Academic Journal of Science and Technology ISSN: 2771-3032 | Vol. 7, No. 2, 2023 31 Application Potential of Layered Double Hydroxides for The Treatment of Persistent Organic Pollutants Tingjuan Feng, Xiao Liu and Yaze Zhi North China University of Science and Technology, Hebei 063210, China Abstract: Layered double hydroxides (LDHs) is a widely used emerging material. With its adjustable composition, other ions or materials can be incorporated on the surface or in the layer to synthesize modified materials with stronger ability to capture target pollutants. Persistent organic pollutants (POPs) exist in air, water and soil for a long time, which not only affect the ecosystem and ecological balance, but also endanger human health. Therefore, it is of great significance to study the application potential of LDHs in the treatment of POPs. The removal mechanism of persistent organic pollutants by LDHs includes adsorption and activated persulfate oxidation. The factors affecting the removal of POPs by LDHs include the characteristics of LDHs itself (including the inherent characteristics of composition, structure, morphology, etc.), coexisting substrates, temperature, etc. Keywords: Layered double hydroxides (LDHs), Persistent organic pollutants (POPs), Modified LDHs, Removal mechanism. 1. Introduction The industrial revolution has changed human life, but also brought some potential hazards. One of the hazards is the production of a large number of chemical pollutants. These pollutants include organic matter, inorganic matter, persistent organic pollutants and heavy metals. Some of these chemicals, known as persistent organic pollutants (POPs), can exist in the environment for a long time due to their semi-volatility[1]. Typical POPs include pesticides, industrial chemicals such as polychlorinated biphenyls (PCBs), perfluorinated compounds (PFCs), bromides, and industrial by-products such as dioxins and furans. Persistent organic pollutants not only affect our ecosystems and ecological balance, but also endanger human health[2].Most persistent organic pollutants exist in the air, water and soil for a long time, and are easily ingested by organisms at the bottom of the food chain[3].And many persistent organic pollutants are fat-soluble substances, which are easily accumulated in adipose tissue and transmitted to the human body through the food chain. Moreover, POPs can also be transferred to the fetus through the pregnancy process.Bjrvang R D[4] measured the concentrations of 9 organochlorine pesticides (OCPs), 10 polychlorinated biphenyl (PCB) congeners and polybrominated diphenyl ether (PBDE) congeners in serum, placenta and fetal tissues (adipose tissue, liver, heart, lung and brain) of 20 stillbirth pregnant women (36-41 weeks of gestation). The results showed that all 22 persistent organic pollutants were detected in all fetal adipose tissue samples. Therefore, it is imperative to study an environmentally friendly and simple method for removing POPs. The existing methods for the treatment of POPs include adsorption, advanced oxidation, bioremediation and physicochemical-biological combination. Among them, the use of layered double hydroxides ( LDHs ) to remove persistent organic pollutants has received extensive attention. Under the background of ' double carbon ', how to treat pollutants with high efficiency and low carbon has become an important issue. LDHs have simple preparation and adjustable composition and structure, which can effectively deal with a variety of POPs. The purpose of this paper is to summarize the properties and synthesis of LDHs, the mechanism of removing POPs in the environment and the factors affecting the removal effect. The application potential of LDHs in the treatment of persistent organic pollutants was pointed out. 2. The Properties and Synthesis of Layered Double Hydroxides (LDHs) 2.1. Properties of LDHs LDHs (Layered Double Hydroxide) is a general term for hydrotalcite and hydrotalcite-like compounds. The general expression is [M1−x 2+Mx 3+ (OH)2]x +(Ax/n n−).yH2O[5].x represents the molar ratio of M3+/(M2++M3+), n is the valence state of interlayer anions, and y is the number of interlayer water molecules. LDHs are composed of multiple MO6 octahedra closely arranged to form a laminate. The interlayer anions and water molecules are sandwiched between the two layers, and the interlayer anions are connected by non- covalent bonds. Part of the divalent metal cation M2+ is replaced by the trivalent metal cation M3+, and the excess positive charge is neutralized by the interlayer anion, and the whole is electrically neutral[6]. The physical and chemical properties of LDHs are very similar to those of clay materials, and they are called ' anionic clay ' because of their anion exchange properties. The main properties of LDHs include : 1 ) Acid-base bifunctionality : LDHs are usually alkaline because the laminate contains abundant hydroxyl groups. Its alkalinity depends on the type of metal ions. LDHs are also acidic due to trivalent metal hydroxides and interlayer anions ; 2 ) Interlayer ion exchangeability : The structural characteristics of LDHs enable the exchange of interlayer ion energy with higher valence anions ; 3 ) Thermal stability : Due to the strong covalent bond and electrostatic interaction, it shows thermal stability in a certain temperature range. Taking Mg-Al-LDH as an example, when it is higher than 200℃ in the air, it will cause structural changes ; 4 ) Memory effect : Under certain 32 conditions, when the oxide obtained by the thermal decomposition of hydrotalcite is immersed in a solution containing anions, its layer structure will be reconstructed and LDHs will be produced again. 