# Batch and Column Adsorption of Phosphorus by Modified Montmorillonite
## 1 Introduction  
Montmorillonite is a soft clayey water-absorbent mineral. It is a hydrous aluminum silicate with perceivable layers and sheets, with the general formula of (Na,Ca,K)\({}_{x+y}\)(Si\({}_{y-x}\)Al\({}_{x}\)) (Al\({}_{2-y}\)Mg\({}_{y}\))-O\({}_{10}\)(OH)\({}_{2}\), where x = 0.05-0.45 and y = 0.05-0.65. Each layer is composed of two types of structural sheets: octahedral and tetrahedral [1, 2]. A unit of montmorillonite contains two tetrahedral layers and an octahedral layer between these two tetrahedral layers. They are turned towards each other by similarly charged oxygen layers, so as to stabilize the aluminum (magnesium)-oxygen-hydroxolayers by the van der Waals forces [3, 4]. Montmorillonite possesses good adsorption and cation exchange capacity (CEC). The Si\({}^{4+}\) in the tetrahedral layers can be replaced by Al\({}^{3+}\), and Al\({}^{3+}\) in the octahedral layers in montmorillonite can be replaced by Zn\({}^{2+}\), Mn\({}^{2+}\), Li\({}^{+}\), or Fe\({}^{3+}\) to form interlayer negative charges [5]. The negative charge is balanced by potassium and sodium ions outside the layered structure. Ions, water, and salts can enter and leave the interlayer of montmorillonite under environmental conditions [6, 7]. In this way, inorganic and organic montmorillonite complexes are formed. A modified adsorbent with a good adsorption effect can be prepared based on the porous property of montmorillonite after modification by anions or/andcations through cation exchange on the surface or interlayer and intercalation of organic compounds into the interlayer. Additionally, montmorillonite has been adopted as a good adsorbent on account of the existence of different types of active sites located on its surfaces, such as ion-exchange sites, Lewis acid sites, and Bronsted sites [5; 8]. The efficiency and effectiveness of the adsorbent are the key factors in adsorption techniques [9]. However, untreated montmorillonite does not effectively remove targeted pollutants during wastewater treatment processes due to the extremely strong hydrophilicity of the silica structure on its surface and the hydrolysis of interlayer cations; thus, its adsorption performance should be improved by modification [10; 11; 12]. Studies have revealed that modification has a certain effect on the adsorption capacity of montmorillonite [13; 14; 15]. [PERSON] et al. performed an XRD analysis of C\({}_{12}\) alkyl polyglycolide quaternary ammonium-modified montmorillonite, suggesting that as the interlayer spacing increased, the number of exchangeable ions between the montmorillonite layers increased, and the specific surface area decreased compared to that of untreated montmorillonite [10]. [PERSON] et al. reported that with an increase in hexadecyltrimethyl ammonium (HDTMA), the specific surface area of the modified montmorillonite decreased and the number of effective sites available for Cr(VI) adsorption also decreased, weakening the strength of purely physical adsorption; meanwhile, the surface electrical properties of the modified montmorillonite changed from negative to positive, reflecting that electrostatic adsorption was enhanced for anionic pollutants [16]. Notably, the pH value of the solution may affect the removal efficiency since the surface charge density of montmorillonite changes with the ionization of adsorbate species [17; 18; 19; 20; 21]. [PERSON] et al. found that the impacts of pH on adsorption of U(VI) on Na-MMt and OH-Al-MMt were greatly dependent on pH, and the amount adsorbed decreased with the increase in the pH from 4.0 to 8.0 [22]. [PERSON] et al. showed that OH-Al-MMt adsorption indicated that the removal of methylene blue dye increased with an increase in the pH, up to a pH of 8 in solution. However, the methylene blue uptake capacity did not change from pH 8 to 10 [23]. Furthermore, temperature variation influences the adsorption rate of modified montmorillonite [24; 25; 26; 27; 28]. [PERSON] et al.