# Retention of the Antibiotic Cofruoxime onto Agricultural and Forest Soils
## 3 Results and Discussion
### Adsorption of CFX and Modeling  
#### 3.2.1 Adsorption Data  
Table S3 (Supplementary Materials) shows CFX adsorption (in umol kg\({}^{-1}\) and percentage) as a function of the concentration of antibiotic added.  
In the soils with corn cultivation, when the highest dose of CFX was added, the adsorption values oscillate between 71.36 and 123.91 umol kg\({}^{-1}\) (Table S3, Supplementary Materials), while in the vineyard soils on schist and slate adsorption is between 101.05 and 122.86 umol kg\({}^{-1}\), and in vineyard soils on granite is between 54.39 and 125 umol kg\({}^{-1}\). Forest soils show a scarce oscillation, between 109.9 and 116.71 umol kg\({}^{-1}\). Analyzing the data expressed as a percentage, a large part of the soils adsorbed more than 75% of the CFX added. In this sense, the range of variation was small in all vineyard soils on schist and slate, and in forest soils (from 79.26 to 96.24%, and between 74.57 and 93.46%, respectively), while a wide oscillation was observed within corn soils (ranging 49.49-99.2%) and vineyard soils on granite (ranging 40.75-99.57%). In other words, some of the soils under study adsorb almost 100% of the maximum concentration of the antibiotic added, while others do not adsorb even half of that concentration.  
In the agricultural soils of the current study, whose pH values vary between 5.01 and 8.02, the differences in adsorption would be mainly related to these pHs. In fact, those soils having the highest pH (M1, M5, VP6, and VP7) show higher adsorption. Similar results were reported by [PERSON] and [PERSON] [28] when studying the adsorption of various cephalosporic antibiotics on magnesium oxide nanoparticles and carbon nanotubes, finding higher adsorption when the pH of the medium was >8. pH is one of the parameters having the highest influence on antibiotic-adsorbent interactions, since it simultaneously affects the chemical speciation of antibiotics and adsorbent surfaces [29]. Both antibiotics and soil components, have functional groups that can undergo protonation/deprotonation reactions, depending on the pH of the solution. The CFX molecule is amphoteric, having two dissociation constants (pka\({}_{1}\) 3.15 and pka\({}_{2}\) 10.97), which can have a positive or negative charge, or be in the form of a zwitterion, depending on the pH of the medium [30].  
At pH above 10.97, CFX is in anionic form, while at pH below 3.97, it is in cationic form, and between both pH values, it is a zwitterion. Regarding the soils in this study, since most have a pH above 5.5, the components with variable charge (1:1 clays, organic matter, and non-crystalline minerals) will preferably be negatively charged. At these pH values, CFX would be as a zwitterion, meaning the positively charged amino groups of the antibiotic would interact electrostatically with the negatively charged adsorbent surfaces of the soils, while the anionic groups (COO-) of CFX would bind to these surfaces through a cationic bridge [31]. The higher the pH of the soil, the more negative charges would be produced in its components, which would explain the influence of pH on these adsorption processes.  
In the case of forest soils, the adsorption of CFX is generally above 70% (Figure S1, Supplementary Materials), despite having a clearly acidic pH (<5). This relevant score for adsorption could be related to the high content of organic matter in these soils (between 12.32 and 16.86%), much higher than that of the agricultural soils of the study. Organic components have a pH-dependent charge and are highly involved in antibiotic adsorption processes [32], giving that at pH < 5, the -COOH groups of organic matter begin to dissociate [33], and then could interact electrostatically with the positively charged groups of CFX.