2.2. Preparation of LDHs The structure of LDHs contains layered double hydroxides and intercalated anions. According to the different intercalation anion addition methods, LDHs can be divided into primary LDHs and modified LDHs. Among them, the primary LDHs are intercalated with inorganic anions ( such as NO3 -, Cl-, CO3 2- ) in the synthetic mother liquor, and the modified LDHs introduce other metals or replace the interlayer anions with other anions in the laminate of the primary LDHs. 2.2.1. Synthesis of primary LDHs The preparation methods of primary LDHs include co- precipitation method, hydrothermal method, sol-gel method and mechanical grinding method. Among them, the co- precipitation method is the most widely used[7]. 1) Co-precipitation method: Co-precipitation method refers to the reaction of mixed metal ion solution and alkali solution, stirring until the precipitation reaction occurs, let the product aging for a period of time, and then filtering, cleaning, drying method. 2) Hydrothermal method: Hydrothermal method is to mix metal salts and urea in an autoclave. After mixing the chemical substances in the solution, they are loaded into the reactor, and the reaction is accelerated under high temperature and high pressure conditions. After cooling, the solution is filtered and dried. 3) Sol-gel method: Sol-gel method refers to the metal organic or inorganic compounds through solution, sol, gel and curing, and then by heat treatment and the formation of oxide or other compound solid method. 4) Induced hydrolysis method: Under certain temperature and pH conditions, the hydroxide solution containing MIII and the salt solution containing MII with the same pH were first prepared, and the two were mixed to induce the hydrolysis of MII to form LDHs precipitation. 5) Mechanical grinding method: the synthetic hydrotalcite raw material metal salt and alkali into high-energy ball mill for high-speed grinding, high-energy mechanical force induced physical and chemical and structural changes in the generation of LDHs method. 2.2.2. 2.2.2 Synthesis of modified LDHs With the in-depth study of layered double hydroxides (LDHs), the advantages of low cost, non-toxicity and structure of LDHs make it widely used in flame retardant, medicine, sewage treatment and other fields. However, the traditional LDHs have low specific surface area, less functional groups, and easy leaching of metal ions to produce secondary pollution, and the adsorption and catalytic effects are not ideal. In order to achieve a more comprehensive demand, researchers are committed to improving the performance of LDHs by transition metal modification, surface modification, anion intercalation and other methods. 1) Transition metal modification Transition metals are usually used as components of LDHs cation layers to activate persulfate. However, the ineffective redox from M(n+1)+ to Mn+ leads to a decrease in catalytic performance. By introducing another transition metal through modification, there is electron transfer between different metals, which can accelerate the original cycle. Zhang[8]prepared transition metal modified Mg-Al-M hydrotalcite by means of hydrotalcite 's ' memory effect '. It is used to catalyze urea and phenol to synthesize salicylamide. The results showed that CHT-Mn had the best catalytic effect, and the synthesis rate was as high as 41.2 %. After repeated use for several times, the catalytic activity remained basically unchanged. It can be seen that the introduction of external metals can also improve the stability of heterogeneous catalysts. 2) Surface modification Surface modification is a common method to change the surface function. Zhang[9]modified MgAl-LDH with anionic surfactants to prepare organic LDHs. The intercalation of anionic surfactants changed the surface properties of MgAl- LDH from hydrophilic to hydrophobic. Surfactants were not only embedded in the interlayer space, but also adsorbed on the outer surface of LDH particles. The interlayer space and outer surface of LDHs are modified by surfactants, which realizes the dual effects of electrostatic interaction and intermolecular interaction, and improves the adsorption of dyes. The experimental data show that the modified Mg-Al- LDHs can be used as a broad-spectrum adsorbent to remove anionic, non-ionic and cationic dyes. 3) Calcination modification The calcination method can also modify LDHs. The main principle is that the calcined LDHs increase many oxygen- containing functional groups, increase the specific surface area, and provide more active sites for adsorption. Huang[10] showed that the surface free energy of LDH film was reduced and some interlayer anions were removed by calcination, which improved the stability and permeability of LDH film. In addition, the performance of LDHs as adsorbent[11], photocatalyst[12] and steel corrosion inhibitor[13]was improved after calcination. 