NSRRC Activity Report 2023

074 NSRRC ACTIVITY REPORT 2023 T his mechanistic study by Biqing Liang’s research group (National Cheng Kung University) confirmed that three-dimensional (3-D) ion exchange is the most important mechanism for heavy metal removal from aqueous solution and immobilization in a bone char (BC) mineral matrix. This research presents a novel and sustainable remediation strategy for cadmium (Cd) removal in wastewater and soil clean-up. Cadmium has high mobility in acidic soil due to its low affinity for soil ligands, making effective Cd remediation and removal challenging. Carbonated hydroxyapatite (CHAp), a mineral found in BC, dissolves readily in mildly acidic solutions due to weaker ionic interaction between Ca 2+ and CO 3 2- than that between Ca 2+ and PO 4 3- . This provides vacant sites on BC for cation sorption, especially for heavy metals with a similar ionic radius to Ca 2+ , including Cd 2+ , Co 2+ , Cr 3+ , Ni 2+ , and Pb 2+ . 1 Here, the research group proposes that millimeter-sized BC chunks and matrix scale ion-exchange mechanisms, other than surface sorption and precipitation, can be explored for high-capacity Cd removal via 3-D incorporation of Cd into the mineral matrix ( Fig. 1 ). Leaching of Ca 2+ from BC chunks after application to fields and aging may further provide vacant sites and porosity for Cd 2+ sorption. So Three-Dimensional Uptake of Cadmium and Spatial Distribution of Cadmium Hydroxyapatite Mineral in Bone Char This study highlights the application of particle bone char for sustainable heavy metal removal in water and soil. It reveals the major working mechanism of action. Fig. 1 : The study confirmed that matrix-scale ion exchange drove Cd removal, rather than complexation and precipitation. This study indicates that pre- leached BC and external Ca and P input could enhance Cd removal from aqueous solution via incorporation into BC. [Reproduced from Ref. 5] far, the most widely proposed mechanisms and research for heavy metal removal utilizing BC have been surface chemical sorption via complexation and precipitation onto BC, with limited investigations exploring ion-exchange mechanisms for heavy metal removal. 2,3 The removal of Cd was explored via 3-D matrix scale incorporation using BC chunks (~2 mm) prepared at 500 °C (500BC) and 700 °C (700BC) in aqueous solution. The Cd incorporation into CHAp was investigated using a series of synchrotron-based techniques ( TPS 21A , TLS 01B1 , TLS 14A1 , and TPS 19A ). It is important to note that conventional X-ray diffraction (XRD) techniques cannot detect trace Cd-phosphate incorporated into HAp due to low concentrations, poor crystallinity, and high similarity in the lattice d-spacing compared to HAp. 4 Furthermore, identifying physicochemical and mineralogical changes of HAp after Cd incorporation using traditional spectroscopic and imaging techniques is challenging due to the similar ionic radii of Ca 2+ and Cd 2+ . Synchrotron-based X-ray nanodiffraction (XND, TPS 21A ) and nano X-ray fluorescence (n-XRF) scanning techniques enable the investigation of subtle elemental and mineral phase changes, allowing investigation of site-specific