Purification and Wastewater Treatment: an overview on nanofiltration Essay

Purification and Wastewater Treatment

A study by UNESCO shows that the disinfection of water at the point where it is used is the most cost effective method of treatment compared to having centralized location for water treatment. The water is also safer and it gives the users freedom to control what they take in.

Nanofiltration

Adsorption

Adsorption is employed as a polishing step to remove organic and inorganic contaminants in water and wastewater treatment. In conventional adsorbents techniques, there has been challenges caused by the limited surface area of active sites, the lack of selectivity, and the adsorption kinetics making adsorption very hard unlike in the case of nanofiltration. Nanoadsorbents offer significant improvement with their extremely high specific surface area and associated sorption sites, short intraparticle diffusion distance, and tunable pore size and surface chemistry.

2.1.1. Carbon based nanoadsorbents

2.1.1.1. Compared to activated carbon, these have shown higher efficiency on adsorption of various organic chemicals which stems mainly from the large specific surface area and the diverse contaminante CNT interactions. In the aqueous phase, the molecules form loose bundles or aggregates due to the hydrophobicity of their graphitic surface, thereby reducing the effective surface area. On the other hand, CNT aggregates contain interstitial spaces and grooves, which

are high adsorption energy sites for organic molecules. Although activated carbon possesses comparable measured specific surface area as CNT bundles, it contains a significant number of micropores inaccessible to bulky organic molecules such as many antibiotics and pharmaceuticals, thus CNTs have much higher adsorption capacity for some bulky organic molecules because of their larger pores in bundles and more accessible sorption sites. CNTs strongly adsorb polar organic compounds due to the diverse contaminante CNT interactions including hydrophobic effect, pep interactions, hydrogen bonding, covalent bonding, and electrostatic interactions.

2.1.1.2. Heavy metal removal.

Oxidized CNTs have high adsorption capacity for metal ions with fast kinetics. The surface functional groups (e.g., carboxyl, hydroxyl, and phenol) of CNTs are the major adsorption sites for metal ions, mainly through electrostatic attraction and chemical bonding. As a result, surface oxidation can significantly enhance the adsorption capacity of CNTs. Several studies show that CNTs are better adsorbents than activated carbon for heavy metals (e.g., Cu2, Pb2, Cd2, and Zn2) and the adsorption kinetics is fast on CNTs due to the highly accessible adsorption sites and the short intraparticle diffusion distance. Overall, CNTs may not be a good alternative for activated carbon as widespectrum adsorbents but as their surface chemistry can be tuned to target specific contaminants, they may have unique applications in polishing steps to remove recalcitrant compounds or in preconcentration of trace organic contaminants for analytical purposes. These applications require small quantity of materials and hence are less sensitive to the material cost.

2.1.1.3. Regeneration and reuse.

Regeneration is an important factor which determines the costeffectiveness of adsorbents. Adsorption of metal ions on CNTs can be easily reversed by reducing the solution pH. The metal recovery rate is usually above 90% and often close to 100% at p