Chemical and environmental analysis of ash derived from the pyrolytic and incomplete combustion of plastic waste in household furnaces

Over the past decade, technological advances have brought a wide variety of new plastic materials to the market, often characterised by highly complex chemical compositions. Some of them are burnt in household furnaces, constituting a hazardous additive to the traditionally employed fuels. The study demonstrated that the combustion of plastics in household furnaces causes a significant increase in air pollution and a 2–3 times greater risk from 2.5 μm particles compared with particles <10 μm. The analyses revealed that the ash from household furnaces has a varied calorific value (9–40 MJ/kg) and medium- or coarse-grained structure, which may stem from incomplete thermal breakdown of fuel components. It is strongly alkaline (pH 11.16–12.85), thus being unsuitable for agricultural use, despite its high organic matter content (7.55–16.82%). The main components of the ash are Si (26%) and Ca oxides (21%), Al, Fe, S and Mg oxides (5–6% each), as well as Na, K, P, Ti and Cl (1–3% each), which together account for 80 wt%. Zn, Mn, Cr, Ba, Zr, Sr, Cu, Rb and Ni compounds constitute slightly over 2%, while Pb, V, Se, Br, Cd, Sn, Sb, Co and Nb are present in trace amounts. The ash also contains high concentrations of volatile aromatic hydrocarbons (1.93–31.80 mg/kg DM; ash samples obtained from polystyrene combustion). Furthermore, the ash shows considerable water leaching of K and Na, as well as Mo, Cr and Hg, which points to a high risk of salinizing both groundwater and surface waters and justifies its classification as hazardous waste. Due to the high amounts of leached Cr (PVC packaging previously containing mineral oils, mixed waste, and artificial leather, rubber and furniture plywood), the ash cannot be disposed of even in hazardous-waste landfills, as it requires prior treatment. The assessment of the ecological risk index (ERI) posed to living organisms ranged from low risk (Pb, Cr and Zn), through moderate risk (As and Cd), to high risk (Hg). Cumulative ERI calculated for all the ash samples allows for classifying the entire group of these materials as presenting considerable ecological risk .