Phytoremediation of Aluminum and Iron from Industrial Wastewater Using Ipomoea aquatica and Centella asiatica

: Heavy metals accumulation from industrial wastewater even at low concentrations can cause long term cumulative health e ﬀ ects. To overcome this problem, phytoremediation is an alternative method to treat industrial wastewater. In this study, Ipomoea aquatica and Centella asiatica were used as phytoremediation plants for removing aluminum (Al) and iron (Fe) from industrial wastewater. The results showed that the regression value ( R 2 ) for all metal concentrations (mg / L) over treatment day is positive and similar to R 2 = 1. This result indicated that the metal concentration exhibits a good relationship for reﬂecting the decrease in the metal concentration with the proportion of treatment day. It was found that I. aquatica accumulates higher Al and Fe contents than C. asiatica . The translocation factor of both plants was found to be greater than 1, implying that both plants can accumulate and extract heavy metals from industrial wastewater.


Introduction
The rapid increase in industrial activities and the increase in the human population have resulted in the increase in water, air, and soil pollution [1,2]. Hence, some efforts to conserve and preserve the environment are required. Untreated effluents resulting from industrial activities, which contain a high content of heavy metals, can pollute water sources [3,4]. Morais et al. [5] and Shuhaimi et al. [6] have independently reported that heavy metals are the main environmental pollutants, and refining, smelting, and plating intermediate industries represent one of the contributors.
In general, industrial wastewater is often treated by a combination of physical, chemical, and biological treatment methods. Such a combined treatment can guarantee the maximum elimination of various pollutants, such as organic and inorganic pollutants. However, the treatment methods are expensive, which is not feasible in the industrial sector. A number of treatment methods have been developed to remove heavy metals from wastewater [7][8][9][10]. Phytoremediation is an alternative technique that can be employed to treat industrial wastewater for reducing water pollution [11].
As a more cost-effective, environmentally friendly technology, phytoremediation is a nature-based treatment, particularly employed for treating contaminated soil, groundwater, and surface water [12].
reported by Syuhaida et al. [25]. Seedlings were collected from a selected waterbody at Universiti Kebangsaan Malaysia (UKM), Bangi, Selangor. The seedlings were washed and soaked in distilled water, followed by sowing on cotton wool. Cotton wool was used as a platform to float plants in the tank, and this method also was applied for wastewater treatment in a synthetic reservoir. Figure 1 shows two reservoirs with a dimension of 1 m × 0.5 m × 0.30 m, which were used to grow I. aquatica and C. asiatica. Thirty-gram samples of each plant were placed in the reservoir filled with 6 L of wastewater for the simulated extraction of heavy metals.
Appl. Sci. 2020, 10, x FOR PEER REVIEW 18 of 18 plants in the tank, and this method also was applied for wastewater treatment in a synthetic reservoir. Figure 1 shows two reservoirs with a dimension of 1 m × 0.5 m × 0.30 m, which were used to grow I. aquatica and C. asiatica. Thirty-gram samples of each plant were placed in the reservoir filled with 6 L of wastewater for the simulated extraction of heavy metals. This section may be divided by subheadings. It should provide a concise and precise description of the experimental results, their interpretation as well as the experimental conclusions that can be drawn. The growth of seedlings took about 20 days in an open environment to ensure optimum growth. The samples were subsequently transferred into a secondary reservoir filled with wastewater. Next, the samples were subjected to the selected treatment at 24°C for 10 days, as has been reported by Ton et al. [31]. At this stage, the water quality and plant conditions were monitored and recorded. A total of 48 samples for heavy metal extraction were obtained, including 12 samples of water for the determination of the heavy metal content in water after the treatment. The plants were harvested, and the water was analyzed at time intervals of 0, 2, 4, 6, 8, 10 days after exposure for 10 days, as has been reported by Ladislas et al. [32].

