Study On Synthesis And Application Of Sodium Alginate-based Flocculant

Flocculation process is widely used in water treatment and sludge dewatering because of its low cost,high efficiency,simple operation and other advantages.The efficiency of flocculant determines the final treatment effect.However,studies have shown that there are some problems in the commonly used flocculants.The residual metal ions of inorganic flocculants will cause secondary pollution,which is harmful to human health.And synthetic polymer organic flocculants have the disadvantages of toxic residual monomers,difficult biodegradation and carcinogenic degradation products.Therefore,natural polymer flocculants have attracted extensive attention due to their biodegradability and environmental friendliness.However,natural polymer organic materials have some shortcomings such as low flocculation efficiency,poor stability and large dosage.In this paper,sodium alginate（SA）based organic flocculant（SA-PDMC）was prepared by grafting methacrylatoethyl trimethyl ammonium chloride（DMC）onto SA molecule with SA as the framework.The flocculation performance and mechanism of SA-PDMC alone and combined with PAC in surface water treatment and sludge dewatering were studied.The main research contents and conclusions are as follows:The preparation conditions of SA-PDMC were optimized by single factor method.When SA-PDMC was used in water treatment,the optimal reaction conditions were 1.0 g SA reacted with 15 ml DMC and 0.5 g KPS at 70℃ for 4 h（SA-PDMC 1）;when SA-PDMC was used in sludge dewatering,more monomers（20 ml）were required to participate in the reaction at a relatively lower temperature（60℃）with less initiator（0.25 g）（SA-PDMC2）.This is because the adsorption bridging in sludge dewatering is more dependent on the molecular weight of SA-PDMC than that in water treatment.When SA-PDMC1 was used to treat synthetic surface water and actual lake water separately,the flocculation effect first increased and then decreased with the increase of dosage.Under the optimal dosage,the concentration of UV₂54 and DOC of simulated surface water decreased by 72.47%and 23.55%,respectively.For actual lake water,the UV₂54 and DOC concentrations can be reduced by 21.76%and 23.96%,respectively.Meanwhile,with the increase of dosage,the anti-fragmentation ability of flocs increased and the recovery ability decreased.The reason is that the proportion of adsorption electric neutralization and adsorption bridging changes with the dosage.When the dosage is low,adsorption electric neutralization plays a major role.The proportion of adsorption bridging becomes larger at lower dosage.In addition,by studying the effect of pH on SA-PDMC1 treatment of synthetic water samples,it was found that due to the protonation and deprotonation of HA and the response of SA conformation to pH,the flocculation effect increased with the increase of pH,and tended to be stable at pH 6.0-10.0,so SA-PDMC1 has a wide range of pH applications.PAC and SA-PDMC1 were combined to treat the actual lake water and analyzed by response surface method.The synergistic effect of SA-PDMC1 and PAC on the removal of organic matter was obvious when the dosage was small,and the effect was 1+1>2.This is mainly because the adsorption bridging effect of SA-PDMC1 was a beneficial supplement to the adsorption neutralization of PAC.After optimization,the optimal dosage of PAC was 11.9 mg/L and SA-PDMC1 was 1.2 mg/L.And the removal rates of turbidity,UV₂54 and DOC were 95.96%,32.42%and 35.42%,respectively,and PAC dosage was reduced by 40%compared to it worked aloneWhen SA-PDMC2 was used in sludge dewatering,the protein substances in extracellular polymeric substances（EPS）could be removed by adsorption bridging and adsorption electric neutralization.With the increase of dosage,the sludge dewatering performance became better.When the dosage was 5‰,the sludge specific resistance and moisture content of sludge cake reached the best,which were 2.87 × 10¹²m/kg and 82.85%respectively,which decreased by 79.8%and 5.48%compared with the original sludge,respectively.At the same time,the added cationic SA-PDMC2 can neutralize the negative charge groups on the surface of sludge particles,which reduces the negative charge on the surface,reduces the repulsive force between the sludge particles,and thus realizes the sludge agglomeration,the flocs particle size increases and the apparent viscosity decreases.The study on the effect of sludge initial pH on the dewatering effect shows that it is better not to adjust the pH.On the one hand,the initial pH influence the properties of sludge,EPS has a lot of amino and carboxyl,the two groups show different properties and ionization effect in different pH conditions.With the increase of pH,the protons adsorbed by amino group will gradually change into protons released by carboxyl group ionization,resulting in the increase of surface negative charge,the enhancement of repulsion between sludge particles,and the decrease of sludge dewatering performance.On the other hand,it will affect the conformation of SA-PDMC2.With the increase of pH,SA-PDMC2 gradually changes from curly state to stretched state.It is beneficial to play the role of adsorption bridging and improve the sludge dewatering performance.The experimental results show that under the comprehensive influence of the two aspects,the sludge dewatering performance is the best when the pH is not adjusted.