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Principle, Application and Development Prospect of Heavy Metal Removal Technolog

n recent years, plant extracts have been widely developed and utilized, but the problem of heavy metal pollution has endangered its safe application. Therefore, the effective removal of heavy metal pollution from plant extracts has become an urgent problem to be solved and a hot research topic at home and abroad. At present, the widely used heavy metal removal technologies include flocculation precipitation method, adsorption method, molecular sieve method, etc. Other new technologies and materials are gradually applied to remove heavy metals from plant extracts, such as microbiological method, nanotechnology, ion/molecular imprinting technology, bionic materials, etc. The principle and application of heavy metal removal technology in plant extracts in recent years were reviewed, the characteristics of each method were summarized, and the development trend and prospect of heavy metal removal technology in plant extracts were prospected. Heavy metals refer to metal elements with elemental density greater than 4,500 kg/m3 under standard conditions. Common heavy metal elements include lead (Pb), mercury (Hg), cadmium (Cd), chromium (Cr), arsenic (As), silver (Ag), copper (Cu), gold (Au), iron (Fe), etc. Because heavy metals cannot be biodegraded in the environment and can be enriched in organisms through food chains, they can even be converted into more toxic chemical forms [1-2]. Therefore, heavy metal ions will interfere with and damage the normal physiological and metabolic functions of the organism to varying degrees after entering the organism, thus causing the organism to show poisoning and even death in severe cases [3-6]. It is generally believed that heavy metal ions enter the organism and interact with the active sites or some inactive sites of macromolecular substances such as proteins, nucleic acids and enzymes in the organism, thus affecting the normal physiological functions of biological macromolecules [7]. Plant extract refers to the active components (such as phenolic acid, flavone, essential oil, alkaloid, polysaccharide, etc.) [8] obtained from plants (all or part of plants) through extraction, separation and purification by certain solvents and methods. It has the advantages of stable content, no pollution, no residue, easy absorption, etc. [9]. At present, plant extracts have been widely used in food additives, food preservation, aquaculture, pesticide, medicine and other fields. Heavy metals in plant extracts mainly come from plant raw materials [10-11]: ① raw material growth environment, including soil, industrial "three wastes" pollution, pesticide and fertilizer pollution, etc. (2) Plant's own genetic characteristics, and some plants have enrichment effect on heavy metals; (3) Heavy metal pollution during collection, transportation and storage of plant raw materials. The control and removal of heavy metal pollution from plant extracts are of great significance for their further development and utilization. 1 Removal of Heavy Metals from Plant Extracts during Processing The research found that the extraction method, extraction solvent, extraction time, extraction times, extraction temperature, solid-liquid ratio, etc. of plant extract in the extraction process will affect its heavy metal content. Panlan et al. [12] extracted active components of Glycyrrhiza uralensis with 80% ethanol and compared the changes of heavy metal content before and after extraction. The results showed that the changes of heavy metal content were not significant, indicating that less heavy metal was dissolved out by ethanol. Supercritical fluid extraction technology is a new extraction technology formed by combining metal matching reaction with supercritical fluid extraction technology. Before extracting effective components from plants, supercritical extraction technology is used to purify plant raw materials, which can effectively reduce the content of heavy metals in the extract. Zhang Huifen and others [13] used supercritical CO2 extraction technology to purify heavy metals in Rehmannia glutinosa and Epimedium brevicornum. The content of heavy metals after purification was lower than relevant standards, and the relative content loss of effective components after purification was less than 5%. Ye Feifei et al. [14] used supercritical extraction technology to extract coix seed bran oil from coix seed bran. The yield of coix seed bran oil was 24%, and the content of heavy metals such as lead, arsenic and mercury was very low, which indicated that the method could obtain effective components in coix seed bran and reduce the extraction of heavy metals. 