Use of Aloe vera as an Organic Coagulant for Improving Drinking Water Quality

: The coagulation–flocculation–sedimentation process is widely used for removal of suspended solids and water turbidity reduction. The most common coagulants used to conduct this process are aluminum sulfate and ferric sulfate. In this paper, the use of Aloe vera as a natural-based coagulant for drinking water treatment was tested. The bio-coagulant was used in two different forms: powder as well as liquid; the latter was extracted with distilled water used as a solvent. The obtained results showed that the use of the natural coagulant ( Aloe vera ) in both powder (AV-Pow-der) and liquid (AV-H 2 O) forms reduced the water turbidity at natural pH by 28.23% and 87.84%, respectively. Moreover, it was found that the use of the two previous forms of bio-coagulant for drinking water treatment had no significant influence on the following three parameters: pH, alkalinity, and hardness. The study of the effect of pH on the process performance using Aloe vera as a bio-coagulant demonstrated that the maximum turbidity removal efficiency accounted for 53.53% and 88.23% using AV-Powder and AV-H 2 O, respectively, at optimal pH 6. water on several parameters such as pH, total hardness, salinity, the concentrations of organic matter increased. This experimental study has that Aloe vera can be successfully used as a natural coagulant for drinking water treatment. Both AV-Powder and AV-H 2 O can improve the final drinking water quality. Therefore metal-based coagulants, such as Aluminum sulfate, Ferrous sulfate, Ferric Chloride, etc., can with with-out the process performance. This study opens new perspectives of research on the development of natural coagulants to treat surface water, groundwater, wastewater, and industrial water, as well as on extraction processes to obtain liquid coagulants from Aloe vera , and finally on the sol-vent recovery procedure after extracting process. However, further investigations in pilot and full-scale plants are necessary in addition to a cost–benefit analysis to comprehen-sively understand the potential of bio-coagulants in water treatment. Author Contributions: Conceptualization, A.B., K.D., A.K., A.P. and F.P.; methodology, A.B., A.K., A.P., C.G., G.C. and F.P.; formal analysis, A.P.; investigation, A.B. and A.P.; data curation, A.B., D.K., R.B. and A.P.; writing—original draft preparation, A.B., A.K., A.P., C.G., G.C., F.P. and A.P.; writing—review and editing, D.K., R.B. and A.P.; supervi-sion, D.K., A.K., C.G. and A.P.; project administration, D.K. All authors have read and agreed to the published version of the manuscript.


Introduction
Water treatment plants are facilities designed to correct the characteristics of influent water to the plant and make it suitable for its final use (e.g., drinking water, productive process water, discharged water, etc.). Different treatments are therefore needed as both influent water and final uses vary widely [1]. In general waters to be treated are grouped as follows: (i) municipal wastewaters that are characterized by a high organic content as well as nutrients, and their treatment is mainly conducted with biological processes [2][3][4][5] and other alternative methods [6]; (ii) industrial wastewaters that are mainly loaded with potential toxic elements (PTEs), and their treatment is performed with several processes, usually chemical, but also biological such as bioremediation [7], and (iii) surface waters and groundwater, which generally represent the main source of drinking water. However, these sources are in most cases loaded with suspended solids and colloids. The presence of these solids increases the water turbidity. For the removal of suspended solids and the reduction of turbidity, several physicochemical processes can be applied [8,9]. Among them, the coagulation-flocculation-sedimentation process is the most widely used [10,11].
Currently, in Algeria as well as in many other countries in the world, aluminum-or iron-based coagulants are the most used in drinking water treatment [12][13][14]. They are used for their effectiveness in removing suspended and colloidal solids, reducing water turbidity, removing color and, in some cases, also microorganisms. The addition of coagulants with trivalent metal ions as Al2(SO4)3 or FeCl3 in raw water, allows flocs composed of Al(OH)3 or Fe(OH)3 to form and precipitate at neutral or acid pH [1]. The formed flocs entrap the negatively charged colloids and promote their separation from water during the sedimentation phase [1,15].
Despite the performance and cost-effectiveness of these metal-based coagulants, they require, in most cases, pH and alkalinity adjustments to properly treat the water [11,16]. Moreover, when Al2(SO4)3 is used as a coagulant, an excess of dosage, an inefficient coagulant use, and unexpected changes in water chemical composition can result in a high concentration of dissolved Al 3+ in treated water [1,13]. Such residual metal concentration can be responsible for several health diseases such as Alzheimer's disease, neurotoxic disturbances, and cancer [17][18][19].
Moreover, the use of metal-based coagulants affects the quality of the sludge resulting from the coagulation-flocculation-sedimentation process. Such sludge is loaded with metals such as Al 3+ and Fe 3+ and its final disposal can require high costs and be a source of environmental concerns [20,21].
On the basis of such critical aspects, recent studies have focused on evaluating sustainable and eco-friendly natural coagulants as an alternative to metal-based coagulants for the coagulation-flocculation-sedimentation process [14].
As regards the adsorption and neutralization phenomenon, ions of opposite charges are absorbed on the surface of colloids resulting in the reduction of the thickness of the electrical water double layer around colloids [35,36].
As regards the adsorption and bridging phenomenon, several natural compounds such as cellulose, proteins, starch, and polysaccharides, are characterized by their high molecular weight and, in addition, have multiple electrical charges throughout their atom chains. These compounds show a long chain length and are capable of destabilizing the negatively charged colloidal solids [36]. Several researchers have developed a theory on chemical bridges, useful to explain the empirical behavior of these compounds [35]. Such a phenomenon is due to the formation of bridges between colloidal solids thanks to coagulant molecules: the free sites of several coagulant molecules adhere to the colloidal solids, and coagulants bind colloids by means of hydrogen bonds, coulumbic attraction force, Van der Waals forces or ion exchange processes [10,36].
To evaluate the efficiency of natural coagulants for destabilizing colloidal particles, two water parameters were considered: turbidity and suspended solids concentration. The aim of this paper is to investigate the potential of Aloe vera as a coagulant for drinking water treatment. This natural element was selected according to its effectiveness of treatment and its worldwide availability.

