Excess adsorption of phosphoric acid from extremely acidic solutions by covalent organic framework EB-COF:Br

The industrial waste streams with excess phosphorus acid (H₃PO₄) was generated in semiconductor industries that has extremely low pH rendering commercial adsorbents unfit for adsorption recycling of valuable materials. This study for the first time use covalent organic framework EB-COF:Br to adsorb phosphoric acid from extremely acidic H₃PO₄ solutions at pH ranging 0.86 to −0.65. The EB-COF:Br could maintain structural stability with these extremely acidic solutions. At 25 °C, 95% adsorption could be completed within 10 min of contact; while the adsorption capacities of EB-COF:Br from 75% H₃PO₄ solution ranged 6520‒6980 mg-H₃PO₄/g during 25–45 °C. The adsorption is regarded isoenthalpic with no heat effects. Water washing is efficient for H₃PO₄ desorption, and the washed COF can be reused as an efficient adsorbent under extremely acidic environment. The interactions between (‒N⁺=) and (‒C=O) groups, and part of the (‒NH‒) groups participated in the adsorption process with adsorbed H₃PO₄ molecules. The adsorbed H₃PO₄ molecules then formed H-bonding to trap the excess free H₃PO₄ molecules to the internal cavity of the COF.