Experimental study on the performance of modi fi ed fl occulant ( PEI-T ) for removal of turbidity and heavy metals from aqueous solution

The current study was focused on the evaluation of the performance of a modified flocculant (PEI-T) for the removal of turbidity and heavy metals from wastewater. Thioglycolic acid was introduced into the molecular chain of cationic flocculant polyethyleneimine (PEI) through an amidation reaction to form a new flocculant PEI-T. Results revealed that: (a) PEI-T and PEI had similar turbidity removal effect, but PEI-T had a good function of catching heavy metal copper and mercury; (b) when Cu2þ and turbidity-causing substances coexist in a water sample, Cu2þ and turbidity-causing substances promote the removal of each other and the residual concentration of Cu2þ and turbidity decrease further; and (c) the presence of Ca2þ and Mg2þ can promote the removal of copper ions by PEI-T.


INTRODUCTION
Metal ions in water cannot be degraded easily (Namasivayam & Ranganathan ; Schwarzenbach et al. ). Due to their toxicity and bioaccumulation in the food chain, it is of great significance to effectively reduce the concentration of metal ions in water (Özverdi & Erdem ). Selective removal of metal ions from simulated wastewater by sulfonated calix[4]arene intercalated with layered double hydroxide (SC4A-NO 3 -LDH) was reported in a previous study (Reda & Zhang ). The authors reported that SC4A-NO3-LDH exhibited high adsorption performance towards metal ions with selectivity order of Fe(III) > Cu(II) > Pb(II) > Ag(I) for heavy metal ions, and Eu(III) > Nd(III) > La(III) for rare earth metal ions. Kinetics of Fe 3þ , Cu 2þ and Eu 3þ were reported to follow the pseudosecond-order reaction, suggesting chemisorption.
Copper-containing wastewater mainly comes from metal mining and smelting, metal processing, machinery manufacturing, electroplating, circuit board production and other industries. With the advancement of industrial processes and the intensification of human activities, a large number of copper-containing wastewaters are being discharged by industrial enterprises and this will lead to the deterioration of the water environment, and affect human health, drinking water safety and seriously damage the aquatic ecosystem.
When irrigated with wastewater containing copper, copper can accumulate in soil and crops, causing harm. Therefore, copper-containing wastewater must be treated effectively.
The treatment methods for copper-containing wastewater mainly include chemical precipitation, electrolysis, adsorption, ion exchange, membrane separation and so on. The conventional treatment of copper is chemical precipitation with lime or sulfide, but this kind of treatment method requires a large amount of reagent, needs to adjust the pH value, and the amount of sludge is large. Moreover, the complexing agent in the wastewater will interfere with the precipitation of copper ions, so it needs to be removed by pretreatment (Meng & Hu ). The harm from mercury-containing wastewater is well known. In the current study, the traditional flocculant was modified by adding groups able to capture heavy metals, so the traditional flocculant could effectively capture heavy metals in water while removing turbidity. There are many primary and secondary amines in the molecular chain of the polymer flocculant polyethyleneimine, which have high reaction activity. However, amidation is a very common chemical reaction. In this study, amines and carboxylic acids were amidated by appropriate means, and sulfhydryl groups were connected into the long chain of polyethyleneimine. The reaction formula is as follows (Equation (1)): There are many types of amidation reaction. The EDC has a linear structure, which has been widely used in the condensation of the carboxyl group and primary amine. The use of EDC as an activator of amidation makes the reaction easy to control, has fewer side reactions and the product is easy to purify. Therefore, the current study was focused on the evaluation of the performance of a modified flocculant (PEI-T) for removal of turbidity and heavy metals and the activator EDC.HCl.

Modification experiments
In the reaction vessel, a certain amount of PEI solution, catalyst and TGA solution were added, and the reaction was stirred at room temperature. In order to determine the best conditions and change the influencing factors (EDC, nFEI: nTGA, PEI, pH, time), the PEI-T was characterized by the removal effect of Cu 2þ , and the subsequent reaction was carried out after the optimization. Finally, the best conditions for preparing PEI-T were obtained at the following conditions: room temperature, nPEI: nTGA ¼ 1:2, PEI concentration was 4%, time t ¼ 12 h, catalyst EDC ¼ 6 ml, initial pH ¼ 2.5.

