Dynamics of Water-Soluble Metals in Soil Moistened with Citrus Wastewaters Depends on Soil Reaction and Organic Acids

The demand for water for civil and industrial use is diminishing the availability of such a valuable environmental resource for agricultural purposes. Thus, for the next generation, it is imperative to find alternative water sources for crop irrigation. The citrus agroindustry utilizes a large amount of water for processing fruit (e.g., essential oil extraction, fruit washing). Wastewaters produced by citrus industry (CWWs) are rich in organic matter and mineral nutrients, thus making them potentially usable for crop irrigation. Conversely, due to their high content of organic acids and low pH, they may increase the availability of soluble metals, in the form of both plant nutrients and contaminants. The aim of this study was to evaluate the effect of CWWs on the dynamics of soil water-soluble metals and pH. To this end, CWWs from the processing of lemons, oranges, and tangerines at three different doses were used. CWWs were analyzed to investigate type and amount of organic acids. Soil water-soluble metals (Na, Mg, Al, K, Ca, Fe, Co, Ni, Cu, Zn, and Cd) and pH were determined at days 1, 3, 7, 21, and 28 after the addition of CWWs. Citric, ascorbic, oxalic, tartaric, acetic, and malic acids were found in CWWs, with citric acid being two orders of magnitude more concentrated than the other acids. After the addition of CWWs, soil pH promptly decreased from 7.2 to at least 5.3 depending on the type and concentration of CWWs. Concurrently, the concentration of almost all investigated metals sharply increased within 7 days after the addition of CWWs. Then, it decreased, reaching values similar to that of the control. The increase in metals availability as a consequence of pH decrease was ascribed to different causes: the exchange reaction between H+ and cations adsorbed onto colloid surfaces, the addition of organic matter by CWWs that stimulated microbial activity, and the quantity and type of organic acids added via CWWs. In conclusion, the obtained results suggest that the use of citrus wastewater for irrigation purposes could be a valid solution, with them being rich in plant nutrients and easily mobilized elements such as Ca, Mg, K, and Na. Further research is needed to refine the understanding of the impact of CWWs in the long term and to develop targeted strategies for managing industrial wastewater in agriculture.