Materials and Methods
This study was conducted according to the sequence reported in Figure 1.

Analytical Methods
Raw water was collected from the drinking water treatment plant of Oued El Athmania (36°14′35.40″ N; 6°17′6.00″ E), Algeria. Table 1 reports the following elements: (i) some chemical parameters measured in raw water; (ii) the used measurement instrument or method, and (iii) the corresponding quality thresholds set by the Algerian water quality law [37].
Measurements of water turbidity were performed using a Turbidimeter (HANNA Code: HI 98713, Hanna instruments, Cluj-Napoca, Romania). Water pH and salinity were determined with a multi-parameter instrument (Jenway model 3540, Camlab, Cambridge, United Kingdom). The total alkalinity, total hardness, and organic matter content in water were detected according to the standard methods for water and wastewater characterization [38].

Preparation of the Bio-Coagulant
The Aloe vera was collected from fields located in the city of Mila (36°27′1.01″N; 6° 15′51.98″ E), which is located in the North East of Algeria. Prior to use as a coagulant, Aloe vera was treated as follows: (1) washed by tap water; (2) dried at a relatively low temperature of 50°C to prevent active component content decomposition; (3) ground; (4) sieved through a 0.35 mm pore size sieve as shown in Figure 2.
After this treatment, 25 g of Aloe vera powder was added to 1000 mL of distilled water and mixed for 20 min by a magnetic stirrer to extract active coagulating agents. After 30 min of settling, the supernatant was filtered through a standard filter (porosity < 8 μm), and the filtrate was used as a liquid coagulant [10, 12,13].

Raw water characterization
Natural coagulant processing Characterization of natural coagulant (FTIR.XRD)

Characterization of Aloe vera
Aloe vera was characterized using both FTIR and XRD analysis.

Fourier-Transform Infrared Spectrophotometry (FTIR)
The infrared spectrum of Aloe vera was obtained in the wavelength range between 4000-500 cm −1 with a Fourier-Transform Infrared Spectrometer (Shimadzu Code: HI 98713, Shimadzu, Cluj Napoca, Romania). Figure 3 shows the infrared spectra of Aloe vera. The strong stretch at 3385.1 cm −1 was due to the bound OH stretching vibration [39,40]. The CH group was observed at 2927.7 cm −1 and 2858.3 cm −1 [41,42].The bands at 2360.7 cm −1 and 2148.6 cm −1 could be representative of C≡N and C≡C, respectively [12].
The carbonyl function (C=O) and CH3 primary aromatic amines were detected at 1635.5 cm −1 and 1423.4 cm −1 , respectively [31,43]. These peaks confirm the presence of NH groups in amides [44] and the carboxyl group (COOH) [31,45], which are responsible for water turbidity removal.
The presence of CN stretching (aromatic primary amine stretch) was confirmed by the peak at 1326.9 cm −1 [46].
The bands at 1257.6 cm −1 and 1018.3 cm −1 indicate the presence of COO stretching Carboxylic acid salt [46]and CO of ester [31], respectively.
During the coagulation-flocculation-sedimentation process conducted with Aloe vera, its functional groups (the carboxyl group) act as adsorption sites for the suspended and colloidal solids [45]. In addition, the amide groups in Aloe vera form intermolecular bonds between the suspended solids and the coagulant, thus increasing the efficiency of the coagulation process [44].

