Chromium liquid waste inertization in an inorganic alkali activated matrix: Leaching and NMR multinuclear approach

A class of inorganic binders, also known as geopolymers, can be obtained by alkali activation of alumi-nosilicate powders at room temperature. The process is affected by many parameters (curing time, curingtemperature, relative humidity etc.) and leads to a resistant matrix usable for inertization of hazardouswaste. In this study an industrial liquid waste containing a high amount of chromium (≈2.3 wt%) in theform of metalorganic salts is inertized into a metakaolin based geopolymer matrix. One of the innova-tive aspects is the exploitation of the water contained in the waste for the geopolymerization process.This avoided any drying treatment, a common step in the management of liquid hazardous waste. Theevolution of the process - from the precursor dissolution to the final geopolymer matrix hardening - ofdifferent geopolymers containing a waste amount ranging from 3 to 20% wt and their capability to iner-tize chromium cations were studied by: i) the leaching tests, according to the EN 12,457 regulation, atdifferent curing times (15, 28, 90 and 540 days) monitoring releases of chromium ions (Cr(III) and Cr(VI))and the cations constituting the aluminosilicate matrix (Na, Si, Al); ii) the humidity variation for differentcuring times (15 and 540 days); iii) SEM characterization at different curing times (28 and 540 days);iv) the trend of the solution conductivity and pH during the leaching test; v) the characterization of theshort-range ordering in terms of T O T bonds (where T is Al or Si) by29Si and27Al solid state magic-anglespinning nuclear magnetic resonance (ss MAS NMR) for geopolymers containing high amounts of waste(10–20% wt). The results show the formation of a stable matrix after only 15 days independently on thewaste amount introduced; the longer curing times increase the matrices stabilities and their ability toimmobilize chromium cations. The maximum amount of waste that can be inertized is around 10 wt%after a curing time of 28 days.