# Adsorption of Ni2+ and Cd2+ from Water by Calcium Alginate/Spent Coffee Grounds Composite Beads
## 1 Introduction  
Heavy metals can be described as metallic elements that have a relatively high density compared to water, but the definition also includes metalloids, such as arsenic. The use of heavy metals in industrial, domestic, agricultural, medical, and technological applications have led to their release in the environment with potential effects on human health and the environment [1]. Known heavy metals are cadmium, chromium, mercury, lead, cobalt, nickel, and so forth; these metallic ions are toxic and pose severe effects on human health. Cadmium is released to the environment in industrial processes such as smelting, alloy manufacturing, and pesticides. Anthropogenic activities such as improper disposal of unused paints, fertilizers, and Ni/Cd batteries are also sources of cadmium contamination [2]. Cadmium affects cell proliferation, differentiation, and apoptosis, therefore interacting with DNA repair mechanisms, the generation of reaction oxygen species (ROS) and the induction of apoptosis [3]. On the other hand, nickel does not raise the same environmental concern than cadmium and it is widely used, mainly in stainless steel. Nevertheless, nickel compounds are classified as human carcinogens based on increased respiratory cancer risks [4]. The International Agency for Research on Cancer (IARC) has determined that some nickel compounds are carcinogenic to humans and that metallic nickel may possibly be carcinogenic to humans. The Environmental Protection Agency of the United States (EPA)has determined that nickel refinery dust and nickel subsulfide (Ni\({}_{3}\)S\({}_{2}\)) are human carcinogens. EPA recommends that drinking water should contain no more than 0.1 milligrams of nickel per liter of water (0.1 ppm) [5]. EPA has set the limits of cadmium in drinking water in 0.005 ppm. Activated carbon and ion-exchange resins are commonly used to remove pollutants but they are expensive materials, therefore the elimination of pollutants from water has been focusing recently in the research of low-cost alternatives, such as forestry wastes, agricultural residues, marine materials, accessible minerals, or waste minerals. Waste minerals from construction work such as dolomite (cadmium sorption of 89.9 mg/g) are inexpensive adsorbents and they do not have any other practical application [6]. In the same way, synthetic mineral adsorbents made from waste material such as zeolite synthetized from fly ash product of power generation by coal (copper sorption of 4.99 mg/g) is another example of a raw material without additional applications [7]. Accessible clay minerals such as glaucoma (cadmium sorption of 2.596 mg/g) have the advantage that are natural, locally available, commonly occurring materials and could be reused as a sustainable and environment-friendly building material [8]. A good adsorbent should have high surface area, high adsorption capacity, mechanical stability, and an easy regeneration. The research in low-cost adsorbents could bring a good adsorbent that is an inexhaustible, cheap, and, mostly, a non-hazardous material [9]. The agricultural residues are an example of low-cost adsorbents; the agricultural by-products are another cause of environmental concern because their disposal requires careful treatment as other pollutants. Coffee drinks are one of the most popular beverages worldwide; therefore, a significant quantity of by-products is generated. A significant amount of literature on sorption of pollutants by coffee residues is available, but comparative analysis between studies is difficult due to the absence of a uniform protocol. However, one characteristic that is always observed is the rapid initial phase of adsorption that is predominant throughout the process [10]. This is an added value of the use of coffee residues for the removal of pollutants.