XRD Analysis
The crystalline structure of the powdered coagulant was evaluated by an X-ray diffractometer using Cu with wavelength K-Alpha (1.54) at 40 kV and 30 mA with a scan analysis. The spectra of Aloe vera were obtained in the range of 10-90 degrees.
X-ray diffraction analysis (XRD) is used for the determination of crystalline phases in natural material. In general, these materials can be classified in three phases: amorphous, crystalline or semi-crystalline [47] Figure 4 shows the XRD pattern of Aloe vera powder. The percentage of crystallinity was 38%.
The peaks between 15 and 40 degrees could be representative of various substances such as lipids, proteins, carbohydrates, and ash [48,49].
The specific nature of this coagulant allows contaminants to enter the coagulant surface [50]. According to the obtained results, it was noticed that the different coagulation agents present in Aloe vera facilitate the removal of suspended solids and negatively charged colloidal particles, thus promoting the turbidity reduction and, as consequence, water quality improvement [47,51].

Experimental Procedure
The coagulation-flocculation-sedimentation process was carried out using a Jar test equipment (LI-JTA-125, LABARD, labard instruments, Bengale, India) to evaluate the performance of the natural coagulant: powder (AV-Powder) and liquid extracted with distilled water (AV-H2O) Aloe vera were both tested at several pH values and coagulant dosages since pH is one of the main parameters to be optimized in a Jar test experiment [22].
A Jar test was conducted according to the following sequence of operations: (i) six 500 mL beakers were filled with raw water; (ii) a defined coagulant dose was added; (iii) beakers were mixed rapidly at a mixing speed of 160 rpm for 3 min (coagulation); (iv) beakers were slowly mixed at mixing speed of 30 rpm for 20 min (flocculation); (v) the formed flocs were allowed to settle for 30 min (sedimentation). At the end of the coagulation-flocculation-sedimentation process, the supernatant from each beaker was characterized in terms of turbidity, pH, total alkalinity, total hardness, salinity, and organic matter content. Turbidity removal efficiency (TRE) was calculated using Equation (1)

Results and Discussion
In the present experimental study, the effect of bio-coagulant concentration on turbidity removal efficiency, water pH, total alkalinity, total hardness, salinity, and organic matter content was investigated. In addition, the effect of the pH adjustment was taken into account in further experiments as discussed in the following sections. Figure 5 shows the effect of the coagulant dosage on the water turbidity removal efficiency using two coagulants: AV-Powder and AV-H2O. Different coagulant dosages were tested in the ranges 0.1-50 mg/L and 0.1-2 mL/L for AV-Powder and AV-H2O, respectively.

Effect of Bio-Coagulant Dosage on Water Turbidity Removal
The obtained results show that the AV-Powder was responsible for a water turbidity reduction of 28.23% as shown in Figure 5a. This can be explained by the low solubility of AV-Powder in water. Conversely, the water turbidity reduction was more pronounced when AV-H2O was added to water, achieving a reduction rate of 87.84%. Several papers from the international literature have confirmed the positive effect of bio-coagulants on water turbidity reduction when they are added in powder or liquid form; the latter was extracted by distilled water. For instance, the study published by Benalia et al. (2019), proves a turbidity reduction in drinking water by 71.6% and 84.77% from an initial value of 13 NTU when acorn was used as a coagulant in powder and liquid form, respectively [12].
A further study, conducted by Gandiwa et al. (2020), showed that water turbidity reduction was enhanced when liquids extracted from Moringa Oleifera and Cactus Opuntia with distilled water were used as bio-coagulants. The authors demonstrated that water turbidity was reduced to 3.2 and 3 NTU from the initial value of 29 NTU [52].  Figure 6 shows the effect of the coagulant dosage on water pH and alkalinity. There was a slight variation in pH and total alkalinity values when the two coagulants, i.e., AV-Powder and AV-H2O, were added to water. This result can be related to the organic nature of the bio-coagulant [12]. As mentioned in the literature, the activity of natural coagulants is due to the long chains of proteins contained in their molecules, which are responsible for pollutant aggregation and floc formation [48]. On the contrary, metal-based coagulants hydrolyze in water and release H + ions, resulting in a pH decrease. For such reason, coagulation using chemical-based coagulants requires pH adjustment after coagulation, thus increasing the costs of chemical reagents used in water treatment [53].  [20,[54][55][56]. Figure 7a,b show the effect of coagulant dosage on total hardness variations in treated water when AV-Powder and AV-H2O were used. The sum of the magnesium (Mg 2+ ) and calcium (Ca 2+ ) ion equivalent concentration represents the total hardness. Such a value was almost independent of the coagulant dosage. This result is due to absence of any interaction between ions (Ca 2+ and Mg 2+ ) responsible for the total hardness and the bio-coagulant used.