4) Anion intercalation modification Benefiting from the high anion exchange capacity, anion intercalation is also one of the methods to improve the performance of hydrotalcite-like materials[14]. The intercalation of inorganic anions can usually be achieved directly by co-precipitation method. However, some organic anion intercalation reactions are not easy to be obtained directly by co-precipitation method. The precursor can be prepared by co-precipitation method, and then the interlayer anion exchange is carried out to obtain the ideal product[15]. Zhang[16] synthesized L-cysteine intercalated LDHs adsorbent by intercalation modification and used for the removal of heavy metals (Pb ( II ), Cu ( II ) and Cd ( II ) ) in water. Through experiments and characterization, it was found that the interlayer of LDH was successfully inserted, and the functional groups of thiol ( -SH ), amino ( -NH2 ) and carboxyl ( -COOH ) were introduced, which provided a large number of adsorption sites for the removal of heavy metals. 5) Modification of composite materials LDHs allow the incorporation of other materials on the surface or in the layer to form a composite material that can selectively capture the target pollutants, which can improve its performance to a certain extent. Periyasamy[17] synthesized chitosan / hydrotalcite composites by hydrothermal and in-situ precipitation methods for the treatment of chromium in farmland. The results showed that the adsorption effect of the composite on chromium was 33 higher than that of a single component, reaching 35.75 mg / g. HTLcs-SBA-15-x composites were synthesized by co- precipitation method using mesoporous material SBA-15 as carrier[18]. When the amount of hydrotalcite is 40wt %, the adsorption effect of CO2 is better. The composite material not only has a complete mesoporous structure, but also has layered HTLcs in the pores, and the increase of specific surface area greatly improves the adsorption effect.  3. The Mechanism of LDHs Treatment of Pops 3.1. Adsorption mechanism Adsorption treatment of small molecule pops is a simpler method. The adsorption mechanism of LDHs includes ion exchange, surface complexation, precipitation and electrostatic interaction. Generally, the adsorption process has three steps : external diffusion, intraparticle diffusion and chemical adsorption. Hu[19] studied the adsorption mechanism of perfluorooctanoic acid and found that when the initial concentration of perfluorooctanoic acid was low, intraparticle diffusion was the only factor restricting the adsorption rate. Increasing the initial concentration to a higher level, both external diffusion and intraparticle diffusion become steps that limit the adsorption rate. Others have found that the adsorption effect is related to the hydrophobicity of the adsorbed material[20]. 3.2. Oxidation mechanism of activated persulfate Sulfate-based advanced oxidation processes ( SR AOPs ) have attracted much attention in the degradation of refractory organic pollutants. Oxidants with peroxy or peroxy groups, such as H2O2, peroxymonosulfate ( PMS ), peroxydisulfate ( PS ), etc., usually form free radicals under certain conditions, which are used to oxidize and degrade pollutants[21]. However, H2O2 as a free radical precursor has many disadvantages, such as short life, narrow working PH range and high cost. Persulfate AOPs have become a more popular pollutant removal method due to their advantages of strong oxidation, high free radical generation rate, diverse activation methods, and low reaction conditions. According to previous studies, there are free radical and non-free radical mechanisms for persulfate activation to eliminate pollutants. 3.2.1. Free radical mechanism The free radicals in persulfate oxidation are mainly SO4•−, ∙OH and •O2‾, and their production requires energy and electron transfer reactions to split persulfate bonds. LDHs / LDO and its composites can provide sufficient active sites to activate persulfate, promote the cleavage of peroxide bonds and the production of free radicals. The existence of active sites is usually related to transition metals and their oxides, surface hydroxyl, carbonyl ( C = O ), defect structure, and sp2 hybrid carbon. In other words, the controllability of the composition of LDHs complexes can increase the active sites, thereby improving the activation ability. The activation mechanism of transition metals and their oxides and surface hydroxyl active sites is discussed below : (1) The activation mechanism of transition metals and their oxides can be described as the following equation[22]: S2O8 2-+Mn+→M(n+1)++SO4·-+SO4 2- (1) HSO5 -+Mn+→M(n+1)++SO4·-+OH- (2) M(n+1)++HSO5−→Mn++SO5•−+H+ (3) In addition, the catalytic performance of single metal LDHs is low due to the accumulation of metal ions in high valence state. The synergistic effect between metal ions of multi- metal LDHs is helpful to solve this problem. (2) Surface hydroxyl groups The surface hydroxyl group reacts with persulfate to cleave the O-O bond to generate free radicals. 