Harvesting and Sample Preparation
In this study, the plants in each reservoir were harvested according to the specified time. Then, the plants were rinsed with tap water, followed by distilled water, to wash any remaining dirt and heavy metal residue from the shoots and roots, followed by drying on a filter paper. Figure 2 shows the harvested and separated plants. The wet sample weight was determined on a digital weighing scale, and the reading was recorded. The plants were cut into three parts: stems, roots, and leaves. Then, the samples were dried in an oven at 70-85°C until a constant dry sample weight was obtained. The samples were dried for 2 days to achieve constant weight, and the reading was recorded. The samples were crushed and ground into a fine paste according to the roots, stems, and leaves by using a mortar and pestle. The samples were subjected to hot plate acid digestion.

Industrial wastewater Industrial wastewater
Centella asiatica Ipomoea aquatica This section may be divided by subheadings. It should provide a concise and precise description of the experimental results, their interpretation as well as the experimental conclusions that can be drawn.
The growth of seedlings took about 20 days in an open environment to ensure optimum growth. The samples were subsequently transferred into a secondary reservoir filled with wastewater. Next, the samples were subjected to the selected treatment at 24 • C for 10 days, as has been reported by Ton et al. [31]. At this stage, the water quality and plant conditions were monitored and recorded. A total of 48 samples for heavy metal extraction were obtained, including 12 samples of water for the determination of the heavy metal content in water after the treatment. The plants were harvested, and the water was analyzed at time intervals of 0, 2, 4, 6, 8, 10 days after exposure for 10 days, as has been reported by Ladislas et al. [32].

Harvesting and Sample Preparation
In this study, the plants in each reservoir were harvested according to the specified time. Then, the plants were rinsed with tap water, followed by distilled water, to wash any remaining dirt and heavy metal residue from the shoots and roots, followed by drying on a filter paper. Figure 2 shows the harvested and separated plants. The wet sample weight was determined on a digital weighing scale, and the reading was recorded. The plants were cut into three parts: stems, roots, and leaves. Then, the samples were dried in an oven at 70-85 • C until a constant dry sample weight was obtained. The samples were dried for 2 days to achieve constant weight, and the reading was recorded. The samples were crushed and ground into a fine paste according to the roots, stems, and leaves by using a mortar and pestle. The samples were subjected to hot plate acid digestion.

Hot Plate Acid Digestion and Atomic Absorption Spectrometry
First, 1 g of sample was added into a porcelain dish and dried in a furnace at 80 • C. After cooling, the samples were digested in 5 mL of a 10% hydrochloric acid (HCl) solution. Next, the porcelain dish was placed on the hot plate, and samples were heated to obtain a clear solution. Then, 10 mL of 20% nitric acid (HNO 3 ) was added, and the samples were heated at 100 • C for 1 h. Then, the digested solution was cooled to room temperature and filtered by using a 0.45-µm cellulose filter paper. The filtered samples were purified with Millipore water to obtain a concentrated solution of 50 mL. The samples were then analyzed by atomic absorption spectrometry (AAS, AA-6800 Shimadzu, Japan). AAS was employed to determine the heavy-metal concentration of both plants as follows (Equation (1)):

Hot Plate Acid Digestion and Atomic Absorption Spectrometry
First, 1 g of sample was added into a porcelain dish and dried in a furnace at 80°C. After cooling, the samples were digested in 5 mL of a 10% hydrochloric acid (HCl) solution. Next, the porcelain dish was placed on the hot plate, and samples were heated to obtain a clear solution. Then, 10 mL of 20% nitric acid (HNO3) was added, and the samples were heated at 100°C for 1 h. Then, the digested solution was cooled to room temperature and filtered by using a 0.45-μm cellulose filter paper. The filtered samples were purified with Millipore water to obtain a concentrated solution of 50 mL. The samples were then analyzed by atomic absorption spectrometry (AAS, AA-6800 Shimadzu, Japan). AAS was employed to determine the heavy-metal concentration of both plants as follows (Equation The extracted heavy metals were analyzed by AAS by the manual injection method. The The extracted heavy metals were analyzed by AAS by the manual injection method. The accuracy of AAS measurements was verified by using a prepared solution and blank according to each heavy metal group. Validation was performed by calibration using a blank. A calibration curve was obtained, reflecting the standards of the heavy metal solution group.

