Fluoride laden wastewater remediation - adsorption strategy

Fluoride laden wastewater remediation - adsorption strategy
Activated alumina, clinoptilolite, and activated magnesium oxide are the predominantly utilized adsorbents for fluoride elimination. Recently, hydroxyapatite and zirconia exhibiting exceptional fluoride adsorption capability have also been documented. These adsorbents can be utilized to process wastewater with a fluoride concentration of 10 mg/L to sub 1 mg/L, fulfilling the standards for potable water.
The adsorption methodology typically involves incorporating the adsorbent into a packed column and utilizing dynamic adsorption, which is user-friendly and exhibits a consistent fluoride removal effect. However, it possesses the following drawbacks:
(1) Limited adsorption capacity.
Commonly employed adsorbents such as clinoptilolite and activated alumina exhibit modest adsorption capacities, spanning from 0.06 to 2 mg/g. The newly reported fluoride adsorption capacity of hydroxyapatite can attain 3.5 mg/g, whereas the fluoride adsorption capacity of active magnesium oxide is 6-14 mg/g, but it is susceptible to loss during utilization. The adsorption capacity of fluoride adsorbent primarily composed of rare earth zirconia can escalate to 30 mg/g.
Despite these novel adsorbents being relatively costly, their adsorption capacity diminishes gradually post-treatment and can be recycled, representing a promising development trajectory. Fly ash incorporates activated alumina, which can also be utilized to treat fluorine-laden wastewater. It can be directly incorporated into the wastewater for waste treatment, with minimal expense. However, the disadvantage is that the fluoride adsorption capacity is modest and the dosage is substantial. Typically, 40-100 mg/L needs to be added to meet the discharge standard of fluoride content in the effluent.
(2) The processing water volume is modest.
When the fluoride ion concentration in water is 5 mg/L, each kg of adsorbent can generally only treat 10-1000 L of water, and the adsorption duration is generally above 0.5 h. The adsorption methodology is only suitable for scenarios where the water volume is limited, such as drinking water treatment.
Comparison of several defluorination technologies
(1) The chemical precipitation method can be employed to treat high concentration fluorine-laden wastewater. When the initial concentration of fluoride ions is 1000-3000mg/L, the final concentration post-lime treatment can reach 20-30 mg/L. This method is straightforward to operate and has low treatment expenses. However, due to the sluggish settling rate of sludge, it is necessary to incorporate calcium chloride or other coagulants to expedite the precipitation. The primary method to decrease fluoride ion concentration is to augment the concentration of calcium ions and maintain a high pH to enable calcium fluoride to precipitate. Additionally, the combined use of phosphates, magnesium salts, aluminum salts, etc. is more efficacious in removing fluoride than employing calcium salts alone.
(2) The flocculation precipitation method exhibits poor fluoride removal effect on high concentration fluorine-laden water, and the concentration of sulfate ions in the treated water is relatively elevated.
(3) Adsorption method is suitable for deep treatment of potable water with modest water volume. Most adsorbents perform an anion exchange role, hence the fluoride removal effect is pronounced. However, specialized treatment agents and specific equipment are necessitated, and the treatment cost is often higher than that of precipitation method, and the operation is intricate. The utilization of novel adsorbents such as hydroxyapatite activated magnesium oxide rare earth metal oxide can enhance the treatment effect.
(4) For high concentration fluorine-laden wastewater, two-step treatment is often required. Initially, lime is utilized for precipitation to reduce the fluoride content to 20-30 mg/L, and subsequently adsorbent treatment is employed to reduce the fluoride content to below 10 mg/L.
(5) Considering the significant disparities in the types, quantities, fluorine content, and other pollutants of fluorine-laden wastewater, it is imperative to select treatment methods based on actual conditions and local circumstances. Special emphasis is placed on the comprehensive management of waste to waste.
(6) During the treatment of fluorine-laden water, various defluorination mechanisms may transpire concurrently. Conducting research on the mechanism of fluoride removal will contribute to the enhancement of existing fluoride removal processes and the development of novel methods for fluoride removal.