Studies On The Preparation And Wastewater Treatment Performance Of Modified Alginate Flocculant

Wastewater treatment using flocculant is one of the main ways to solve the current problems of sewage treatment,and it is also a hot spot of green chemistry research.Compared with traditional sewage treatment methods,flocculant technology has many advantages,such as high efficiency,environmental protection,energy saving,easy operation and so on.Natural organic polymeric flocculants are favored for their low cost,no secondary pollution,environmental friendliness,high selectivity and renewability.At present,chitosan,cellulose and starch-based flocculants are the main flocculants reported in literatures.Sodium alginate is a natural anionic polysaccharide polymer,having advantages of renewable,rich resources and environmental friendly.At the same time,therer are a plenty of functional groups on the molecular chains.Sodium alginate is an ideal flocculant for water treatment.However,the study of sodium alginate flocculant is rare at present.In this paper,four new modified sodium alginate flocculants were prepared by modifying sodium alginate with different chemical methods.The products were characterized by infrared spectroscopy,¹H NMR,visible-ultraviolet spectroscopy,X-photoelectron spectroscopy,elemental analysis,thermogravimetric analysis,Zeta potential test and scanning electron microscope.The flocculation performance of flocculants for heavy metal ions,organic pollutants and charged particle pollutant in wastewater was studied.The effects of pH value of wastewater solution,initial concentration of pollutants and dosage of flocculants on the flocculation were investigated.The flocculation mechanism and flocculation kinetics for different pollutants were explored.The main conclusions were as follows:1.A series of new nano-flocculants SADC with different weight increase rates were successfully prepared using sodium alginate?SA?as the base material and dodecylamine?DC?as the modifier.It was found that the amphiphilic nanomicelles with hydrophobic core and hydrophilic shell formed in water solution.The removal rates of Pb²⁺and BPA by the flocculants reached up to 97.20%and 88.60%respectively,showing excellent flocculation ability.The higher the pH value of wastewater,the higher the removal rate of pollutants by SADC;the higher the initial concentration of pollutants in wastewater,the higher the removal rate.The flocculation mechanism of SADC is the chelation of oxygen in carboxylic acid group-COO^-,hydroxyl group-OH with Pb²⁺.But the flocculation mechanism is the solubilization of BPA by nano-micelles.The mechanism of SADC flocculation of Pb²⁺can be described by Langmuir isothermal model equation,while the mechanism of SADC flocculation of BPA can be described by Freundlich isothermal model equation.The kinetic equations of heavy metal Pb²⁺and BPA flocculation by SADC are suitable to be described by quasi-second-order kinetic equation.The new nano-flocculant can simultaneously remove heavy metal ions and organic pollutants from wastewater solution without secondary pollution,and is environmentally friendly.It provides a more potential solution for removing heavy metal ions and organic pollutants in wastewater,and may become a green environmental friendly wastewater treatment agent with good environmental benefits.2.A series of amphoteric sodium alginate flocculants SA-CTA with cationic and anionic groups were successfully prepared by using sodium alginate as the base material and 3-chloro-2-hydroxypropyl trimethyl ammonium chloride?CTA?as reagent by one pot.Cationic groups of-N⁺?CH₃?₃ were introduced into the flocculants.The flocculants have good flocculation properties for heavy metals such as Pb²⁺,Cd²⁺.The flocculation selectivity for mixed metal ions is Hg²⁺>Pb²⁺>Cu²⁺>Zn²⁺.The flocculation mechanism is the chelation of oxygen elements in carboxyl-COO^-,hydroxyl-OH with heavy metal ions.When the pH value of wastewater solution decreases,the flocculation performance also decreases.The flocculant SA-CTA also has excellent removal ability for negative charged ions in wastewater.Under the action of coagulant promoter CaCl₂,the removal rates of humic acid?HA?and kaolin in wastewater exceed 95%and 99%,respectively,and are not affected by solution pH value.The flocculation mechanism is electroneutralization and net sweeping.The adsorption isotherm equation of SA-CTA for heavy metals,humic acid and kaolin can be described by Langmuir isotherm adsorption model,and the flocculation process can be described by quasi-second-order kinetic model.The preparation materials of the new amphoteric flocculant are easily available,and the synthesis method is simple.It can simultaneously remove the cation and anion pollutants in the wastewater solution,without secondary pollution,and the post-processing is simple.The amphoteric properties of flocculant greatly expands its application range in the field of wastewater treatment,and may become a more potential green and environmental friendly wastewater treatment agent.3.Sodium alginate was modified through three-steps using SA as a material,triethylenetetramine?TETA?and carbon disulfide?CS₂?as modifier reagents.Amino and dithiocarboxyl groups were introduced to the molecular chain.A series of new sodium alginate chelating flocculants with different substitution degrees of dithiocarboxyl?-C?=S?-S-?were prepared.The removal rates of heavy metal ions such as Pb²⁺?Cd²⁺?Cu²⁺?Zn²⁺?Hg²⁺by the flocculant SA-TETA-S were 93.33%,96.21%,86.40%,60.88%and58.11%,respectively.The order of preference for ions in mixed metal ion is Hg²⁺>Pb²⁺>Cu²⁺>Cd²⁺>Zn²⁺.The lower the pH value of the solution,the lower the flocculation performance;and the higher the initial concentration of heavy metals,the better the flocculation performance.The flocculation mechanism includes the chelation of O,N and S with heavy metals,the electroneutralization and the net trapping and sweeping.The flocculation process for metal ions can be described by Langmuir isothermal adsorption model,and accords with Quasi-second-order dynamic model.The new chelating flocculant can effectively remove heavy metal ions from wastewater solution,which is environmentally friendly and has no secondary pollution,and the post-treatment is simple.It provides a potential new method for the removal of heavy metals from wastewater.4.A series of new sodium alginate chelating flocculants SA-TSC with different degrees of substitution were successfully prepared by using SA as the base material,thiourea?TSC?as the modifier,1-[3-?dimethylamino?propyl]-3-ethylcarbodiimide?EDC?and 1-hydroxy-2,5-pyrrolidinedione?NHS?as catalysts.Thiourea groups were introduced to in the molecular chain.The saturated adsorption capacity of flocculant SA-TSC for heavy metals Pb²⁺,Cd²⁺,Cu²⁺reached up to 684.9 mg/g,245.1 mg/g and 168.0 mg/g,respectively.The order of preference for ions in mixed metal ions is Pb²⁺>Cd²⁺>Cu²⁺.When the pH value increases from 2 to 7,the removal rate of heavy metal ions increases at the same time.With the increase of initial concentration of heavy metal ions in wastewater,the removal rates of Pb²⁺,Cd²⁺and Cu²⁺also increased.The flocculation mechanism for the heavy metal ions is the chelation of S,N and O elements with heavy metals.The flocculation process of metal ions can be described by Langmuir isothermal adsorption model,which conforms to quasi-second-order kinetic model.The new chelating flocculant is easy to prepare raw materials,simple synthesis method,and is suitable for industrial production.It can efficiently remove heavy metal ion pollutants from wastewater solution,without secondary pollution.It provides a new green environmental friendly method for the treatment of wastewater containing heavy metal ions.The aims of this work is to explore the preparation conditions and application scope of the new sodium alginate flocculant through studying the synthesis and properties of the modified sodium alginate flocculant,reveal the flocculation mechanism of the flocculant for different pollutants,and provide a theoretical and practical reference for the development of the new sodium alginate-based wastewater treatment agent and its applications.