Lead immobilization through thermal incorporation and framework intercalation

Abstract

This study aims to explore the potential lead-hosting structure candidates and develop efficient lead removal strategies for heavy metal treatment. In this study, Pb immobilization was achieved through incorporation by thermal treatment and intercalation by specific adsorbents. A low-temperature thermal immobilization based on the reaction mechanism forming the melanotekite (Pb2Fe2Si2O9) crystal phase was investigated to stabilize Pb-containing waste, and a feasible adsorption-stabilization strategy was also developed to treat the spent adsorbents. The well-controlled thermal scheme was applied to treat two typical Pb-containing wastes - cathode ray tube (CRT) funnel glass and waste PbZrxTi(1-x)O3 (PZT) piezoelectric ceramic. The synergetic effects of CaCO3 and Fe2O3 played a vital role in immobilizing amorphous Pb in the CRT funnel glass and promoting its beneficial use for ceramic industry. Additionally, the combination of phosphate treatment with controlled thermal scheme offers a possibility for treating the waste PZT piezoelectric ceramic. A new chemical-durable matrix with the chemical formula PbZr(PO4)2 was developed in the sintering process and its crystal structure was solved by synchrotron techniques and Rietveld refinements. With the increase of Ti contents in the trigonal PbZrxTi(1-x)(PO4)2 (0.5 ≤ x ≤ 1) crystal systems, a reduced Pb leachability was observed which might be due to the lower lattice formation energy. The excellent chemical durability suggests an alternative waste form for dealing with the waste PZT piezoelectric ceramics. Furthermore, the effect of isomorphous substitution on crystal structure and leaching behavior was investigated in the pyromorphite (Pb10(PO4)6Cl2) structure. The replacement of phosphate by silicate and sulfate substantially changed the lattice parameters and Pb dissolution, and a reduced apparent activation energy might be attributed to the increased Pb leaching in the SiO44-/SO42- dominated structures.

To further promote Pb immobilization in the solution system, two halide (chloride and bromide) intercalated graphitic carbon nitrides were successfully prepared by molten salt synthesis. The layers of the carbon nitrides (CN) are stacked in analogue to graphite with the halide ions located in the center of the interlayers. The interlayer space of Br-intercalated CN expanded compared with Cl-intercalated CN, which could be explained by the larger ionic radius of Br (IRBr=1.96 Å and IRCl=1.81 Å). The intercalation of halide ions provides effective bonding site for Pb(II), which greatly enhances Pb immobilization in the carbon nitride network. The results of adsorption kinetics and isotherm study showed both of the materials were chemical rate controlling and with monolayer adsorption behavior. The Pb adsorption capacities in the halide-intercalated graphitic carbon nitrides (LiCl-CN-4h: 172.41 mg/g; BrCN600: 307.69 mg/g) were greatly enhanced compared with other typical carbon-based materials, and this concept could be well utilized for heavy metal treatment in environmental remediation.