Functionalization Of Tannic Acid(TA)-3-aminopropyltriethoxysilane(APTES) Coating And Its Application In Water Treatment

In recent years,surface/interface modification based on mussel bionics has been widely applied in the field of water treatment.Recently,our research group developed tannic acid(TA)-3-aminopropyltriethoxysilane(APTES)coating(TA-APTES coating),which has excellent adhesion property similar to that of polydopamine(PDA),and rich micro-nano structure,which is conducive to the preparation of functional materials with excellent properties.In addition,TA and APTES are cheap and conducive to industrial applications.On the basis of the above,the secondary reactivity,stability and expansibility of TA-APTES coating in water treatment were further studied.The secondary reactivity of the coating is of great importance to the extensibility of the coating.Therefore,we first investigated the secondary reaction activity of the coating,and on this basis prepared a variety of functional materials for water treatment:adsorption materials,catalytic materials,oil-water separation materials.The results show that TA-APTES coating has a large amount of phenolic hydroxyl and a small amount of amino,and has secondary reactivity similar to PDA coating,which provides a good platform for further functionalization of the coating and preparation of various functional materials.PDA coating was selected as the comparison,and the same secondary modification method and steps were used to prepare the functional materials of TA-APTES coating and PDA coating.Research shows that:under the same conditions,based on the TA-APTES coating the function of the material performance is far better than that of PDA coating materials,concrete comparative results are as follows:(1)Through the TA-APTES coating and contains a lot of amino polyethylene imine reaction of adsorption material of rosy,methylene blue,orange and yellow G,amaranth compared to the same after processing of PDA adsorption material performance respectively by 80.1%,96.2%,86.1%,98.6%;(2)The silver-carrying nanoparticle catalytic material prepared by in-situ reduction of silver ions based on TA-APTES coating can convert 4-nitrophenol to 4-aminophenol in 10 min,with an efficiency of up to 98%,while the PDA based catalytic material downloaded silver under the same conditions can only achieve 60%conversion;(3)After TA-APTES coating reacts with octadecyltrimethoxysilane(ODS),it can be directly converted into super hydrophobic coating(water contact angle is more than 150°,and rolling angle is less than 10°),which can realize efficient separation of oil and water mixture.However,PDA coating after the same treatment does not have super hydrophobicity(water contact angle is only about 100°),so it cannot separate oil and water mixture well.Further research and analysis found that the rough surface composed of a large number of nano-spheres unique to TA-APTES coating provided more reaction interfaces for the adsorption and catalytic processes,while the large number of micro-nano structures caused by the rough surface provided a prerequisite for the preparation of super-hydrophobic surface,and finally gave the functional materials obtained more excellent properties.The stability of coating is crucial to the practical application of coating,so we further studied the stability of TA-APTES coating.TA-APTES coating will lose its superhydrophilic and underwater superhydrophobic after being exposed to air for a period of time(30 days or so);In addition,in acidic solutions(pH<3)or alkaline solutions(pH>12),TA-APTES coating will gradually decompose.Through a series of characterization,the reasons for the poor stability of TA-APTES coating were revealed:(1)When TA-APTES coating was placed in the air for a long time,the phenolic hydroxyl group on the coating surface was gradually oxidized to quinone,leading to the decrease of coating hydrophilicity and the loss of superhydrophilic and underwater superhydrophobic;(2)When TA-APTES coating is in an acidic environment,the amino in the coating is protonated,and with positive charge,with the decrease of pH value,the degree of protonation is more serious,leading to the increase of electrostatic repulsion inside the coating,and eventually overcome the intermolecular forces and hydrogen bonds inside the coating,leading to the decomposition of the coating;Similarly,when the coating is in an alkaline environment,part of the phenolic hydroxyl group in the coating will be deprotonated,resulting in negative charge and electrostatic repulsion.With the enhancement of alkalinity and deprotonation,electrostatic repulsion will be enhanced,and finally decomposition will overcome the intermolecular force and hydrogen bond inside the coating.Based on the above findings,we proposed an effective strategy to improve the stability of TA-APTES coating by using ferric ion treatment.Ferric ions can and coating of phenolic hydroxyl and amine complex,effectively inhibit the oxidation of phenolic hydroxyl group,reduce the protonation of amino in acidic environment and the deprotonation of phenolic hydroxyl in alkaline environment,ultimately significantly improve the stability of coating in air and acid and alkali solution:after dealing with the ferric iron ion coating exposed in air 1 month or in a strongly acidic or alkaline solution immerse 48 hours,still can maintain its excellent oil pollution prevention performance and good oil-water separation performance,and the characteristic of the micro-nano structure will not be damaged.We further use(TA-APTES)-Fe(Ⅲ)to transform PVDF film,fibrous nylon,sponge,cotton,wood,filter paper and other materials into photothermal composite materials for solar seawater desalination.Taking(TA-APTES)-Fe(Ⅲ)coated sponge as an example:the composite photothermal material has extremely high light absorption characteristics(>90%)in the wavelength range of 250-2500nm.The evaporation capacity of pure water under one solar light intensity(1kwm-2)can reach 1.43 kg m-2 h-1,and the photothermal conversion efficiency reaches 87%.We investigated its desalination capability of simulated seawater and the purity of condensed water collected exceeded the international standard for drinking water.In addition,the(TA-APTES)-Fe(Ⅲ)coating can also give photothermal materials excellent anti-oil pollution properties,so that it can maintain a stable water evaporation efficiency when treating oily wastewater.In addition,the photothermal materials obtained by TA-APTES-Fe(Ⅲ)coating have strong stability,acid and alkali resistance,washing resistance and circulation freeze-thaw resistance.More importantly,(TA-APTES)-Fe(Ⅲ)coating has excellent universality(can be loaded on various material surfaces),facilitating the structural design of photothermal materials,thus further improving the water evaporation efficiency.We used the coating to prepare wood-based photothermal materials,and by designing different structures,the evaporation rate of their water increased from 1.43 kg m-2 h-1 to 1.8 kg m-2 h-1(under one solar intensity),which is much higher than the properties of wood-based photothermal materials reported so far.