Preliminary kinetic assessment of nitrate adsorption using construction and demolition waste materials

This study presents a preliminary kinetic assessment of nitrate (NO₃⁻) adsorption using construction and demolition waste (CDW) materials as low-cost adsorbents. Two residues were evaluated: Cariri Stone Waste (CSW) and Red Ceramic Waste (RCW), both abundantly generated in northeastern Brazil. The materials were processed to a particle size between 1.7 and 2.0 mm and characterized through point of zero charge (pHpzc) determination and batch kinetic experiments at 25 °C. The pHpzc analysis revealed distinct surface-charge behaviors: RCW exhibited a broad charge transition range (1.09–7.16), reflecting its heterogeneous mineral composition and indicating limited electrostatic favorability for nitrate adsorption at environmentally relevant pH values. In contrast, CSW presented a well-defined pHpzc of 7.16, suggesting favorable adsorption under slightly acidic to near-neutral conditions typical of groundwater. Kinetic results showed rapid nitrate uptake, with most adsorption occurring within the first 20 min of contact. Maximum adsorption capacities at 120 min were 1.50 mg·g⁻¹ for RCW and 3.89 mg·g⁻¹ for CSW. The pseudo-second-order model provided the best statistical fit for both materials, while diffusion analyses (Boyd and Vermeulen models) indicated that intraparticle diffusion predominates during the initial adsorption stage, followed by a combined diffusion regime. Although equilibrium isotherms and thermodynamic analyses were beyond the scope of this exploratory study, the results demonstrate that CSW, in particular, exhibits kinetic behavior comparable to mineral-based adsorbents reported in the literature, with relatively short contact times. These findings provide a screening-level evaluation of CDWs as potential adsorbent materials and support further investigation into their reuse for sustainable nitrate removal applications.