Flocculation experiments
A six-line stirrer was taken as the main experimental equipment, 400 ml copper or turbidity water samples were prepared, different test conditions were adapted, rapid agitation (120r < min À1 ) was taken for 2 min, slow agitation (40R < min À1 ) was taken for 10 min, and then allowed to stand for 15 min, a pipette was used to suck up the supernatant 2 cm from the liquid level to determine the content of copper or turbidity. The experiment was repeated 3 times, and the final average value was taken, and lastly the relative error of the experiment was ensured as 5%.

Experiment methods
The residual concentration of Cu 2þ and Hg þ was measured by an atomic absorption spectrophotometer, and the turbidity was measured by turbidity meter (APHA ).

Data analysis
Outcomes of the present study were statistically analyzed by ANOVA and Origin 8.1 was used for preparing the figures.

RESULTS AND ANALYSIS
Characterization of PEI-T and description of product properties PEI-T was precipitated and washed with acetone several times. After filtration and vacuum drying (50-), both PEI-T and PEI were analyzed by infrared spectroscopy respectively with potassium bromide tablets. The functional groups of PEI-T were determined by comparison. Figure 1 shows the infrared spectra of PEI and PEI-T, respectively. The upper spectrum is for PEI-T and the lower spectrum is for PEI. It can be seen from Figure 1 that PEI is transformed into PEI-T with a wave number of 1,667.14 cm À1 , indicating that PEI-T contains a tertiary amide group and an absorption peak with wave number of 2,562.96 cm À1 is generated, indicating that the PEI-T molecule already contains thiol group.
The sulfur content of PEI-T 2 g was measured by an elemental analyzer. The results of elemental analysis show that the content of sulfur in PEI-T is 0.198%. FTIR and elemental analysis showed that TGA was successfully attached to PEI. The product PEI-T was milky white, liquid, pH ¼ 3.12, density 1.16 g/ml, solid content 4.31%.
Performance of turbidity removal before and after modification Distilled water was added, the pH was adjusted to 5, and different amounts of PEI and modified flocculant PEI-T In addition to its turbidity function, PEI-T could also capture and remove heavy metals. It can be seen from  . This experiment did not compare the copper removal effect of PEI, because PEI has no effect on copper.     (3) and (4).
4RCOCH 2 SH þ Cu 2þ ! RCOCH 2 S À Cu(I) À SCH 2 COR + RCOCH 2 S-SCH 2 COR (4) The lower the Cu 2þ concentration, the closer the PEI-T to Cu 2þ reaction ratio is to 2: 1 (mg • L À1 ), and the larger the initial Cu 2þ concentration, the smaller the reaction ratio between PEI-T and Cu 2þ (<2). At the same time, -NH 2 on the parent chain and the nitrogen atom on the equivalent group will coordinate with H þ in the solution, and will also undergo an ion exchange reaction with Cu 2þ .

Effect of pH on PEI-T removal of Cu 2þ
A water sample containing 25 mg • L À1 Cu 2þ was taken to start the experiment. The pH value of the raw water was adjusted, and the effect of pH value on the removal of Cu 2þ by PEI-T was studied. The experimental pH range was set between 3.0 and 5.0, given that Cu 2þ hydrolyzes at a higher pH and the acidity of general heavy metal wastewater.
Results presented in Figure 5 reveal that the copper removal rate was lower when the pH was lower and the copper removal rate was higher when the pH was higher. As a whole, as long as the PEI-T dosage was increased, the removal rate of Cu 2 þ could also reach the maximum value. The reason is that when the pH was low, the equilibrium shown in formula (2) shifted to the right. The thiol group in PEI-T mainly existed in the form of -SH, which was not easy to chelate with Cu 2þ , and the excessively high H þ concentration changed the Cu (II)/Cu (I) standard potential (Huang ), which enables H þ to oxidize Cu þ to Cu 2þ . Since the stability of the complex of Cu 2þ and PEI-T was lower than that of Cu þ and PEI-T, the Cu 2þ removal rate was reduced. At higher pH levels, the equilibrium shifted to the left and the molecule was dominated by the negatively charged form -S-, which easily chelates with Cu 2þ and Cu þ , making Cu 2þ removal easier.

