Recent studies on eggshell as adsorption material

Abstract: China is the largest egg production and consumption country in the world, more than 40% of the world's eggs produced in our country every year. In a strong contrast to the large egg production, only limited number of egg was exported to other countries, more than 90 percent of the egg was consumed domesticly, thus there are large quantities of eggshell waste produced by the egg industry, food industry, hatcher, restaurant, canteen and household every year. Eggshell waste is a serious matter in egg industy and will induce environmental pollution , as the residual egg white, which attract rats and other vermin. Disposing eggshell waste can be costly for egg and food industries, thus, recycling and reuse of eggshell waste are essential for the improvement of industry and protection of the environment. There are many reuse and disposal methods for waste eggshells, for example, to produce fertilizer, animal feed ingredients, discarded in municipal dumps and so on. Besides those methods stated above, eggshell has a large potential value in the remediation of polluted environment as a low-cost , easily obtained adsorbent and has received increasing academic attention. We analyzed the relationship between eggshell structure and adsorption, processing techniques , adsorption target , application and influencing factors of the eggshell as an absorbent in the remediation of polluted water and soil. On base of the published papers, we should pay more attention to the potential value of eggshell. The adsorbability of eggshell is on the base of its natural porous structures and high contents of calcium. Through the surface complexation and ion exchange, eggshell can adsorb many metal ions in different valence states, including Cu2+, Zn2+, Fe2+, Fe3+, Pb2+, Cd2+, Cr3+, Cr6+, As3+, As5+, Au+, Au3+, Ag+, Hg2+, Mn2+, Ni2+, U6+, Co2+, Pt2+, Pd2+ and in polluted water and soil matrix. Eggshell has good adsorbability to the organic dyes in aqueous solutions, including congo red, crystal violet, methyl orange, methylene blue, reactive red, malachite green, ethyl orange ,brilliant green direct violet, basic violet and so on. Eggshell is also reported to be able to absorb fluoride, phosphate, lignosulfonate compound, pancreatic lipase, COD, malathion and so on. Much studies focused on the adsorbability of eggshell in aqueous solutions compared with soil matrix. Only a few studies applied eggshell to immobilize heavy metals in the polluted soils. Matrix effects are different, depending on the targets that absorbed by the eggshell. Readers will benefit from this paper, especially in understanding the characteristics of the eggshell adsorbent, thus value the waste eggshell in future. Moreover, several typical cases have been introduced, which studied the processing techniques and modification of eggshell in order to improve the adsorbability of eggshell to different targets. For example, cleaning, grounding, calcination, developing synthetic hydroxyapatite, synthesis of nano-hydroxyapatite, iron oxide coated eggshell powder, synthesis of alumina supported carbon composite and so on. We conducted this work and prepared this paper in the purpose of promoting the study and application of waste eggshell as an adsorbent in the remediation of polluted environment, advancing the resue of waste eggshell.