Experimental study on the performance of modified flocculant (PEI-T) for removal of turbidity and heavy metals from aqueous solution Open Access

Flocculants are efficient water treatment chemicals, which are widely used in the actual water treatment process. Flocculation is a very important process (Mutairi et al. 2004; Chang 2011; Yang et al. 2012) in solid-liquid separation of water treatment to produce the flocculation phenomenon through collection of solid particles in suspension. Compared with other water treatment methods, flocculation has the advantages of being more effective, and offering easier operation and low cost (Vilarrasa-García et al. 2017; Santos et al. 2018; Song et al. 2020). Due to the rapid industrialization, urbanization, environmental protection requirements, and enhanced environmental awareness, green treatment of wastewater has become a research hotspot in the field of environmental protection (Duan et al. 2019; Li et al. 2019). The study of natural non-toxic polymers has received increasing attention from researchers. At present, many researchers are engaged in the research and development of new flocculants for more effective, economic and sustainable treatment of wastewaters.

Metal ions in water cannot be degraded easily (Namasivayam & Ranganathan 1998; Schwarzenbach et al. 2016). 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 2006). Selective removal of metal ions from simulated wastewater by sulfonated calix[4]arene intercalated with layered double hydroxide (SC4A-NO₃-LDH) was reported in a previous study (Reda & Zhang 2019). 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 Fe³⁺, Cu²⁺ and Eu³⁺ were reported to follow the pseudo-second-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 2000). 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.

Flocculation can easily and effectively remove 80–95% of suspended solids and 65–95% of colloidal substances (Fang et al. 2018; Ma et al. 2019). The organic polymer flocculants mainly adsorb colloidal particles in water to the molecular chain of the flocculants through the adsorption bridging effect to form flocs (Fang et al. 2018; Zhang et al. 2018; Wang et al. 2019). Compared with inorganic flocculants, organic polymer flocculation has the advantages of good stability, lower dosage, wide application range, strong adsorption and bridging ability, fast flocculation speed, small impact of pH value and temperature, and less sludge. Its flocculation effect is affected by molecular weight, charge density, and dosage, mixing time, flocculant stability and other factors (Bolto & Gregory 2007; Lu et al. 2014; Fang et al. 2018; Wang et al. 2019).

Polyethyleneimine (PEI) is a traditional polymer flocculant, which can be used to remove colloidal substances and fine suspended solids in water but has little effect on dissolved heavy metal ions in water (Fosso-Kankeu et al. 2016). In this study, sulfhydryl (-SH), which has a strong coordination effect on heavy metals, was introduced into the traditional flocculant polyethyleneimine through reaction, so the traditional flocculant can not only remove turbidity, but also capture heavy metals. In consideration of the challenges and cost control of the access reaction, thioglycolic acid TGA (HOOCCH₂SH) containing -SH was added into the long chain of polyethyleneimine (PEI) by amidation reaction to form modified polyethylene thioacetamide (PEI-T). This expanded the function of the traditional flocculant, and can effectively capture and remove heavy metals in the water while removing turbidity. By adding the modified flocculants, not only can the turbid substances in water be removed, but also the heavy metals in the water can be collected and removed, which enlarges the scope of application of the flocculant and reduces the process of water treatment.

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.

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 Cu²⁺, 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 = 1:2, PEI concentration was 4%, time t = 12 h, catalyst EDC = 6 ml, initial pH = 2.5.

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⁻¹) was taken for 2 min, slow agitation (40R < min⁻¹) 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%.

The residual concentration of Cu²⁺ and Hg⁺ was measured by an atomic absorption spectrophotometer, and the turbidity was measured by turbidity meter (APHA 2005).

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

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⁻¹, indicating that PEI-T contains a tertiary amide group and an absorption peak with wave number of 2,562.96 cm⁻¹ 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 = 3.12, density 1.16 g/ml, solid content 4.31%.

Distilled water was added, the pH was adjusted to 5, and different amounts of PEI and modified flocculant PEI-T were added. These results are presented in Figure 2. Results revealed the slight difference between modified PEI-T and the original flocculant PEI in the turbidity removal effect (Figure 2). As a flocculant, they almost have the same effect due to having the same turbidity removal mechanism. PEI is a cationic organic polymer flocculant. The molecule contains a large number of primary amine groups (-NH²), tertiary amine groups (-NH-) and quaternary amine groups. It could adsorb H⁺ with positive charge, and mainly promoted flocculation and sedimentation of colloidal substances in water by reducing the turbidity of the colloidal substances through ‘adsorption neutralization’ and ‘adsorption bridge’ (Wang et al. 2017). The mechanism of PEI-T turbidity removal is the same as that of PEI.

In addition to its turbidity function, PEI-T could also capture and remove heavy metals. It can be seen from Figure 3 that with the increase in the pH value, the removal rate of turbidity by PEI-T decreased. In general, the pH value had a certain effect on the turbidity removal of PEI-T, but it was small. For the modified flocculant PEI-T, there was an equilibrium of ionization and hydrolysis in water (Equation (2)). Results revealed that when the pH is low, the equilibrium is shifted to the right, and the cationic flocculant fully exerted adsorption and electric neutralization. When the pH is high, the equilibrium shifts to the left, and the negative charge is the key form in -S-, and the adsorption neutralization can be inhibited. These results are consistent with the findings of Augustine et al. (2019).

At pH 5, PEI-T was added to water samples with different concentrations of Cu²⁺, and the residual Cu²⁺ concentration in the water was measured after flocculation and precipitation. The experimental results are shown in Figure 4. This experiment did not compare the copper removal effect of PEI, because PEI has no effect on copper.