2 Removal Technology of Heavy Metals from Plant Extracts 2.1 flocculation precipitation method Flocculation precipitation method refers to adding flocculant to the plant extract solution to be treated, and the metal oxide generated after hydrolysis generates electrostatic binding effect on dissolved heavy metal ions, so as to reduce its concentration in the solution, and at the same time form easy-to-precipitate flocs with large density and volume, so as to precipitate them from the solution [15]. The mechanism mainly includes four theories: compression of electric double layer, electric neutralization, adsorption bridging and precipitation netting [16]. There are many kinds of flocculants, including chitosan, adsorption resin, natural cellulose, tannic acid, gelatin, etc. Chitosan has been widely used to remove heavy metals from plant extracts due to its wide source, low price, good biocompatibility and degradability. Wu Meiyuan et al [17] used chitosan to remove heavy metals from Hericium erinaceus polysaccharide extract. The results showed that when the chitosan concentration was 5 g/mL, the removal rate of Pb and As in polysaccharide exceeded 90%, and the removal rate of Hg and Cd exceeded 80%. Ren Yong et al [18] used EDTA modified chitosan magnetic adsorbent to remove low concentration of heavy metals in Angelica sinensis extract. The results showed that it had high removal rate of Cu, Cd, Pb and so on in low concentration in the extract, and had little effect on its pharmacodynamic components. Studies by Cheng Hongxia et al. [19] and Pan Yufang et al. [20] found that chitosan has a certain adsorption capacity for heavy metals in Chinese medicine extracts, and can be used for removing heavy metals from Chinese medicine extracts. Luo xuan [21] prepared a kind of methyl methacrylate grafted crosslinked chitosan microsphere through the reaction of glutaraldehyde crosslinked chitosan microsphere (CCR) and methyl methacrylate (MMA) and was used to remove heavy metals in fruit and vegetable juice. The results showed that the removal rates of Cd2+, Hg2+, Pb2+ were 55.9%, 64.0% and 77.1%, respectively. 2.2 adsorption method The adsorption method utilizes the specific adsorption of heavy metal ions on the surface of the adsorbent to achieve the purpose of removing heavy metals. Adsorption can be divided into physical adsorption and chemical adsorption, physical adsorption through van der Waals force, chemical adsorption through chemical bonds [22]. At present, the commonly used adsorption materials include macroporous resin, ion exchange resin, silica gel, metal adsorbent, etc. 2.2.1 resin adsorption method (1) Macroporous resin adsorption: Macroporous resin is a high molecular polymer with a three-dimensional structure inside and a large pore diameter and specific surface area. It is insoluble in organic solvents such as acid, alkali, ethanol, propane and hydrocarbons, and is stable to oxygen, heat and chemical reagents. The adsorption theory of macroporous resin is mainly divided into adsorption kinetics, adsorption thermodynamics, adsorption structure-activity relationship and adsorption selectivity [23]. Macroporous resin adsorption method is generally applied to adsorption of macromolecular substances, such as phenols, alkaloids, saponins, flavonoids, etc. However, in recent years, some scholars have also found that macroporous adsorption resins can be used to remove heavy metals from plant extracts and have little effect on the effective components. Wang Xianliang et al [24] found that D402 and D401 macroporous resins can be used to remove excessive heavy metals from crude extracts of traditional Chinese medicine, and have little effect on the content of flavonoids. Cheng Xiaoliang et al. [25] compared the removal effect of D751 and D403 macroporous chelating resins on heavy metals in ginkgo biloba extract, which can make the content of heavy metals in the extract lower than the national limit and the loss rate of effective components lower than 6%. Liang Hesheng et al [26] used D751 macroporous adsorption resin to remove Cu, Pb and Cd from Radix Isatidis extract. The results showed that column passing speed, initial concentration of heavy metals, concentration of Radix Isatidis aqueous extract and resin regeneration had little effect on removal of Pb and Cd. Both removal rates were very high, but they had certain effect on removal rate of Cu. The content of main components in Radix Isatidis extract after resin treatment did not change much. Chen Lai Yin et al. [27] invented a method for reducing heavy metals including As and Pb in instant tea by using macroporous adsorption resin and chelating resin. The results showed that the removal rate of heavy metal ions such As As and Pb reached 50%, and the loss rate of tea polyphenols was less than 5%. At present, there are few reports on the direct application of macroporous resins to remove heavy metals from plant extracts. The main reason is that the adsorption effect of macroporous adsorption resins is easy to be affected. Moreover, the types and specifications of adsorption resins are various, which need to be optimized when determining the process conditions. The requirements on the process conditions are higher, the operation is more complicated, and the macroporous adsorption resins are easy to affect the effective components of the extract.

