Abstract
Elevated concentrations of fluoride ions (F–) in natural groundwater are a worldwide problem. Discarded oyster shells were ground to <100 µm particle size to produce oyster shell powder (OS). A subset of the OS was heated to produce calcined oyster shell (COS). A subset of the COS was further treated with 1 M phosphoric acid to produce phosphoric-acid-treated oyster shell (POS). OS and COS were combined with phosphoric acid (1.6 mM and 3.2 mM) to produce OS + P (oyster shell with phosphoric acid) and COS + P (calcined oyster shell with phosphoric acid). OS and COS removed 46% and 50% (10 g/L of sorbent dose) but POS, OS + P and COS + P removed 96%, 100% and 76% (1 g/L of sorbent dose) when the initial concentration of fluoride was 10 mg/L. The sorption kinetics of POS, OS + P and COS + P followed second-order reaction rates, and sorption isotherms of all sorbents were well-described by the Freundlich sorption isotherm. These results indicate that oyster shells can be an effective sorbent for fluoride removal, with the added benefit of re-use of a waste product.
Introduction
Fluoride (F–) concentration in drinking water is an important parameter to determine water quality for drinking purposes1. Fluoride in drinking water has both beneficial and detrimental effects, depending on its dose. At low concentrations, it protects against tooth decay2. By contrast, the ingestion of water with a fluoride concentration above 1.5 mg/L causes dental and skeletal disorders3. Groundwater in many areas around the world has high levels of fluoride concentration4. The World Health Organization (WHO)4 recommends a fluoride concentration of less than 1.5 mg/L in drinking water. Because some groundwater naturally has high levels of F–, millions of people worldwide are affected by drinking this water. A study by UNICEF reported that fluorosis is endemic in more than 25 countries around the world. The natural concentration of fluoride in groundwater depends on geological characteristics of the aquifer, the physical and chemical characteristics of soil and rock, and depth of the wells. All these factors contribute to variable concentration of F–, which generally ranges between 1 mg/L and 44 mg/L, throughout the world5.
Many technologies, based on sorption, ion exchange, chemical precipitation, reverse osmosis and electro dialysis have been used to remove excessive fluoride from drinking water. Among them, sorption is one of the most common technologies because of ease of operation and cost effectiveness4. Many materials such as hydroxyapatite, activated alumina, limestone, activated carbon, zeolite, calcite, and clay have been used for fluoride sorption6. Low cost materials such as limestone, quick lime, bentonite, kaolinite, china clay and calcite have been tested for fluoride removal and proven to be effective7. However, at low F– concentrations, these materials lose their fluoride removal capacity and they are not suitable for drinking water because of limited fluoride sorption8. So it is important to identify new materials that are inexpensive and efficient for fluoride removal. We hypothesize that oyster shells meet these design criteria for fluoride removal from water.
Oyster shells are produced in large amounts from the marine food industry in China as well as in some other countries9. They can potentially be used as an inexpensive material for water treatment. Oyster shells are a rich source of calcium carbonate10,11,12 that can be used for fluoride removal13. Calcium carbonate can be used to produce hydroxyapatite which has been shown to be effective for fluoride removal from water14. Phosphoric acid–enhanced limestone has improved fluoride removal efficiency and is a low-cost material15. In composition, oyster shells resemble limestone and therefore have the potential for significant fluoride removal from water. However, to date, no studies have investigated the equilibrium and kinetic sorption of fluoride to oyster shells treated to optimize performance.
This work studies the feasibility of using oyster shells as a sorbent for fluoride removal from water. The sorption of fluoride by different forms of oyster shell fragments from aqueous solution was measured in a series of equilibrium and kinetic batch experiments and for different water chemistries. To our knowledge, this is the first study to evaluate the use of waste oyster shells for F– removal from water with consideration of thermodynamic and rate effects.
*Read the full article online at http://fluoridealert.org/wp-content/uploads/kim-2020.pdf
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