I Residui del trattamento di potabillizzazione delle acque come adsorbenti dell'arsenico in suoli contaminati

In this study we determined, at different pH values (pH 4, 7 and 9), the ability of a Fe- and an Al-based water treatment residues (WTRs) to accumulate As(V) from an aqueous solution and to define the mechanisms which regulate the sorption of this anion. The Fe-WTR showed a greater As(V) sorption capacity with respect to Al-WTR at all the pH values studied [e.g. 1.65 mmol•g-1and 1.02 mmol•g-1of As(V) sorbed at pH 4 respectively]. Sequential extraction of As(V)-WTR suggests that the main mechanism governing the sorption of As(V) likely involved the formation of inner-sphere surface complexes [Fe/Al–O–As].The capacity of Fe- and Al-WTR to decrease the mobility and bioaccessibility of As in a polluted sub-acidic soil (~2267 mg•kg-1d.m.) was also evaluated. The addition of both sorbents at two application rates (1 and 2% w/w) induced a shift of the labile As fractions towards different ones more strongly retained by the components of WTRs (particularly by the Fe/Al amorphous (hydr)oxides), with the consequent decrease of the As mobility. All the treatments favoured a decrease (even if not significantly) of As bioaccessibility, evaluated through the SBET (stomach phase extraction at pH 1.5) and the SBRCG (intestinal phase extraction at pH 7.0) analyses, which simulate the gastrointestinal processes.The results presented show that both WTRs can be useful at decreasing the mobility and bioaccessibility of As, so favouring a certain recovery of the functionality of polluted soil.