The lower the Cu²⁺ concentration, the closer the PEI-T to Cu²⁺ reaction ratio is to 2: 1 (mg • L⁻¹), and the larger the initial Cu²⁺ concentration, the smaller the reaction ratio between PEI-T and Cu²⁺ (<2). At the same time, -NH₂ on the parent chain and the nitrogen atom on the equivalent group will coordinate with H⁺ in the solution, and will also undergo an ion exchange reaction with Cu²⁺. This process could promote the removal rate of copper (Equation (5)).

(5)

A water sample containing 25 mg • L⁻¹ Cu²⁺ 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 Cu²⁺ by PEI-T was studied. The experimental pH range was set between 3.0 and 5.0, given that Cu²⁺ 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 +} 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 Cu²⁺, and the excessively high H⁺ concentration changed the Cu (II)/Cu (I) standard potential (Huang 1998), which enables H⁺ to oxidize Cu⁺ to Cu²⁺. Since the stability of the complex of Cu²⁺ and PEI-T was lower than that of Cu⁺ and PEI-T, the Cu²⁺ 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 Cu²⁺ and Cu⁺, making Cu²⁺ removal easier.

A water sample containing Cu²⁺ 25 mg • L⁻¹ 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 Cu²⁺ but no turbidity was compared for the case with turbidity but no Cu²⁺, as shown in Figure 6.

It can be seen from Figure 6 that in the case where Cu²⁺ and turbidity coexisted, the removal rates of Cu²⁺ and turbidity were higher than those when they existed separately, especially for turbidity removal, so the removal rate of Cu²⁺ and turbidity substances can be improved. The reason may be, on the one hand, that Cu²⁺ can neutralize the excessive negative charge of both the turbidity substances and the flocculant, reducing the repulsion between them. The flocculant can effectively play the role of ‘adsorption bridging’; therefore, the turbid substances in the water can be combined into a large number of flocs. On the other hand, due to the sweeping and net-catching effects of a large number of flocs, the copper ion complex precipitates rapidly. In addition, due to the large specific surface area of the floc, a portion of the soluble copper ions will be adsorbed on the floc surface and further removed as the floc is precipitated. The effect on turbidity was large because the first-formed heavy metal chelate can fully play the role of electric neutralization and sweeping net capture of turbid substances, and strengthen flocculation.

Figure 7 reveals the effect of Ca²⁺ and Mg²⁺ on PEI-T flocculation and the copper removal effect at pH5. The effects of Ca²⁺ and Mg²⁺ were studied at different dosages of PEI-T. The finding presented in Figure 7 reveals that the presence of Ca²⁺ and Mg²⁺ promoted the removal of Cu²⁺. The main reason is that the sulfur atom, which is mainly composed of -S- in the PEI-T molecule, was not easy to coordinate with the alkali metal or alkaline earth metal, is difficult to coordinate with Ca²⁺, Mg²⁺, and has poor stability after coordination (Song 1990). Therefore, Ca²⁺ and Mg²⁺ would not compete with Cu²⁺ for thiol ligands. In the case where -S- was chelated with Ca²⁺, there would still be a certain amount of negatively charged groups on the polymer, and the electrostatic repulsion between them would not be conducive to flocculation and precipitation. When the presence of Ca²⁺ and Mg²⁺ in the solution increased the ion concentration, the electric double layer compressed due to the electric neutralization. Due to this, the electric double layer was reduced in thickness, reducing the zeta potential, weakening the repulsion effect and accelerating the speed of flocculation, which allowed Cu²⁺ to be removed more effectively. Earlier studies also revealed that this effect was more significant when the ion number was high (Shi & Zhang 1989), therefore Mg²⁺ and Ca²⁺ promoted the copper removal rate more than Na⁺. The promotion effect of Mg²⁺ on Cu²⁺ removal in solution was more obvious than that of Ca²⁺ because the effective nuclear charge number of Mg²⁺ in the solution was higher than that of Ca²⁺ (Hu & Huang 2001).

The PEI-T with different concentrations was added to water samples with different concentrations of Hg²⁺ (50 mg • L⁻¹, 100 mg • L⁻¹) and the removal rate of Hg²⁺ in the water after flocculation and precipitation were measured to compare the effect of Hg²⁺removal. It is evident from Figure 8 that the removal rate of Hg²⁺ increased with the increase in the dosage, and PEI-T had the optimal dosage for water samples with different concentrations of Hg²⁺. Excessive low or high dosage would affect the flocculation effect.

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 table, it can form a strong covalent bond with the atom that provides the electron pair. It can form a very stable complex 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.

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 Cu²⁺ and turbidity by PEI-T was high whereas the effluent concentration was low. The pH value has a certain effect on Cu²⁺ 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 Cu²⁺ removal effect was better at higher pH level. When PEI-T was used to remove the coexisting system of heavy metals and turbidity, the presence of heavy metals and turbidity in wastewater promoted the removal of each other. The presence of Na⁺, Mg²⁺, and Ca²⁺ in wastewater can promote the removal of heavy metals by PEI-T. The floc particles formed by the PEI-T treatment wastewater are large and settle quickly. The PEI-T product is susceptible to oxidation and should be stored at low temperatures.

This research was co-funded by the Universities and Colleges Innovation Ability Improvement Project of the Gansu province, China (No. 2019B-193), the National Natural Science Foundation of China (No. 41967043), and the Scientific and Technological Project of Construction of the Gansu province, China (No. JK2020-30).

All relevant data are included in the paper or its Supplementary Information.