry, ship building ,road runoff and in paints industry Chlor-alkali plants, fluorescent lights, fluorescent lamps ,dental amalgams ,thermometers, barometer, electrical appliances, , thermal power plants. Burning of fuel, Alloys, as dopent , arsenical pesticides ,herbicide , in electronic industry , thermal power plants and preservatives of wood Mining, electronic industry ,copper plating and smelting operations, Metal plating, smelting process,combustion of fossil fuel, electroplating, thermal power plants, battery industry, road runoff. Ni/Cd batteries, e-waste, paint sludge, incinerations and fuel combustion, road runoff. Smelting, electroplating, road runoff.
The demand of membrane distillation (MD) has increased since last few decades for numerous applications. The membrane used in MD is hydrophobic; therefore, the focus has been emphasised on the development of a suitable membrane with desired microstructure. In this study, the flat sheet hydrophobic membrane of suitable properties has been casted with various additives such as water, ethane-di-ol, and propan-2-ol in dope solution using a non-solvent induced phase separation (NIPS) technique. The effect of water content in dope solution has been studied on casted membrane porosity and contact angle. The maximum contact angle and porosity were found to be 96° and 53.23% at 4 weight percent of water content in dope solution of PVDF polymer and di.methyl.acetamide as solvent. It was found that SEM micrograph when ethane-di-ol and propan-2-ol are used as an additive shows more finger-like pores and nodules, respectively, in the microstructure of the casted membrane. Furthermore, synergistic effects using water with other additives were also identified using SEM micrograph of casted membrane and it was observed that water with ethane-di-ol and propan-2-ol form contact angle of 98° and 105°, respectively, for 2 weight percent each additive in dope. In this study, the membrane was also cast by dissolving PVDF powder in di.methyl.acetamide solvent with lithium chloride and the effect of the temperature difference between coagulation bath and film temperature was investigated using an SEM micrograph. Overall, it was found that water content and temperature difference aid in developing hydrophobic porous membrane of desired properties for MD applications.
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