Translocation Factor and Accumulated Heavy Metal Amount
TF is a ratio of the ability of plants to translocate heavy metals from roots through shoots [33]. TF < 1 indicates that heavy metals are accumulated in the roots and vice versa. TF is expressed as follows (Equation (2)): Heavy-metal concentration of stems (mg/kg) Heavy-metal concentration of roots (mg/kg) The amount of heavy metals accumulated in plants was determined from the changes in shoots, stems, and roots, which was measured according to the biomass that can be harvested under controlled conditions [32].

Statistical Analysis
Data were collected and sorted by using IBM SPSS Statistics version 22 software. ANOVA test was conducted to measure the difference in the amount of the heavy metals accumulated in the leaves, stems, and roots from day 0 until day 10 of treatment. Analysis was also conducted to compare changes in the heavy metal accumulation of the leaves, stems, and roots and the effect of the selected plant species on the heavy metal content of water.

Quality Control
All of the apparatuses used in this study were soaked overnight in a 5% HNO 3 solution, followed by rinsing with distilled water and drying before use. For each measurement, the reading was measured three times to obtain an average value. For the heavy-metal concentration analysis, 15 mL of each sample solution was added into a vial for flame atomic absorption spectroscopy analysis. The amount of the absorbed plants was analyzed on an AAS (AA-6800 Shimadzu, Japan). The HNO 3 solution was used as a blank for the plant sample, which was digested without additional plant materials. The samples were measured in triplicate for each injection by AAS, and the analysis was conducted for every 2-3 samples.

Initial Characteristics of Wastewater
Wastewater was collected from the paper mill at the effluent channel, which is the final channel before being discharged into the waterbody. The in-situ parameters were recorded by using a YSI multiprobe, whereas other physico-chemical analyses were determined in the laboratory. Then, 1 L of the wastewater sample was collected and analyzed by inductively coupled plasma mass spectrometry for the determination of the heavy metal content. Results indicated that the heavy metal content is in compliance with the industrial effluent discharge standards as stated in the Environmental Quality (Sewage and Industrial Effluents) Regulation. Table 1 summarizes the results obtained from the analysis of the physico-chemical and heavy metal contents. According to the results obtained herein, Al and Fe were selected for the simulation of wastewater remediation to examine the ability of the selected aquatic plants to extract heavy metals. Then, wastewater was treated by using 30 g of I. aquatica and C. asiatica in a 6-L reservoir in the open air for 10 days. Figure 3 shows readings for the Al content accumulated by I. aquatica for 10 days of treatment. Readings recorded on day 0, 2, 4, 6, 8, and 10 were~0.30, 0.13, 0.12, 0.10, 0.08, and 0.04 mg/L, respectively. On the final day of the treatment, the heavy-metal concentration of the wastewater decreased up to 87%. A significant drop was recorded on the second day of treatment, with~57%.   The present study found evidence of the ability of C. asiatica and I. aquatica as phytoremediation plants for removing Al and Fe. These results were in good agreement with those reported previously, indicative of the ability of C. asiatica and I. aquatica for removing various pollutants. C. asiatica and I. aquatica were assessed in previous studies for its ability to remove various pollutants and water quality parameters, e.g., chemical oxygen demand (COD), biochemical oxygen demand (BOD), and ammoniacal nitrogen (NH3-N) [34], cadmium [35], lead [36], nitrate [37], total suspended solid (TSS) [38,39], copper [40], phosphate [41,42], nitrogen and phosphorus [43], and chromium (VI) [44]. Based on the previous studies, C. asiatica was found to be more efficient to remove most of the pollutants   On the final day of treatment, the heavy-metal concentration decreased by 83%. A significant drop was recorded on the second day of treatment, which was~33%. Figure 3 also shows readings for the Fe content for 10 days of treatment. Readings recorded on day 0, 2, 4, 6, 8, and 10 were~1.09, 0.90, 0.83, 0.78, 0.79, and 0.60 mg/L, respectively. The heavy-metal concentration decreased by 45% on the final day of treatment. A significant drop was recorded on the second day of treatment, which was~17%.

