## 1 Introduction  
In recent years, with the rapid growth of the economy, a shortage of resources has gradually emerged. At the same time, it has also caused the deterioration of the global environment. To protect the environment and to promote sustainable economic and social development, the reusing of renewable biomass resources has attracted scientists' attention. The extensively studied renewable resources include biological polysaccharides such as starch, cellulose, agar, and chitin; plant proteins such as soybean protein, wheat protein, and corn protein; and other animal proteins such as casein, collagen, elastin, keratin, and silk fibroin, etc. [1]. Different from the earlier direct-use renewable resources to manufacture goods, the current researchers are devoted to developing new materials from waste biomass to achieve high-value utilization.  
Worldwide, the poultry meat processing industry produces about 4.0 \(\times\) 10\({}^{7}\) tons of feather by-products each year [2]. However, those by-products are ineffectively reused;only a small amount of feather was processed into low-value products such as feather powder [3] and fertilizer [4]. The most of it was discarded, possibly causing environmental pollution and even disease transmission [5]. Hence, feather as a cheap, renewable biomass resource, with a large amount of solid waste produced by poultry industry, is a potential candidate of substitute material for fossil resources.  
Although research on the extraction, chemical composition, physicochemical properties of feather keratin (FK) has been conducted since the early 20 th century [6; 7], using discarded feather as a keratin source to develop new keratinized materials appeared only in the last twenty years [8; 9]. Currently, FK can be prepared into membrane [8], hydrogel [10], fiber [11; 12], biomedical materials [13; 14], and daily chemical materials [15], etc. However, the modification of FK has focused on using polymer grafting or compositing [16; 17], little research has been conducted using small molecules to modify FK [18]. Therefore, using small molecules to modify feather keratin has broad prospects. Gallic acid (GA, 3,5,7-trihydroxybenzoic acid) is a hydrolytic product of biological tannic acid. GA is a natural organic acid containing a phenolic ring, which has a variety of biological activities such as antibacterial and antioxidant properties [19]. It is widely used in many fields such as printing, dyeing, leather chemical industry, biomedicine, and so on. Because it has three hydroxyl groups and one carboxyl group, it was believed to be suitable for preparing functional polymer materials [20; 21; 22].  
In this work, GA was employed to functionalize the FK. In the reaction, EDC-NHS was used as the mild and non-toxic activation system [23]. The GA could be bonded to FK through amide bonds. At the same time, the amide bonds between FK chains could also be formed, and the covalent disulfide bonds (-S-S) between FK chains would be formed again. Thus, GA-modified FK gel (GA-FK gel) was prepared. The obtained GA-FK gel was characterized by IR, SEM, TGA, and XRD methods. And GA-FK gel was applied to remove the Fe(III) in aqueous solution, the adsorption properties are investigated in detail. It is expected that FK-based functional environment-friendly polymer materials will be developed, to realize the efficient recycling and high-value utilization of discarded biomass.