3.2.2. Non-free radical mechanism It was found that some SR AOPs did not play a role in the removal of pollutants through SO4 • − and · OH, mainly non-radical mechanisms such as singlet oxygen, mediated electron transfer and direct oxidation. (1) Singlet oxygen Studies have shown that there are usually two ways to produce 1O2. One is produced by the self-decay of PMS, and the other is produced by photochemical processes. The self- decay process is usually inefficient and cannot be used as the main way[23].In addition, carbonaceous materials can catalyze persulfate to produce 1O2, and some carbon materials composite hydrotalcite can produce 1O2 through this way, thereby removing target pollutants. (2) Mediated electron transfer Under this mechanism, the catalyst usually acts as an electron transfer conductor to promote the transfer of electrons from pollutants to persulfate[24].These catalysts usually have good electrical conductivity and can form charge transfer complexes with persulfate for organic oxidation. 4. Factors Affecting the Treatment Effect 4.1. Characteristics of hydrotalcite itself The characteristics of hydrotalcite itself include composition (metal ions, interlayer anions, functional materials), structure, morphology and other inherent characteristics. These characteristics determine the density of active sites that play a role, which in turn affects the removal efficiency of pollutants. The morphology of LDHs produced by different synthesis methods is different. Huang[25] synthesized Zn / Al-LDHs by ion exchange method, reconstruction method and one-pot method, and the morphology showed characteristic sheet, layered and irregular plate structure, respectively. The methyl orange adsorption test was carried out on the hydrotalcite prepared by three synthesis methods. The results showed that the one- pot method had the largest adsorption capacity. The magnetic material mixed with hydrotalcite can improve its reusability. For example, Wang[26] doped Fe2O3 magnetic material to synthesize magnetic flower-like hydrotalcite-like materials. By testing its reusability, it was found that after 5 cycles of adsorption, the fluoride removal performance of the adsorbent was still maintained at 80.93 % of the fresh adsorbent. The removal effect of LDHs pollutants doped with magnetic materials is significantly improved. 34 4.2. Coexisting substrates In polluted water bodies, in addition to the target pollutants to be removed, there are usually some coexisting substrates. These coexisting substrates mainly include inorganic anions and natural organic matter (NOM). Inorganic anions (such as NO3 −, HCO3 −, Cl-, PO4 3- ) often exist in water even at low concentrations. They can reduce catalyst performance and affect pollutant degradation by buffering solution pH, capturing ROS, and neutralizing electrostatic bonds between reactants. Xiao[27] pointed out that anions such as Cl-, HCO3 − and HPO4 2-inhibited MNZ degradation. There are two reasons for this phenomenon : one is the consumption of free radicals ( • OH and SO4 • - ) by these coexisting ions, and the other is that they increase the pH of the solution.Xu[28] prepared nano-zero-valent iron-loaded peanut shell biochar to degrade emerging pollutants PPCPs in water, and investigated the effect of inorganic anions on the degradation system. It was found that inorganic anions inhibited the degradation of tetracycline, and the inhibition intensity of inorganic anions on TC removal was HCO3 ->Cl->HPO4 2->SO4 2->NO3 -. NOM is a complex organic chemical substance that naturally exists in all water bodies. The content and characteristics of NOM in different water sources are very different. However, it can be determined that NOM interacts with ROS, thereby reducing the removal rate of target pollutants.  4.3. Temperature According to the principle of thermodynamics, the increase of temperature can improve the reaction kinetics[29]. In the adsorption process, as the temperature increases, the entropy value in the system increases, the chance of collision between LDHs and target pollutant molecules increases, and the utilization rate of adsorption sites also increases. When exploring the effect of temperature on the adsorption of tetracycline, it was found that the adsorption curve with temperature as the abscissa and the adsorption amount as the ordinate showed a trend of increasing first and then stabilizing[30]. For the activated persulfate oxidation system, high temperature may lead to the thermal activation of PMS or PDS, thereby increasing the removal rate. However, high temperature may bring some negative effects, such as the damage of the carrier medium and the decomposition of the catalyst, resulting in an increase in metal leaching rate and a decrease in reusability. Therefore, combining the properties of the carrier and the catalyst, moderately increasing the degradation temperature is helpful to improve the reaction efficiency. 5. Summary As an ' anionic clay ', layered double hydroxides can be used to synthesize new modified materials that can selectively capture target pollutants through some modification methods such as anion intercalation, insertion of transition metals and addition of composite materials. Studies have shown that the removal mechanism of persistent organic pollutants by LDHs includes adsorption and activation of persulfate oxidation. For small molecule POPs, adsorption method is generally used to remove them. For macromolecular POPs that are difficult to remove completely, LDHs are usually used as activators to activate persulfate and remove pollutants. Among them, the catalytic activation persulfate oxidation system includes free radical pathway and non-free radical pathway. 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