**1. Introduction**

In recent years there has been growing concern regarding F− and Cr(VI) pollution, due to both substances being transported into effluents from industries related to the extraction of minerals, foundries, dyes and pigments, semiconductors, and glass manufacturing [1,2]. These effluents can reach surface- and ground-waters by direct discharge or after passing through soils. Authors such as Rafique et al. [3] or Kumar et al. [4] indicate that there is a global hazard as regards fluoride and chromium pollution, taking into account that their permissible limits in drinking water (1.5 and 0.05 mg L−1, respectively, as per the World Health Organization) are widely exceeded in occasions, some of them referenced for countries such as India, China, USA, Mexico, or Argentina.

A F<sup>−</sup> concentration between 0.5–1.0 mg L−<sup>1</sup> in drinking water can be considered beneficial for bones and teeth, but it can cause fluorosis and even neurological damage when it is higher than 1.5 mg L−<sup>1</sup> [5]. In the case of Cr, although Cr(III) is indispensable in low quantities, Cr(VI) is considered highly toxic due to its mutagenic, carcinogenic, and teratogenic potential [6]. Given that it is extremely improbable that a ban will be implemented in the short term in order to remove F− and Cr(VI) from industrial use (mainly in aluminum, textile, or leather tanning factories), it is of main importance to

determine the capacity of soils to retain both anions, aiding to prevent their entry into waterbodies and plant uptake, as well as to develop low-cost methodologies to increase soil retention capacity and to remove these toxics when they reach waters [4].

Although different methodologies have been developed to remove F− and Cr(VI) from waters, such as precipitation, electrocoagulation, ion exchange, or electro-dialysis, the use of adsorbent materials has been considered as the most economical and sustainable alternative [2]. Previous works have dealt with individual adsorption of F− and Cr(VI), separately, both in soils and in different waste materials [3,7–13]. In addition, some studies focused on simultaneous retention of F− and Cr(VI), using adsorbents such as a chitosan-alginate aluminum complex (CSAlg-Al) [4], or synthetic mesoporous alumina [2]. However, there is not enough information on simultaneous retention of these two anions in different soils, as well as on the adsorbent capacity of different byproducts derived from industrial activities.

Galicia (NW Spain) is one of the geographical areas affected by activities which cause environmental pollution by F− and Cr(VI) (mining, aluminum factories, glass, dyes, leather tanning), and where it is also easy to obtain locally different by-products that could be used as low-cost sorbents for both pollutants [10–13]. Taking into account this previous background, the objectives of this work are: (1) to determine the capacity of forest and vineyard soils to adsorb F− and Cr(VI) simultaneously; (2) to determine the adsorption capacity for both anions of byproducts from forestry (oak ash, pine bark, and pine sawdust), from agriculture (hemp waste), from mining (pyritic material), and from the food industry (mussel shell). The results of this study could aid to solve environmental issues due to both pollutants in waters and soils, and at the same time promote the productive recycling of by-products.

### **2. Materials and Methods**
