# Gelatin-Siloxane Hybrid Monoliths as Novel Heavy Metal Adsorbents
## 1 Introduction  
Heavy metal pollution has become a globally recognized environmental issue due to the extensive anthropogenic activities connected with rapid industrialization [1; 2; 3]. The main sources of heavy metal release into environment are wastewaters from chemical industries such as metal plating facilities, mining operations, battery manufacturing, fertilizer, paper or metallurgical industries, pesticides and tanneries [4]. Heavy metal ions such as lead, chromium, cadmium, arsenic, mercury, nickel, zinc and copper are hazardous above certain ppm concentration and toxic for human health. These contaminants do not biodegradable, show high solubility in the aquatic environments and tend to accumulate into body of living organisms. They have already been found in the food chains of aquatic species [1; 4; 5]. Long-term exposure to heavy metals causes defects to organs (e.g., kidney, lungs, liver, eyes) and biological entities such as the nervous system, immune system, reproductive system and respiratory system. It may contribute to several degenerative diseases of central nervous system, as well as damage of cardiovascular and gastrointestinal systems. Heavy metals increase the risk of anaemia, cancer, nausea, organ failure, loss in bone mineral, and tumour induction [1; 2; 6; 7]. Research has shown that copper, nickel, lead, and arsenic have been observed to be toxic even at low concentrations [4], therefore effective methods of their removal from ecosystem are of importance. In the light of the above, the World Health Organization (WHO) established guidelines for drinking-water quality and recommended maximum permissible limits for selected heavy metals. Similar standards for drinking water have been developed also by the United States Environmental Protection Agency (USEPA) and the European Union (EU) [4]. Recently, different attempts have been made to solve the problem of effective heavy metal ions removal from wastewater. These methods include chemical precipitation, ion-exchange, adsorption, membrane filtration, coagulation-flocculation, bioremediation, flotation and electrochemical methods [1; 2; 5]. Among them adsorption has gained in importance as an effective way to treat industrial waste effluents, offering several advantages like low-cost, availability, profitability, and ease of operation. It serves as an alternative to conventional methods such as precipitation or coagulation. [2; 3]. Moreover, while adsorption is sometimes reversible, once employed adsorbent can be regenerated by a suitable desorption process [2; 4]. In principle, adsorption is a mass transfer process by which a substance is transferred from the liquid phase to the surface of a solid, and becomes bound by physical and/or chemical interactions [5]. A number of materials have been reported as adsorbents such as activated carbon, carbon nanotubes, chitosan, gelatin, zeolites, clay, or waste products from industrial operations i.e., fly ash, coal, and oxides. The use of natural materials for heavy metal removal offers some advantages as they are inexpensive and locally available [1; 8]. To a large extent, the adsorption ability of various materials can be attributed to their morphology. The effectiveness of the removal of heavy metals is dependent on the surface area and adsorption capacity [5]. Thus, fabrication of highly porous structures, also using natural biopolymers is an interesting approach for the preparation of novel adsorbents. An interesting and wide group of heavy metal porous adsorbents that can be applied for water purification are aerogels. Depending on the chemical composition (organic, inorganic or both), and the synthetic route applied for the preparation, such materials exhibit tuneable morphology and efficiency [9]. Other examples of effective adsorbents are described by [PERSON] et al. [10] who developed highly porous inorganic materials derived from metal-organic frameworks suitable for Hg(II) elimination from aqueous solutions.