Biological Calcium Carbonate With Hierarchical Organic–inorganic Composite Structure And Its Modification Enhancing The Removal Of Heavy Metals From Wastewater

With the development of mining industry and the increasing discharge of wastewater,the pollution of heavy metals is getting serious.The problems not only directly harm the health of human but also cause the deterioration of our environment.Therefore,how to effectively deal with the heavy metals in wastewater is an important subject of scientific research.Among many disposing methods?physical methods,chemical methods and biological methods?of heavy metal removal,adsorption is one of the most effective and economic methods.Geological calcium carbonate?geo-CaCO₃?is used extensively in water treatment and environmental remediation due to the fact that it is environmentally friendly,inexpensive and effective in neutralizing acidity and immobilizing heavy metals.However,geo-CaCO₃ normally has a dense bulk structure,and relatively low surface area,as a result,it can be difficult to achieve the water discharge standards in removing heavy metals from wastewater and the output sludge can be enormous.Therefore,alternative materials with better performance are needed to solve the abovementioned problems.In contrast,biological calcium carbonate?bio-CaCO₃?with unique microstructure and organic functional groups is expected to outperform and substitute geo-CaCO₃ in heavy metal removal.Meanwhile,bio-CaCO₃ as wastes can be converted into resources.This work focused on the issue of Pb???removal and Eu???recycling in wastewater and investigated the mechanism of heavy metals removal by bio-CaCO₃ and its modification counterparts.The main research contents and innovation points are as follows:?1?Waste oyster shells were used as raw materials of bio-CaCO₃,and its physical and chemical characterizations were analyzed through X-ray powder diffractometer?XRD?,Scanning Electron Microscope?SEM?,Transmission Electron Microscope?TEM?,Fourier Transform Infrared Spectroscopy?FTIR?and Brunner-Emmet-Teller?BET?study.In addition,we investigated the mechanism of the immobilization of Pb???by bio-CaCO₃ in contrast to geo-CaCO₃.The study revealed that the maximum adsorption capacity of bio-CaCO₃ for Pb???was three times that of geo-CaCO₃,reaching up to 1667.67 mg/g.Extensive mechanism research showed that the hierarchical porous organic–inorganic hybrid structure of bio-CaCO₃ expedited the dissolution of CaCO₃ to provide abundant CO32-active sites and facilitated the permeation and diffusion of Pb???into the bulk solid phases.Furthermore,the examination of Pb???removal ability of bio-CaCO₃ after calcination indicated that the organic functional groups of bio-CaCO₃ also facilitated the immobilization of Pb???into CaCO₃ particles,however the contribution was less than 1/3 of the total adsorption capacity.Therefore,the major contribution was from the hierarchical porous structure of bio-CaCO₃.?2?Bio-CaCO₃ was successfully functionalized by mussel-inspired polydopamine?PDA?chemistry.However,the weakness of dopa in alkaline is its auto-oxidation,which renders dopa unreliable for adhesion on inorganic materials.The study revealed that the functional groups of the organic matters in bio-CaCO₃ were found to be able to effectively inhibit the auto-oxidation of dopa in alkaline environments,forming a uniform and tight PDA coating on bio-CaCO₃.In comparison with the mussel-inspired PDA functionalized geo-CaCO₃,Fe3O4 and Si O2,the mussel-inspired PDA functionalized bio-CaCO₃ exhibited much more superior Eu???adsorption capacity in aqueous solutions,reaching up to 151.52 mg/g.Based on the SEM,Energy Dispersive Spectrometer?EDS?and X-ray Photoelectron Spectroscopy?XPS?analyses,the possible Eu???adsorption mechanism of bio-PDA was proposed as the coordination of Eu???with the abundant catechol and amine/imine groups in the PDA coating.?3?Bio-CaCO₃ was modified with different temperatures of calcination?600,700,800 and 900??,and Eu???adsorption experiments by different temperatures modified bio-CaCO₃ were studied.The results indicated that the higher temperature,the better effect of the samples for Eu???recycling,which was due to the replacement reaction between Ca?OH?2 and Eu???.