Fluoride leaching from tuff breccia and its removal by natural and commercial adsorbents

In Japan, the concentration of fluoride (F^-) leached from rocks, such as tuff breccia, excavated in tunnel construction projects often exceeds the Japanese environmental standard of 0.8 mg/L. Because of this, proper disposal methods are necessary for managing F^--bearing excavated rocks. One effective solution based on circular economy is the use of an adsorption layer system. This system can simultaneously prevent the migration of F^- released from excavated rocks and allow the recycling of this construction waste material. To determine the most suitable material for the disposal of excavated F^--bearing tuff breccia from a tunnel construction in Hokkaido, Japan, four types of natural geological materials (S-1, S-2, S-3, and S-4) obtained near the tunnel construction site, as well as three types of commercial adsorbents (calcium (Ca), magnesium (Mg), and CaMg adsorbents) were selected for evaluation. The batch adsorption test results showed that S-1 and S-4 had high adsorption capacities for F^-, and the adsorption process followed the Langmuir isotherm. The adsorption of F^- to the natural adsorbents was strongly influenced by the pH and the presence of bicarbonate ions (HCO₃^-), but unaffected by chloride (Cl^-) and sulfate (SO₄^2-). There was also a strong positive correlation between the abundance of amorphous aluminum (Al) and iron (Fe) extracted and the adsorption of F^-, indicating the importance of ion exchange reactions associated with surface OH^- in immobilizing F^-. Meanwhile, the Mg-bearing adsorbent exhibited the highest adsorption affinity for F^- among the commercial adsorbents. This was attributed to adsorption through electrostatic interactions and coprecipitation with magnesium hydroxide (Mg(OH)₂) formed during the hydration of magnesium oxide (MgO). To effectively incorporate these adsorbents into the adsorption layer system, parameters such as permeability and residence time need to be determined in order to maximize the retention of F^- through adsorption, ion exchange and coprecipitation reactions.