## 1 Introduction  
Platinum group metals (such as Pd, Pt, Au, Ru, Ir, and Os) can be naturally found at very low quantities in the earth's crust, and they are regarded as precious metals. In general, plenty of the precious metals have been used as catalytic elements by diverse industrial fields like automobile, petrochemical, and electronic components industries, although their availabilities are limited and they are expensive. After use, liquid or solid wastes, including a large amount of the precious metals, have been discharged from many industries. Such release of the metals without recovery is unprofitable for companies, and the released chemicals can play roles as toxicants in environments [1]. Therefore, recovery processes for precious metals from the industrial wastes should be an important practice. If recovery processes for metals are methodologically established, the waste solution containing precious metals can be regarded as secondary sources. Indeed, many researchers reported that some precious metals could be recovered using various techniques such as solvent extraction [2; 3], chemical precipitation [4; 5], ion exchange system [6; 7], and chelation [8]. However, there were several drawbacks to these techniques, such as poor selectivity, difficult stripping, low recovery efficienciesor high capital costs for establishing the processes, and disposal costs for chemical wastes used for recovery [9; 10]. Therefore, the use of these techniques should be limited [11]. To overcome the drawbacks, adsorption-based recovery of precious metals from wastewater is a promising process, because the process is effective and economical for treating aqueous effluents, even at low concentrations of metal. However, the efficiency of the process is strongly dependent on the employed sorbent. Among several sorbents' types (e.g., bead, capsule, fiber, etc. ), fibrous-type ones have been attractive because they have large surface areas, implying that plenty of functional groups can be potentially activated by chemical modifications. As a result, these characteristics can contribute to higher adsorption capacities and faster adsorption rates [12]. Until now, many types of fibrous agents--for example, activated carbon fiber [13], cotton fiber [14], glass fiber [15], and polymer fiber [16]--have been presented. Currently, polyacrylonitrile (PAN), with a molecular formula of [C\({}_{3}\)H\({}_{3}\)N]\({}_{n}\), as a raw material has been highlighted for designing a sorbent because it is abundant--a high amount (i.e., >64 million tons per year) has been discharged--and its stability in water is very advantages for the metal recovery process since metals or contaminants are mainly in aqueous phases. Other benefits of PAN fiber is its high strength, chemical resistivity, good mechanical properties, abrasion resistance, and electrostatic accumulation [17]. Because all fiber-form sorbents do not have good adsorption capacities, surface modification of fiber-formed sorbents are essential, as the case may be. In general, the technologies for functionalization of fiber can be divided into two categories: physical and chemical methods. The first case includes surface fibrillation and electric discharge, which can be used to change the structural and surface properties of the fiber and thus improve the mechanical bonding on the matrix. The latter can be conducted by hydrolysis, chemical coating, or activation [18]. In this study, we report the application of PAN fiber for a sorbent preparation for recovery of precious metals (i.e., Pd(II) or Pt(IV)) from aqueous phases. The PAN fiber has cyanide groups, but the groups are not effective binding sites for the negatively charged metal ions [19]. Therefore, the surface of PAN fiber was physically and chemically modified by firstly spinning PAN-dimethylsulfoxide mixture in distilled water and secondly amidoximation to generate the binding sites on the surface for anionic metals.