# Low-Cost RSAC and Adsorption Characteristics in the Removal of Copper Ions from Wastewater
## 1 Introduction  
Water contamination through heavy metal ions is an environmental problem of great concern [1]. Adsorption is one of the most efficient methods to remove noxious heavy metal ions, especially for wastewater with large volumes and low heavy metal ion concentrations [2]. Adsorption is spontaneous and the basic principle is that the surface energy of substances could change the concentration at the phase interface. Adsorption usually relies on some adsorbent materials with a large specific surface area and a high surface energy to remove heavy metal ions [3, 4]. Adsorption has two major advantages: the reaction rate is fast, and no other reagents are needed. Therefore, adsorption is regarded as an important and promising method for addressing heavy metal ions such as copper in wastewater.  
The key issue of adsorption is the adsorbent. Adsorbents with good adsorption performance have such qualities as: a fast adsorption reaction rate, stable physical and chemical properties, good solid-liquid separation, an economical cost, easy recovery and regeneration, and reusability [5]. However, industrial adsorbents could not meet all of these qualities. Therefore, the core of the adsorption method focusing on treating wastewater with heavy metal ions is to find efficient adsorbents at a low cost [6, 7].  
Currently, common adsorbents in the water treatment domain include activated carbon, fly ash, etc. Activated carbon has a large surface area, fast filtration rate, stable structure, large adsorption capacity, a wide range of applications, and good adsorption performance. However, activated carbon has a short service life, high sludge treatment cost, and low recycling performance [8, 9]. Fly ash, a waste product from power plants that use coal as the main fuel, has a loose and porous structure. It can intercept pollutant molecules and bind pollutants to the active sites on the surface by a chemical bond, resulting inexcellent adsorption [10]. Whereas, [PERSON] et al. [11] have also considered fly ash a low-cost material for adsorption. It has been found that steel slag has a good adsorption effect on copper, nickel and zinc ions, which mainly relies on the generation of hydroxide complexes [12]. Under optimal conditions, the adsorption efficiency of modified steel slag for uranium was 98% [13]. Activated white clay is an adsorbent made from clay minerals by inorganic acidification and other means, and dried by water rinsing. It is mainly made of bentonite clay as a raw material; its appearance is presented as a milky white powder, which is a non-toxic, tasteless, odorless, strong adsorption; and it can adsorb colored substances and organic matter [14]. Bentonite is considered to be an excellent adsorbent for Cu\({}^{2+}\), and the maximum adsorption capacity was 248.9 mg/g [15].  
The residue and soil adsorption composite (RSAC) particle consists of solid waste and natural minerals. The raw materials used in the preparation of RSAC granules are fly ash, steel slag, activated white clay, bonding agent, and porogenic agents. The main raw materials are fly ash, steel slag, and activated white clay, the first two of which are industrial waste substrates [16].