Flocculation effect and interaction between Cu 2þ and turbidity
A water sample containing Cu 2þ 25 mg • L À1 and turbidity of 127NTU was taken, and the dosing concentration of PEI-T for flocculation experiments was changed at pH 5. A water sample containing only Cu 2þ but no turbidity was compared for the case with turbidity but no Cu 2þ , as shown in Figure 6.
It can be seen from Figure 6 that in the case where Cu 2þ and turbidity coexisted, the removal rates of Cu 2þ and turbidity were higher than those when they existed separately, especially for turbidity removal, so the removal rate of tively. Earlier studies also revealed that this effect was more significant when the ion number was high (Shi & Zhang ), therefore Mg 2þ and Ca 2þ promoted the copper removal rate more than Na þ . The promotion effect of Mg 2þ on Cu 2þ removal in solution was more obvious than that of  Ca 2þ because the effective nuclear charge number of Mg 2þ in the solution was higher than that of Ca 2þ (Hu & Huang ).

Effect of modified flocculant on mercury removal
Effect of dosage of PEI-T on Hg 2þ removal The PEI-T with different concentrations was added to water samples with different concentrations of Hg 2þ (50 mg • L À1 , 100 mg • L À1 ) and the removal rate of Hg 2þ in the water after flocculation and precipitation were measured to compare the effect of Hg 2þ removal. It is evident from Figure 8 that the removal rate of Hg 2þ increased with the increase in the dosage, and PEI-T had the optimal dosage for water samples with different concentrations of Hg 2þ . Excessive low or high dosage would affect the flocculation effect.
The PEI molecule underwent a complexation reaction with Hg 2þ in the form of coordination bonds and covalent bonds to form a stable insoluble heavy metal complex with a cross-linked space network structure to precipitate PEI-T-Hg, and also had a redox reaction (Cabrera & West ). The reaction formula was as follows (Equation (6)).
2RCOCH 2 SH þ Hg 2þ ! RCOCH 2 S À Hg(II) À SCH 2 COR þ 2H þ 4RCOCH 2 SH þ Hg 2þ ! RCOCH 2 S À Hg(I) The removal rate of Hg 2þ began to decrease when the dosage was higher than the optimal dosage. This trend occurred because after adding an excessive amount of PEI-T, the excess polymer's long chains protect the colloidal material and prevent particles from colliding with each other. The removal rate started to decline because the small particles were not easy to settle and the PEI-T molecules did not participate in the reaction. The -SH dissociation was negatively charged, causing electrostatic repulsion of the chelate, weakening the collisions between the particles, resulting in a decrease in the flocculation effect and a cause of the reduction in the removal rate of Hg 2þ .
Since the sulfur atom has an empty 3D orbit and is easily polarized, it has a high affinity for heavy metal ions.
As mercury belongs to the Group II elements of the periodic with the mercapto group, which has a strong affinity for mercury. Therefore, the dosage of PEI-T used in the mercury removal experiment is much lower than the dose required for copper removal.

CONCLUSIONS
The synthesis of PEI-T is simple and easy. PEI-T is a highly efficient water treatment agent that can remove heavy metals and turbidity in actual wastewater due to the rapid settlement of floc particles. PEI-T has a good removal effect on copper and mercury wastewater at different concentrations. The removal efficiency of Cu 2þ and turbidity by PEI-T was high whereas the effluent concentration was low. The pH value has a certain effect on Cu 2þ and turbidity removal by PEI-T, but not significant. Results revealed that the turbidity removal effect was better when the pH value was low, whereas the Cu 2þ removal effect was better at higher pH level. When PEI-T was used to remove the coex-