Mercury emission reduction has become a global consensus due to the physiochemical properties of mercury and its side effects on humans. Adsorption is considered as a potential Hg²⁺ ions removal method. Chitosan is a natural Hg²⁺ ions adsorbent, and its Hg²⁺ ions adsorption capacity and efficiency can be improved by preparation of modified derivatives from physiochemical methods. Hg²⁺ ions adsorption by chitosan and its derivatives is now assumed to occur through several single or mixed interactions:the amino group (-NH₂) and hydroxyl (-OH), which adsorb Hg²⁺ ions mainly through chelate, ion exchange or electrostatic force. The protonated amino group of chitosan and C=N group (Schiff base) of its derivatives may are the main selective functional groups responsible for Hg²⁺ ions adsorption. In this study, we review the research progress of Hg²⁺ ions removal by chitosan and its composites in the field of water treatment, and introduce the means of chitosan physiochemical modification (e.g. freeze drying, electrostatic spinning, crosslinking or grafting) as well as composite mercury removal by new carbon materials (e.g. carbon nanotubes, graphene oxide) in latest research. The removel efficiency and influential factors of Hg²⁺ ions removel by chitosan and its derivatives are discussed in detail. Finally, we discuss the research prospects of chitosan adsorbent materials in treatment of mercury pollution.

Contents
1 Introduction
2 The mechanism of mercury removal by chitosan and its derivatives
2.1 The influencing factors of mercury removal by chitosan and its derivatives
2.2 The mechanism of selective mercury removal by chitosan and its derivatives
3 The study of chitosan and its derivatives removal of mercury
3.1 Physical modification of chitosan
3.2 Chemical modification of chitosan
3.3 Novel chitosan composite sorbents
3.4 The effect of mercury removal by different modification methods
4 Problems and prospects