*2.1. Materials*

In Galicia (NW Spain), some environmental problems related to F− and Cr(VI) pollution have been previously pointed out [10–13]. In fact, the soils and sorbent materials used in this work to study F− and Cr(VI) competitive adsorption were the same previously described when performing individual adsorption tests for these two anions [13], in addition to pine sawdust, also previously described [12]. Specifically, in the current work we used samples of forest and vineyard soils, pyritic material, fine mussel shell, pine bark, oak ash, hemp waste, and pine sawdust. Taking into account that detailed descriptions for all these materials were previously published in the referred works, data on it are included in Supplementary Material. In fact, specific references regarding Supplementary Material are also included [14–37].

### *2.2. Methods*

### 2.2.1. Characterization of the Soil Samples and Sorbent Materials

Details on all methods used to characterize soils and by-products, as well as the results of those procedures, are shown in Supplementary Material. Specifically, total C and N contents, pH in distilled water, pH of the point of zero charge (pHpzc), exchangeable Na, K, Ca, Mg, and Al, effective cation exchange capacity (eCEC), total P, total concentrations of Na, K, Ca, Mg, Al, Fe, Mn, As, Cd, Cr, Cu, Ni, Pb, and Zn, non-crystalline Al and Fe (Alo, Feo), and particle-size distribution for forest and vineyard soils. Also, infrared spectroscopy was used to determine the main functional groups present in each soil and byproduct.
