The growing demand for freshwater, combined with increased wastewater generation as the world population grows and industrial applications develop, makes wastewater recycling an urgent issue.
The presence of heavy metals, which have high and long-term toxicity, limits the recyclability of these waters. Heavy metal concentrations are particularly high in industrial wastewaters, endangering public health and the environment, if discharged without adequate treatment.
How are heavy metals removed from wastewater using flotation?
Chemical precipitation, solvent extraction, reverse osmosis, ultrafiltration, electrodialysis, ion exchange, and adsorption, are all common methods for the removal of heavy metals from wastewater.
Ion flotation can be used to eliminate ions of heavy metals from wastewater. The ion flotation process is based on making ionic metal species in wastewaters hydrophobic, by using surface active agents (surfactants), and then removing these hydrophobic species with air bubbles.
Processes involved in flotation for removal of heavy metals from wastewater
Flotation uses bubble attachment to isolate HMIs from the liquid phase. Dissolved air flotation (DAF), ion flotation, and precipitation flotation are the most commonly used water decontamination processes.
DAF allows bubbles to adhere to particles, forming aggregates that due to their low density, rise to the surface of the water and are easily collected and removed as sludge.
Ion flotation surfactants are used in hydrophobic wastewaters to transfer ionic metal species.
Precipitate flotation is another flotation method that is based on precipitate formation, and subsequent removal by attachment to air bubbles.
Combination of flotation and adsorption for heavy metal removal
Flotation as a separation process has recently received a lot of attention, because of its simplicity, quickness, economy, good separation yields (recovery >95%), for small impurity agent concentrations, a wide range of applications for species with different natures and structures, flexibility and friability of equipment and processing for recovery and production of more concentrated sludge, in smaller volumes.
This process is expected to be used as a clean technology to treat water and wastewater in the near future. For the reasons stated above, combining adsorption and flotation into a unified operation, could be regarded as a critical process.
What is electroflotation (EF)?
This technique is widely used in wastewater treatment and can be used to remove heavy metal ions, from industrial wastewaters. Pollutants, iron, or solid particles adhere to tiny bubbles of H2 or O2 formed on the electrodes of the floatation cell, and float to the surface during the electroflotation process.
Some additives, such as aluminum or ferric salts, are commonly introduced into waters to reduce suspended particles, and complete a natural separation. Water electrolysis forms very fine bubbles of hydrogen and oxygen, towards the reactions when electroflotation is used in aqueous media:
Cathode: 2H2O + 2e− ↔ 2OH− + H2(g)
Anode: H2O ↔ 2H+ + 1/2O2(g) + 2e−
The physical processes of gas evolution can be divided into three categories:
Nucleation, growth, and detachment.
The nucleation of bubbles occurs at electrode surfaces from highly supersaturated solutions. They are removed from the electrode by buoyancy forces or liquid shearing forces, which pull the bubbles away. The size of the bubbles in electroflotation is primarily determined by the nature of the electrode material, and its position in the electromotive series.
The pH of the solution influences bubble size; smaller hydrogen bubbles form in neutral or alkaline media, than in acidic media.
Heavy metals are major pollutants which are not biodegradable. They accumulate in the ecosystem. Furthermore, the persistent nature, toxicity, and accumulation of heavy metal ions in the human body, have become driving forces in the search for new and more efficient water treatment technologies, to reduce heavy metal concentrations in waters.
Because, conventional techniques are unable to meet the growing demand for lower heavy metal levels, in drinking water and wastewaters, implementing technologically advanced alternative water treatments is becoming increasingly difficult.
As a result, the flotation method is a suitable and cost-effective method, for heavy metal removal.
How can we assist?
A wide variety of wastewater treatment options are offered by Netsol Water, including water treatment, wastewater treatment, sewage treatment, effluent treatment, along with advanced physical, chemical and biological methods of wastewater treatment.