Removal of Hazardous Pollutants from Wastewaters: Applications of TiO₂-SiO₂ Mixed Oxide Materials

Table 1

Different removal techniques and its advantage(s) and disadvantage(s).

Removal techniques	Advantage(s)	Disadvantage(s)
Electrochemical oxidation	Does not require auxiliary chemicals, high pressures, or high temperatures.	Low selectivity and low reaction rates.
Biological process	Ecologically favorable process.	High capital and operational cost.; Handling and disposing the secondary sludge pose problems.
Adsorption	Cost-effective method.; Easy availability and operation.; Most profitable process and more efficient than the conventional methods (i.e., precipitation, solvent extraction, membrane filtration, etc.).	Merely removes the pollutants from one phase (aqueous) to another (solid matrix).; Expensive process for regeneration especially if the pollutants are strongly bound to the adsorbents.
Advanced oxidation processes (AOP)
(i) Ozonation	Powerful oxidation technique oxidizes a large number of organic and inorganic materials.	More complex technology and requires high capital/operational cost.; High electric consumption.
(ii) UV	An effective method that typically does not leave any byproducts which are harmful to the environment.	Less effective if the wastewater has high amounts of particulates which can absorb UV light.
(iii) UV/H2O2	An effective technique in the oxidation and mineralization of most organic pollutants.; Ease of formation of ^∙OH radicals.	Less effective, when the wastewater has high absorbance.; High operational cost.
(iv) O3/UV/H2O2	Most effective process due to the fast generation of ^∙OH radicals.; Can treat a wide variety of contaminants.	Needs to compete with high turbidity, solid particles, and heavy metal ions in the aqueous stream.; High operational cost.
(v) Fenton reaction	Simple process.; Easy availability of chemicals.	Production of sludge iron waste and handling the waste pose logistical problems.
(vi) Photo-Fenton reaction	Reduction of sludge iron waste compared to original Fenton reaction.; Effective and fast degradation.	Needs a controlled pH medium for better performance.
(vii) Heterogeneous photocatalysis	Long-term stability at high temperature.; Resistance to attrition.; Low-cost and environmentally benign treatment technology.	Could form byproducts that can be harmful to the environment.; Requires efficient catalysts that can absorb in the visible region.