1. Introduction
Coagulation and flocculation are important preliminary steps in wastewater and potable water treatment facilities. The processes include the removal of non-dissolved and particulate matter, including algae, pathogens, organic materials, minerals, and others. This is achieved by the addition of materials such as ferric chloride, alum (aluminum sulfate), or organic polymers, which destabilize the colloids resulting in small suspended particles agglomerating into larger settleable flocs [
1]. Poly-diallyl-dimethylammonium chloride (poly-DADMAC, PD, CAS 26062-79-3) is a homopolymer also known as polyquarternium-6, with a chemical formula of (C
8H
16NCl)
n (
Figure 1). In commercial formulations, its molecular weight typically ranges from a few hundred to several thousand kDa, depending on the number of monomers (each with a molecular weight of 161.7 Da) [
2]. PD is the most common polymer used in various coagulation processes [
3], including in pulp and paper industry, either alone [
4] or in combination with other polymers [
5]. It was the first cationic polyelectrolyte approved by the Food and Drug Administration of the U.S.A (FDA) for use in potable water treatment [
6]. Recent studies also test its influence when adsorbed on cellulose based products [
7] or for the preparation of specific adsorbing matrices for per- and polyfluoroalkyl substances [
8].
The wide utilization of PD and other organic polymers can be attributed to several advantages. These include their remarkable cationic properties, high charge density that leads to the formation of stronger and easily separable flocs, effective bridging capabilities allowing for higher solid content in the sludge phase, and minimal impact on pH levels, necessitating only a single pH adjustment during the process However, a few disadvantages are associated with the use of PD as a coagulant, mainly regarding the residuals remaining after the process. While PD can serve as an effective coagulant, its presence in the subsequent treatment process might cause severe fouling to the filtration membranes. Although novel membranes with better anti-fouling and anti-microbial properties have been developed [
9,
10], residual PD molecules of even low concentrations can desorb onto the membrane surface, potentially leading to pore blocking [
11]. Fouling depends on the type, charge, size, and concentration of the polymer, and higher molecule-weight polymers have larger fouling potential [
12,
13]. Membrane fouling, particularly in processes like reverse osmosis, microfiltration, and nanofiltration, can lead to reduced water flux and increased energy consumption. It necessitates more frequent membrane cleaning or replacement, subsequently elevating operational costs in water treatment plants [
14]. An additional disadvantage is the reaction of residual PD with disinfection by-products such as chloramine and ozonation, forming carcinogenic nitrosamines [
15,
16,
17,
18,
19]. Therefore, several American and European standardization organizations have limited the residual amount of PD in drinking water at ≤50μg/L ~ 0.310 µM, which is considered to be also toxic to aquatic organisms [
20].
For these reasons, fast and simple quantification of residual PD is required at relatively low limits of detection (LOD), to control and monitor the optimal coagulant dose with minimal negative effects. Chromatography-based analytical methods for quantification of polymers are usually very sensitive to interferences by other organic molecules [
21]. Indeed, a recent study presents a method based on ion-pair chromatography, which accurately measures residual DADMAC monomers [
22] without measuring the amounts of the polymer. Other accurate methods such as gel permeation chromatography, epoxidation, fluorescent tagging, and co-precipitation required advanced, expensive, and long procedures [
2,
3,
23,
24,
25,
26]. A highly sensitive method was developed based on the use of gold nanoparticles [
27], applied in South African treated water [
28] and recently developed to Lovibond color filters [
29], but the preparation of the nanoparticles requires a relatively complicated procedure.
Cationic polyelectrolytes such as PDs can bind to anionic molecules such as tannic acid or anionic dyes like rose Bengal, methyl-orange [
23], Ponceau S [
30], and acid orange [
31] resulting in an insoluble polymer-anion complex that can be separated by centrifugation. This separation method allows the quantification of the polymer concentration by measuring the remaining dye. A similar method was used for quantification of the cationic biopolymer chitosan, based on complexation with Cibacron Brilliant Red 3B-A [
32]. ]. However, in all those studies LODs were two to three orders of magnitude higher than the requirement.