 
(2) Ion exchange resin: Ion exchange resin is a functional polymer material containing ion exchange groups in the cross-linked polymer structure. The adsorption process of heavy metal ions is shown in fig. 1, which can be divided into five main parts [28]: ① a static liquid film where heavy metal ions diffuse to the resin surface; (2) heavy metal ions diffuse to the resin; (3) heavy metal ions diffuse into the resin; (4) performing ion exchange with opposite charges carried by functional groups; ⑤ The exchanged ions diffuse into the solution. Li Zhou et al [29] used cation exchange resin to remove Cd, Pb and Cu from Saviae Miltiorrhizae Radix extract. The results showed that cation exchange resin could remove more than 80% of heavy metals and the recovery rate of effective components was over 90%. Wu Congjun and others [2] used 001×7 strong acid cation exchange resin to adsorb and remove heavy metals in Chinese medicine extract. The results showed that it had obvious removal effect on heavy metals such as Pb, Cd, Cu, Hg, and the recovery rate of effective components in Chinese medicine extract exceeded 90%. Dong Weibing [30] used strong acid cation exchange resin to remove Pb, Cd, Cu and Hg from Se-enriched Salvia miltiorrhiza leaf extract. The removal rate of heavy metals was above 80%. The recovery rates of total phenolic acid, total flavonoids and total tanshinone from Salvia miltiorrhiza were above 90%, and the recovery rate of total selenium was 85.1%. Wei Jixin et al [31] studied the feasibility of using two chelating resins to remove heavy metal ions from Radix Isatidis extract. The results showed that resin A was more suitable for removing heavy metals from Radix Isatidis extract, and the yield loss of dry extract was less than 7%. He Qi [32] invented a polycondensed macroporous beaded weak base ion exchange resin, which can be used to remove divalent heavy metal ions such as Cu2+, Cd2+, Pb2+ from plant extracts. 2.2.2 Silica gel adsorption method Silica gel is a porous amorphous substance, mainly composed of SiO2, which is prepared by reacting sodium silicate with sulfuric acid and performing a series of subsequent treatments. Its molecular formula is MSIO 2N H2O [33]. The original silica gel has small adsorption capacity for heavy metals and poor selectivity, so silica gel needs to be modified and modified to improve its performance [34]. Bonded silica gel with silica gel as carrier and modified by complexing group is widely used for enriching, separating and recovering trace heavy metal elements. The complexing group is connected with the silica gel surface through a carbon chain and generates a certain steric hindrance to prevent the analyte from binding with the silanol group on the silica gel surface; The other end of the complexing group is a ligand atom with the characteristic of adsorbing metal ions and is used for adsorbing heavy metal ions. Among them, the most widely used complexing groups include some simple structure complexing groups containing Ryukyu (-SH) and amino (-NH2), such as Ryukyu propyl, aminopropyl, ethylenediamine propyl, etc. Bonded silica gel containing hydrophobic and amino complexing groups has adsorption effect on heavy metal ions such as Pb2+, Cd2+, Cu2+ and Hg2+, and silica gel containing Ryukyu group also has good selective adsorption ability on arsenite (AsO33+). The application of some bonded silica gel in heavy metal ion enrichment is shown in Table 1[35].