Effect of Bio-Coagulant Dosage on Water pH, Total Alkalinity, total Hardness, and Salinity
Concerning the water salinity content, its value depends on the concentration of the main dissolved ions, calcium (Ca 2+ ), magnesium (Mg 2+ ), sodium (Na + ), chlorides (Cl -), sulfates (SO4 2-), and bicarbonates (HCO3 -). According to Figure 7c,d, the water salinity content generally remained stable when the bio-coagulant dosage was changed.   Figure 8 shows the effect of bio-coagulant dosage on organic matter concentration, and it can be noticed that the concentration of organic matter increased in treated water. This can be due to the natural organic origin of the bio-coagulant that partially dissolved in the aqueous solution during raw water treatment. Actually, the organic matter content after coagulation was higher than the initial value.

Effect of Bio-Coagulant Dosage on Organic Matter Concentration
Similar trends in organic matter variations were found by other researchers, such as

Effect of pH on the Water Turbidity Reduction
The coagulation-flocculation-sedimentation process performance is affected by many parameters, such as coagulant type and dosage, initial water turbidity, pH, temperature, water chemical composition, etc. Among them, pH plays a crucial role as coagulants often are associated with electric charges [59].
Therefore, the coagulation process performance was investigated at different pH values (from 5.5 to 8) and at an initial turbidity of 13 NTU, with the optimal dosages (10 mg/L of AV-Powder and 0.1 mL/L of AV-H2O). pH was adjusted by adding NaOH (1.0 M) or HCl (1.0 M). Results from this study are shown in Figure 9: the most promising process performance in water turbidity reduction was obtained at a pH value of 6 by using AV-H2O and AV-Powder. According to the obtained results, at the optimal pH value, the water turbidity was reduced by 53.53% and 88.23% with AV-Powder and AV-H2O, respectively.   Turbidity reduction (%) pH AV-poudre AV-H20 Table 2 shows the effectiveness of some organic coagulants, in powder as well as liquid form, to remove turbidity from water. Such values are compared to those obtained from the present study.

Conclusions
All the experiments conducted in this study were performed on natural raw water with an initial turbidity of 13 NTU obtained from a drinking water treatment plant. In this work, two forms of bio-coagulants obtained from Aloe vera were used. The obtained results show that the turbidity was reduced to 6.0 NTU at pH 6 and to 1.42 NTU at pH 7.5 when AV-Powder and AV-H2O were used, respectively. It can be highlighted that the residual turbidity from AV-H2O addition was less than 5 NTU and fulfills the quality standards of the Algerian drinking water law [37]. Moreover, the use of natural coagulants in water treatment had no significant effect on several parameters such as pH, total hardness, total alkalinity, and salinity, whereas the concentrations of organic matter increased.
This experimental study has proved that Aloe vera can be successfully used as a natural coagulant for drinking water treatment. Both AV-Powder and AV-H2O can improve the final drinking water quality. Therefore metal-based coagulants, such as Aluminum sulfate, Ferrous sulfate, Ferric Chloride, etc., can be replaced with natural coagulants without reducing the process performance.
This study opens new perspectives of research on the development of natural coagulants to treat surface water, groundwater, wastewater, and industrial water, as well as on extraction processes to obtain liquid coagulants from Aloe vera, and finally on the solvent recovery procedure after extracting process. However, further investigations in pilot and full-scale plants are necessary in addition to a cost-benefit analysis to comprehensively understand the potential of bio-coagulants in water treatment.
Funding: This research received no external funding.

Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.

Conflicts of Interest:
The authors declare no conflict of interest.