In this study we present a simple colorimetric quantification method based on addition of fast green (
Figure 1, FG, ethyl-[4-[[4-[ethyl-[(3-sulfophenyl) methyl] aminophenyl]- (4-hydroxy- 2-sulfophenyl)methylidene]-1-cyclohexa-2,5-dienylidene]- [(3-sulfophenyl)methyl]azanium), and its complexation with PD. FG is an anionic tri-aryl methane food dye known also as “food green 3”, “green 1724” and “E143”. It is stable in a wide range of pH, and has a brilliant blue-green color with an absorption maximum at 624nm, and two additional smaller absorption bands at 420 and 304 nm [
33]. The very high molar absorptivity (ε
624>100000 M
-1cm
-1) enables high sensitivity and yields a procedure suitable for trace quantification of PD with LOD<3 µg L
- 1 (0.018 µM)- one order of magnitude lower than regulation requirements. The primary objective of this research was to establish the proof of concept for the proposed quantification method in a simple, fast, and cost-effective manner. This approach serves as the first step in mitigating PD’s impact on membrane fouling and potential environmental risks. Additionally, we conducted preliminary implementations of the method under different conditions
.
2. Materials and Methods
2.1. Materials
Analytical polydiallyl dimethylammonium chloride (PD; medium molecular weight, 200.000 to 350.000) and fast green (FG) were purchased from Sigma-Aldrich (Jerusalem, Israel). Commercial grade coagulant with 40% PD as an active ingredient manufactured by SNF Ltd. (FLOQUAT
® FL-45) was purchased from Amgal-Depotchem Ltd. (Beer Tuvia, Israel). NC24 clay polymer nanocomposites were prepared as described in the literature [
34,
35,
36] using 10 g/L sepiolite S9 provided by Tolsa S.A. (Madrid, Spain), and 45 g/L FL-45. DKG kaolinite clay polymer nanocomposites [
37] were prepared in a similar way, using 10 g/L MPO kaolinite provided by Agat Minerals and Yehu Clays Ltd. (Yeruham, Israel) and 45 g/L FL-45. 1 mM HCl was prepared from 37% stock (Merck) and used for pH adjustment.
2.2. Complexation/calibration experiments description
In order to prepare calibration curves and demonstrate the method’s applicability, several complexation experiments were conducted, including a wide range of analytic poly-DADMAC with fast green dye Three separate experiments were conducted to examine different analytic PD concentrations. These concentrations included low, medium, and high PD ranges: 0.02-1 µM (0.0322-0.1617 mg L-1), 1-30 µM (0.1617-4.851 mg L-1), and 10-200 µM (1.617-32.34 mg L-1) PD solutions, all of which were dissolved in double-distilled water. To each PD range, 0.8, 20, and 100 µM of FG were added to the solutions in the low, medium, and high series respectively. After the addition of FG, pH values were adjusted to 3-3.5 by adding 20 µl of 1 mM HCl (see section 2.4). Subsequently, all samples were kept at room temperature (23 ± 1 °C) on an orbital shaker (200 rpm) for 1 hour. FG spectra were measured using a 1 cm cuvette in a Cary 60 UV–Vis spectrophotometer (Agilent Technologies, Santa Clara, CA, USA). Absorbance was determined using the absorption bands at 624 and 420nm with a molar absorptivity of 103500 and 11600 M-1cm-1 respectively. To enhance sensitivity, absorbances of the low range FG solutions were measured at 624 nm, utilizing a 5 cm cuvette. A visual representation of the preparation procedure and the experimental setup is available in Figure S1 of the Supplementary Material.
To confirm the formation of the PD-FG complexes, 50 ml samples containing 20 µM FG and a range between 2.5-50 µM PD solutions were prepared as described above, centrifuged, and the sediment was lyophilized (Christ Alpha 1-2 LD Plus, Germany) and measured for the attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectra on a Nicolet iS10 FTIR (Nicolet Analytical Instruments, Madison, WI), using a SMART ATR device with a diamond crystal plate, within a range of wavenumber comprised from 4000 to 525 cm-1 and analyzed with and OMNIC 9.3.32 software (Thermo Fisher Scientific, Madison, WI), including full spectra ATR correction.
