Mercado‑Borrayo, and 3 co-authors, reported in 2018 that the reuse of geothermal wastewater for irrigation is an attractive alternative for supplying water demand in agriculture, due to the high volumes generated in geothermal plants. This application is limited by the presence of toxic semimetals such as arsenic and boron, which generally require high-cost commercial adsorbents for removal. The authors evaluated the the removal mechanism and process optimization of arsenic and boron, present in high concentrations in synthetic solutions and in geothermal wastewater, using metallurgical slags. The effect of pH, initial concentration of arsenic and boron and slag dose were investigated using a 33 factorial experimental design and response surface method to optimize the operating conditions of the removal of pollutants.
Scanning electron microscope analysis illustrated that the removal mechanism consisted in a dissolution–precipitation reaction rather than adsorption. The effluent produced from wastewater at the optimal operating conditions in a two-step process meets the criteria proposed for both metalloids by the US Environmental Protection Agency for water used for irrigation.
The authors concluded that as an important progress with respect to earlier studies, this work demonstrates the feasibility of treating naturally contaminated geothermal wastewater with a two-step process, at the optimized conditions obtained for synthetic solutions of As(V) and B. This produced an effluent with an arsenic content (0.011 mg/L) which meets the criteria proposed by the US Environmental Protection Agency for irrigation. The final boron concentration (10.56 mg/L) was within the range proposed for very tolerant crops (6–15 mg/L). The performance of the two-step process is better than other processes using non-conventional reagents. The spent slag can be stabilized by conventional means (solidification/stabilization)
and does not propose an environmental hazard.
The authors also concluded that removal of the metalloids involves the dissolution of iron-, calcium- and magnesium-containing minerals in the slag and re-precipitation of insoluble products on the surface of the slag, which capture the pollutants in their structure. This observation presents a significant paradigm shift, because physiosorption and chemisorption are more frequently considered as being responsible for the removal of metalloids by slags. A detailed analysis of the products and reactions involved is subject of ongoing research and may allow a more precisely directed process optimization strategy in future projects.
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