Accumulation and colloidal mobilization of trace heavy metals in soil irrigated with treated wastewater
Reuse of treated wastewater for irrigation purposes is worldwide accepted and implemented to face water scarcity and save high quality resources. Although such practice has undoubtable advantages and is certainly more sustainable in respect to the use of fresh water, it is not exempt from severe concerns. Potentially harmful pollutants contained at trace levels in the reused water might indeed impact on the receiving soil and on the crops. Among these pollutants, trace heavy metals (HMs) play a primary role due to their widespread presence in the used water and to their persistence once released into the environment. The fate of HMs in soils can be hardly predicted as mobility mechanisms through soils are extremely diverse and related to highly complex simultaneous phenomena and chemical equilibria. HMs, in fact, as many other contaminants, are not only partitioned between the solid immobile and the water mobile phases. Indeed, colloids and nanoparticles act as a third mobile phase, with their own rheological properties and velocity. This latter aspect has been one of the main focus of the thesis.
In details this thesis describes the results of several experimental tests conducted by irrigating the Organization for Economic Cooperation and Development (OECD) standard soil with real and/or synthetic wastewater, containing HMs in trace level concentrations. For each test a specific soil (e.g. varying the organic matter content) and wastewater composition (e.g. varying the metals concentration, the salinity, the organic matter content, or testing real treated wastewaters) has been chosen in order to evaluate the effects of different conditions on the overall HMs fate. The increase of soil organic matter from 2.5 to 10% linearly enhanced the mobility of Cd, Cu and Ni. The maximum mobility increase obtained comparing soils at 2.5 and 10 % of organic matter was of 35.6, 43.7 and 49.19 % for Cd, Cu and Ni, respectively. In most experiments metals accumulated in the top soil layer (0.5 – 1 cm). Nevertheless, peaks of contamination were detected at different depths in the soil deeper layers and at different leaching time in the soil leachates depending on the metal and on the soil and wastewater characteristics. Peaks of metals in the leachate appeared simultaneously with release of organic matter and/or release of silicates, demonstrating outstanding involvement of colloids in metals transport. Sodium concentration (20 mM) was demonstrated to highly reduce colloidal mobilization whereas more than 95 % of the influent metal was detected in the top layer despite the soil organic matter content. Conversely, low sodium concentrations (1-5 mM) enhanced colloidal and hence metals mobilization. Salinity displayed different effects. The irrigation with real treated wastewater with quite high content of Ca and Mg (111 and 134 mg/L, respectively) resulted in higher average release of silicon from the soil inorganic matrix (8.2 mg/L) compared to the low salinity artificial wastewater (1.9 mg/L). Consequently higher mobilization of Cd, Cu, Ni and Zn was observed when the soil was irrigated with real treated wastewater. An advanced spectroscopic characterization of the soil leachates was performed to identify such colloidal aggregates with the aim of clarifying their nature, chemical properties and aggregation state. The observation of 3D excitation-emission matrix demonstrated in all the leachates samples the presence of fulvic (230-450 nm ex-em fluorescence area) and humic (330-445 nm ex-em) substances. NMR spectra of the leachates resulted poorly resolved in the aromatic region (6 ppm <δ< 8 ppm) due to hydrophobic interactions in the core of the supramolecular structure of humic substances.