Study On The Construction Of Amino-functionalized Materials For Adsorption Of Heavy Metals And Their Application In Removal Of Heavy Metals From Tea Polyphenols

Tea polyphenols,the main functional component in tea,has been widely used due to various physiological activities such as good antioxidant,anti-inflammatory,lowering blood lipid,antiviral,anticancer and so on.However,during the planting,processing,storage or transportation of tea,heavy metals(such as Pb,As,Hg,Cd,and Cu)may be enriched and ultimately remained in tea polyphenols,not only causing potential irreversible damage to human health,but also resulting in restrictions and economic losses in sales,especially in international trade,which has brought great obstacles and challenges to the development of tea industry in China.However,there are few studies on the removal of heavy metals in the industrial production of natural plant extracts such as tea polyphenols in China and abroad.In this study,the content and distribution of heavy metals in tea polyphenols were investigated,and a series of efficient,green,safe and environment-friendly amino functionalized materials were developed for the removal of heavy metals in tea polyphenols based on solid phase adsorption technology,and the loss of tea polyphenols caused by the functionalized materials was evaluated,which provided technical support and prevention and control scheme for harm reduction and attenuation of tea polyphenols.The main results are as follows:1.Determination of heavy metals in tea polyphenols and screening of adsorbentsThe contents of harmful heavy metals such as Pb,Cu,Cd,Hg,and As in tea polyphenols and the distribution of heavy metals during the extraction process were determined by wet digestion combined with inductively coupled plasma mass spectrometry.The results showed that more than 23.36% of the heavy metals in tea were residual in tea polyphenols,and the total residual amounts of the five toxic heavy metals in tea polyphenols were in the range of2.61～3.44 mg/kg,indicating a potential heavy metal residual risk.The results of screening the four adsorbents showed that the removal rates of Pb,Cu,Cd,Hg and As from tea polyphenols by silica gel were 43.21%,32.17%,38.55%,28.62% and 0.65%,respectively,and there was almost no adsorption of tea polyphenols with the loss rate of 0.15±0.03%.Therefore,silica gel was selected as the material for the removal of heavy metals from tea polyphenols,but its adsorption capacity for heavy metals in tea polyphenols was small,and its practical application value was low.2.Removal of heavy metals from tea polyphenols by amino modified silica gelIn order to enhance the adsorption capacity and selective adsorption of silica gel,an aminofunctionalized silica gel(AFSG)was successfully synthesized using(3-aminopropyl)triethoxysilane(APTS)as the amino donor and its physicochemical properties were characterized.The adsorption experiments showed that compared with silica gel,the maximum adsorption capacities of AFSG for Pb,Cu and Cd were 140.226,100.753 and 48.032 mg/g,respectively,and the adsorption and desorption rates remained above 90% after repeated adsorption for six times.The AFSG adsorption column was applied for the removal of heavy metals in tea polyphenols,and the results showed that the total removal rates of Pb,Cu and Cd in tea polyphenols were 82.54%～84.19%,90.09%～92.65% and 70.29%～77.15%,respectively,and the loss of tea polyphenols was less than 6.31%.The infrared spectra before and after adsorption were compared,and the results showed that strong chemical bond was formed between the amino group and heavy metal ions on AFSG,thus removing them.3.Removal of heavy metals from tea polyphenols by polylysine modified mesoporous silica gelIn this study,polylysine(PL),a natural polyamino compound,was chemically grafted onto mesoporous silica gel(MSG)with a larger specific surface area to obtain MSG-PL,and its physicochemical properties were characterized.The large surface area and abundant amino groups of MSG-PL provided more functional adsorption sites for heavy metal adsorption.The adsorption experimental results showed that MSG-PL had high removal capacity and repeatability for heavy metals such as Pb,Cu,Cd and Hg,and the adsorption capacities were117.01,132.88,67.32 and 109.27 mg/g,respectively.The results of infrared spectra before and after adsorption showed that the heavy metals mainly formed coordination complexes with amino and amide groups.The MSG-PL column was applied for the removal of heavy metals in tea polyphenols,and the results showed that the removal rates of Pb,Cu,Cd and Hg in tea polyphenols were 80.99%～85.89%,90.82%～94.66%,77.98%～88.24% and 75.91%～80.36%respectively,and the loss of tea polyphenols was small,only 5.09%–7.96%.However,this material has many preparation procedures,complicated process and relatively high cost.4.Removal of heavy metals from tea polyphenols by polylysine/sodium alginate electrostatic self-assembled fiberBased on the consideration of improving the preparation efficiency of the material and the environment-friendly green process,in this study,the cationic polyelectrolyte PL and the anionic polyelectrolyte sodium alginate(SA)were efficiently prepared into SA/PL fibers by drawing in the aqueous solution using the electrostatic self-assembly technology.The characterization results showed that the diameter and mechanical properties of SA/PL fibers could be regulated by controlling the preparation parameters.The results of X-ray photoelectron spectroscopy analysis(XPS)showed that the abundant carboxyl,amino and amide groups in SA/PL fibers could form strong coordination bonds with heavy metal ions,providing a large number of adsorption sites for their adsorption of heavy metals.The adsorption experiments showed that SA/PL fiber had good selective adsorption for Pb,Cu and Cd with the maximum adsorption capacities of 380.05,169.94 and 72.95 mg/g,respectively.After repeated adsorption for 9 times,SA/PL fiber still had good adsorption efficiency(89.46%～99.08%).The removal rates of Pb,Cu and Cd from tea polyphenols by SA/PL fiber were 90.34%～94.29%,94.71%～97.11% and 93.26%～96.67% respectively,and the loss of tea polyphenols was5.84%～9.04%.5.Removal of heavy metals in tea polyphenols by polylysine modified tea residue microcrystalline cellulose double network hydrogelTo fully utilize a large amount of tea residue waste generated in the extraction process of tea polyphenols,tea microcrystalline cellulose(MCC)with smaller particle size was prepared using tea residue cellulose as the raw material,and PL was covalently grafted onto MCC and then crosslinked with N,N-methylene bisacrylamide/acrylic acid to prepare MCC-PL double network hydrogel(MCC-PLH).The physicochemical properties and swelling properties of the hydrogel were characterized.The results of adsorption experiments showed that it had high adsorption capacity and reusability for heavy metals of Pb and Cu,and the maximum adsorption capacity could reach 366.29 mg/g and 195.09 mg/g,respectively.The adsorption kinetics were combined with XPS analysis to clarify the adsorption mechanism.The amino,carboxyl and amide groups on MCC-PLH formed the heavy metal adsorption sites.In addition,the removal rates of Pb and Cu from tea polyphenols by MCC-PLH were 93.03%～96.15% and95.72%～97.11%,respectively,and the loss of tea polyphenols was 8.44%～11.66%.Finally,After treatment with 4 kinds of heavy metal adsorption materials AFSG,MSGPL,SA/PL and MCC-PLH,all tea polyphenol products meet the various indicators under the existing standards.