# Environmental Behavior and Remediation Methods of Roxarsone
## 4 Adsorption of ROX  
Adsorption of ROX to soil occurs to a similar degree to adsorption of As(V) because of the presence of arsenate. ROX strongly adsorbs to metal oxides and clay minerals in soil, and the physical and chemical properties of soil strongly affect ROX adsorption [62]. Environmental factors that affect ROX adsorption include the mineral type, metal ions, ion strength, pH, and dissolved organic matter (DOM) content.  
Soil minerals are mostly responsible for providing ROX adsorption sites. Adsorption of ROX depends strongly on the characteristics of the soil minerals, including the particle size, specific surface area, hydrophobic partitioning, surface coordination, hydrogen bonding sites, and main adsorption sites, which all play important roles in the adsorption process. Goethite is an iron oxide mineral that is widely present in soil. Goethite has stable chemical properties, a high specific surface area, and high surface charge, so is an effective adsorbent for ROX. Aluminum oxides and hydroxides are widely present in acidic soil and aquatic environments. Fewer studies of arsenic adsorption by aluminum oxides than by goethite have been performed. Silicate minerals are widely present in the aquatic and terrestrial environment. Many water-bearing silicate minerals contain aluminum and magnesium, and the main minerals are kaolinite, illite, and montmorillonite. These minerals and aging of these minerals significantly affects arsenic adsorption, desorption, and oxidation.  
Ions affect ROX adsorption to soil. Metal ions, particularly Cu\({}^{2+}\), Fe\({}^{3+}\), and Zn\({}^{2+}\), can form complex precipitates with ROX and enhance ROX adsorption to goethite [63]. Conventional ions in natural water also affect ROX adsorption. Na\({}^{+}\), K\({}^{+}\), and other monovalent cations affect ROX adsorption through competitive adsorption. Divalent cations, such as Mg\({}^{2+}\) and Ca\({}^{2+}\), decrease the adsorption capacity for organoarsenical by competing for adsorption sites. Divalent cations can also form complexes with organoarsenical or form bridges between organoarsenical and the adsorption substrate to increase the adsorptioncapacity for organoarsenical. Sulfate and nitrate affect ROX adsorption little, but phosphate strongly affects ROX adsorption because As and P have relatively similar properties. It has been found in many studies that phosphate can strongly compete with ROX for adsorption sites [63; 64].  
The pH can affect ROX adsorption by changing the charge state of the adsorbent and the hydrolysis characteristics and stability of ROX. [PERSON] et al. found that the pH affects ROX adsorption by changing the degree of ionization of montmorillonite modified with Fe/La, the morphologies of the ionizable chemicals, and the surface charge of the adsorbent [65]. At a low pH, ROX is poorly dissociated and strongly hydrophobic, which will be conducive for linear distributive adsorption of ROX [4].  
The morphology, mobility, and bioavailability of arsenic can be affected by DOM. DOM can occupy adsorption sites on typical soil minerals (nano alumina, goethite, and kaolin) through electrostatic interactions, decreasing the amount of ROX that becomes adsorbed. ROX can form DOM-ROX complexes through interactions with amino, carboxyl, hydroxyl, and nitro groups in DOM [62; 66]. Binding to DOM markedly changes the chemical and biological reactivities and bioavailability of arsenic, and so affects ROX bioavailability, mobility, and morphology.