2.3. Implementation in water with commercial PD
Commercial PD samples were examined in medium-range concentrations of 2-50 µM (0.323-8.085 mg L-1) PD dissolved in double-distilled water with the addition of 20 µM FG. FG spectra were measured in the UV–Vis spectrophotometer using a 1 cm cuvette as described in section 2.2.
2.4. Influence of pH
To assess the performance of the quantification method in actual water samples, two coagulation-flocculation experiments were conducted. These experiments were carried out using water contaminated with cyanobacteria and cowshed effluents as test samples. The evaluation involved testing both a commercial PD formulation and clay-PD nanocomposites developed within our research group.
2.5. Quantification of poly-DADMAC in coagulation water treatment samples
To evaluate the efficiency of the quantification method in “real” water samples, two coago-flocculation experiments were performed, in cyanobacteria polluted water, and in cowshed effluents. Experiments included testing commercial PD formulation, and clay-PD nanocomposites developed in our research group [
35,
36,
37]
The first coagulation experiment was performed on cyanobacteria culture suspension using concentrations of two poly-DADMAC-based coagulants: industrial PD and kaolinite-PD nanocomposites (DKG). The cyanobacteria suspension consisted of the Microcystis family, which was isolated from Lake Kinneret and cultivated in batch cultures using BG11 medium [
38]. The experiments were carried out in 300 ml glass beakers and included two sets of PD and DKG coagulants. Each set comprised five distinct coagulant concentrations: 14, 28, 42, 53, and 71 µM PD, either in its free form or adsorbed onto kaolinite clay particles (referred to as DKG coagulant). The coagulants were added to 250 ml cyanobacteria suspensions. After the addition, the suspensions were stirred thoroughly for 1 min and left to settle for 1 h.
The second coagulation experiment was performed on cowshed effluents. The wastewater was taken from Kfar Blum industrial cowshed (Upper Galilee, Israel) during July 2023. To determine the required amount of coagulant needed for neutralization [
39] the electrokinetic colloidal charge of the wastewater was measured in a particle charge detector (PCD) (BTG Mütek, PCD-05, Eclépens, Switzerland). Three poly-DADMAC-based coagulants were tested: commercial PD, kaolinite-PD nanocomposite (DKG), and sepiolite-PD nanocomposite (NC). The doses were equivalent to 100% of the requirement to neutralize the colloids (4.3 mM charges). Each experiment was conducted in 300 ml glass beakers, with each treatment having 3 replicates. After the addition, the suspensions were thoroughly stirred for 1 min and allowed to settle for 1 h
In both experiments, turbidity was measured with a LaMotte 2020i turbidimeter before and after the treatment to estimate the efficiency of clarification and coagulation. The residual PD concentrations were quantified using the complexation method as described in section 2.2. Specifically, 10 ml of the supernatant was sampled, and the pH values were adjusted to 3-3.5 by adding 20 µl of 1 mM HCl. Subsequently, suitable volumes of FG from a 1000 µM stock solution were added to each sample, yielding FG concentration of 20 µM. Test tubes were placed on an orbital shaker (200 rpm) for 1 h, and samples were measured in the spectrophotometer as described in section 2.2.
Author Contributions
Conceptualization, G.R.; methodology, G.R., I.L., B.C, and I.M.; software, I.L. and Y.K.; validation, G.R., I.L., and Y.K.; formal analysis, I.L.; investigation, G.R., I.L., B.C, and I.M.; resources, G.R.; data curation, I.L. and G.R..; writing—original draft preparation, G.R., I.L., and B.C..; writing—review and editing, I.L. and G.R.; visualization, I.L. and Y.K.; supervision, G.R.; project administration, G.R.; funding acquisition, G.R. All authors (except I.M. that deceased) have read and agreed to the published version of the manuscript.