Application And Mechanism Of Green Phosphorus-Free Scale Inhibitor In Circulating Water System

In recent years,to alleviate the shortage of water resources,circulating cooling water system is widely employed in industry.However,there are usually many dissolved calcium salts in the circulating cooling water,which form calcium scales on equipment surfaces with increasing cycles,leading to a major challenge.Currently,adding scale inhibitors is one of the most economical and effective methods to solve this problem.Although most phosphorus-containing scale inhibitors exhibit excellent scale inhibition performance,the use of them can also cause the eutrophication of water bodies and destroy the ecological environment.Therefore,developing a new green,phosphorus-free and effective scale inhibitor is an urgent task.Based on the principle of free radical polymerization,this thesis used itaconic acid（IA）and2-acrylamido-2-methylpropane sulfonic acid（AMPS）as functional monomers to modify valonia tannin extract（VTE）,successfully synthesizing a novel green,phosphorus-free modified valonia extract scale inhibitor（MVTE）.The effect of synthesis process and water conditions on the scale inhibition performance of MVTE was investigated using static scale inhibition experiments.The electron microscope（SEM）,X-ray diffraction（XRD）and X-ray photoelectron spectroscopy（XPS）were applied to analyze the crystal morphology,structure and surface elementary composition of Ca CO₃scales,respectively.In addition,molecular dynamics（MD）simulation and quantum chemistry（QC）techniques were utilized to further explore the scale inhibition mechanism,providing a theoretical basis for the development of new green,phosphorus-free scale inhibitors.The main research findings of this thesis are as follows:（1）The single factor method was conducted to optimize reaction conditions of MVTE and its scale inhibition performance on Ca CO₃scales under different water quality conditions was investigated.Results show that MVTE exhibits the best scale inhibition performance when the valonia dosage is 2.5 g,the initiator dosage is 6 wt.%of the total mass of monomers,the reaction temperature is 75℃and the reaction time is 3.5 h.At p H 7,the scale inhibition efficiency of MVTE for Ca CO₃is 98.2%at a concentration of 35 mg/L,which is 36.5%higher than that of VTE.Therefore,the introduction of carboxyl,sulfonic acid and amide groups through modification can increase the active binding sites of MVTE molecules to Ca²⁺and enhance the ability of MVTE to bind to Ca²⁺,thereby better inhibiting the formation of calcium scales and significantly improving the scale inhibition performance of MVTE on Ca CO₃scales.In addition,sulfonic and amide groups greatly improve the salinity and temperature resistance of MVTE,which is crucial for its application in circulating cooling water systems.（2）The scale inhibition performance of MVTE on Ca CO₃scales was used to study using the electrochemical evaluation method.The results indicate that the scale inhibition efficiency of MVTE increases with increasing concentration,up to the maximum value at 35 mg/L.This is consistent with the result obtained in the static scale inhibition experiment,suggesting that using the residual current density（i_r）to evaluate the scale inhibition efficiency of scale inhibitor is feasible and can provide an efficient method for the evaluation of scale inhibition performance of scale inhibitors.（3）The FT-IR results show that AMPS,IA,and VTE successfully participated in the free radical polymerization reaction.The SEM results indicate that after the addition of MVTE,the Ca CO₃scale crystals transform from large-sized cubic calcite and needle-like aragonite into smaller and looser spherical vaterite crystals.The XRD results show that the main action surfaces of MVTE on Ca CO₃scale crystals are the（122）,（221）and（102）planes of aragonite.The XPS results reveal that the carboxyl,sulfonic and amide groups in MVTE can effectively bind with Ca²⁺and adsorb onto the surface of Ca CO₃scales,thereby retarding their normal growth.Therefore,MVTE can hinder the effective collision between Ca²⁺and CO₃^2-,inhibit the formation of calcium scale nuclei and produce electrostatic repulsion on the surface of the Ca CO₃scale crystals or occupy the active sites of calcium scale crystal planes,leading to lattice distortion of the Ca CO₃scale crystals.These effects result in an excellent scale inhibition performance of MVTE.（4）Through molecular dynamics simulation,the interaction between MVTE and the crystal surfaces of calcite and aragonite was explored.The results show that MVTE can overcome its own deformation energy and adsorb onto the surface of Ca CO₃scale crystals,inhibiting the growth of them.After introducing carboxyl,sulfonic acid and amide groups,the binding energies between MVTE and Ca CO₃scales are higher than that of VTE,demonstrating better scale inhibition performance,which consistent with the result of the static scale inhibition experiment.Furthermore,the binding energy of MVTE on the aragonite（122）surface is greater than that on the calcite（104）crystal surface,indicating that MVTE exhibits a better inhibitory effect on the growth of aragonite crystals,which consistent with the result of XRD.RDF results further show that the interaction energy between MVTE and Ca CO₃scale crystals mainly originated from the ionic bonds formed between the O atoms in MVTE and the Ca²⁺ions in Ca CO₃scale crystals,as well as the hydrogen bonds formed between the H atoms in MVTE and the O atoms in Ca CO₃scale crystals.Therefore,through molecular dynamic simulations,the scale inhibition performance of scale inhibitors on Ca CO₃scales can be accurately and rapidly predicted.（5）Through quantum chemical calculations and analysis of the molecular structure of MVTE,the results indicate that the N and O atoms in the MVTE molecule have a higher electronegativity and can provide multiple active sites for strong electrostatic interaction with Ca²⁺,thus delaying the formation of calcium scale crystals.At the same time,the distances between the N and O atoms in the MVTE molecule and the closest calcium ion pairs on the crystal surfaces of aragonite（122）and calcite（104）are similar,which can significantly enhance the adsorption of MVTE on specific crystal surfaces and effectively inhibit the normal growth of calcium scales,thereby improving its scale inhibition performance.The f_k^-values of O84 and N24 in the amide groups of MVTE are the largest,and they preferentially bind with Ca²⁺to form coordination bonds,which endows MVTE with good scale inhibition properties.In addition,compared with VTE,the energy of the highest occupied molecular orbital（HOMO）of MVTE is higher,making it easier for MVTE to provide electrons to Ca²⁺,and the energy gap of MVTE is smaller,making it more active and easier to form a coordination bond with Ca²⁺,thereby effectively hindering the deposit of calcium scales.Therefore,quantum chemical technology can be used for precise calculation of molecular spatial geometry and electronic distribution information,predicting the relationship between the molecular structure of scale inhibitor and their scale inhibition performance.