One-pot hydrothermal transformation of seashell calcium carbonate waste to apatite and doped-apatite particles

Abstract

Waste seashells from the fishery industry represent an important environmental issue. Its valorization by transforming the CaCO3 into apatite micro/nanoparticles may have a positive environmental impact besides an economic profitability since the global apatite market is expected to grow at 6.5% to around USD 3.086,05 million by 2027. Many of methods to transform biogenic CaCO3 into apatite particles are two-step processes involving calcination to CaO at 900-1000 °C followed by titration with a phosphate reagent (typically H3PO4). Here we present a one-step hydrothermal method using oyster shells Crassostrea gigas as model raw material. Shells were treated with NaClO 5% v/v, crushed, milled, and sieved (Ø<45 ¿m mesh), before submitting to hydrothermal conversion. Firstly, we explored the influences of KH2PO4 and K2HPO4, P/Ca molar ratios (0.24, 0.6, and 0.96), and temperature (25-200°C) in the transformation process. The minimum temperatures to obtain full transformation when using KH2PO4 and K2HPO4 for P/Ca molar ratios 0.6 (stoichiometric) were 160 ºC and 120ºC respectively, while for P/Ca ratios 0.96, they were 120 ºC and 80 ºC. The precipitates (Mg2+-doped carbonated-apatite micro/nanoparticles) are the result of a dissolution/re-precipitation mechanism driven by pH variation. Secondly, we explored the influence of osteogenic ions Mg2+, Mn2+, and Co2+ at P/Ca molar ratios 0.6. Full transformation of CaCO3 was obtained at 160 ºC, yielding platy-shaped apatite nanoparticles doped with either 0.22 mol% Mg2+, 0.012 mol% Mn2+, or 0.16 mol% Co2+, and sizes within the range 75-90 nm. Compared to the previous two-step processes, the one reported here is straightforward, one-pot, easily to scale up, and can be operated at relatively low temperatures without any pH adjustment.