Centella asiatica
The present study found evidence of the ability of C. asiatica and I. aquatica as phytoremediation plants for removing Al and Fe. These results were in good agreement with those reported previously, indicative of the ability of C. asiatica and I. aquatica for removing various pollutants. C. asiatica and I. aquatica were assessed in previous studies for its ability to remove various pollutants and water quality parameters, e.g., chemical oxygen demand (COD), biochemical oxygen demand (BOD), and ammoniacal nitrogen (NH 3 -N) [34], cadmium [35], lead [36], nitrate [37], total suspended solid (TSS) [38,39], copper [40], phosphate [41,42], nitrogen and phosphorus [43], and chromium (VI) [44]. Based on the previous studies, C. asiatica was found to be more efficient to remove most of the pollutants compared to I. aquatica. By comparing the efficiency rate of C. asiatica and I. aquatica obtained from the present study, a similar pattern of results was found in Ismail et al. [3] that have identified S. grossus as a hyperaccumulator in the phytoremediation of Al and Fe. It should be noted that, however, the removal percentage rate of pollutants varies depending on the type of wastewater, duration of treatment, concentration of pollutant, etc.  a) Ipomoea aquatica Figure 4 shows the contents of Al in the leaf, stem, and root sections for day 2 to 10 of treatment by I. aquatica. Readings recorded for the leaf section were 973, 332, 261, 27, and −15 mg/kg. Overall, ~1577 mg/kg of heavy metals was accumulated at 10 days of treatment, with an average of 157 mg/kg per day. The highest amount of heavy metals was recorded on the second day of treatment, which was ~62%. On the final day of treatment, the percentage of heavy metals in leaves was the lowest, (a) Ipomoea aquatica Figure 4 shows the contents of Al in the leaf, stem, and root sections for day 2 to 10 of treatment by I. aquatica. Readings recorded for the leaf section were 973, 332, 261, 27, and −15 mg/kg. Overall, 1577 mg/kg of heavy metals was accumulated at 10 days of treatment, with an average of 157 mg/kg per day. The highest amount of heavy metals was recorded on the second day of treatment, which was 62%. On the final day of treatment, the percentage of heavy metals in leaves was the lowest, which was 0.01% of the total amount of heavy metals.

Amount of Heavy Metals in Leaves, Stems, and Roots
The contents of heavy metals recorded for the stem section were 3055, 1157, 130, 88, and 18 mg/kg. In total,~4447 mg/kg of heavy metals was recorded for day 2 to 10 of treatment, with an average content of 448 mg/kg per day. The highest amount of heavy metals was recorded on the second day, which was~69% of the total amount of heavy metals, while the percentage of heavy metals was the lowest on the final day, which was 0.01%. The contents of heavy metals in the root section were 2859, 614, 556, 321, and 175 mg/kg. About 4524 mg/kg of heavy metals was recorded for 10 days of treatment, with an average amount of~452 mg/kg per day. The highest heavy metal content was recorded on the second day, which was~49%. On the final day of treatment, the percentage of heavy metals in leaves was the lowest, which was 0.08% of the total amount of heavy metals. Figure 4 also shows the contents of Fe in the leaf, stem, and root sections for day 2 to 10 of treatment. The contents of heavy metals accumulated in leaves were 113, 190, 499, −56, and 75 mg/kg. Overall,~821 mg/kg of heavy metals was accumulated at 10 days of treatment, with an average of 82 mg/kg per day. The highest content of heavy metals was recorded on the sixth day, which was~61% of the total amount of heavy metals, while the percentage of heavy metals was the lowest on the eighth day, which was 0.07%. The contents of the heavy metals accumulated in the stem section were 1606, 197, 55, −9, and −83 mg/kg. About 1767 mg/kg of heavy metals was recorded for 10 days of treatment, with an average amount of~177 mg/kg per day. The highest content of heavy metals was recorded on the second day, which was~91%. On the final day of treatment, the percentage of heavy metals in leaves was the lowest, which was 0.05% of the total amount of heavy metals. Readings recorded for the root section were 2198, 1089, 824, 621, and 352 mg/kg. Overall,~5084 mg/kg of heavy metals was accumulated at 10 days of treatment, with an average of 508 mg/kg per day. The highest content of heavy metals was recorded on the second day, which was~43%. On the final day of treatment, the percentage of heavy metals in leaves was the lowest, which was 0.07% of the total amount of heavy metals.
(b) Centella asiatica Figure 4 shows the contents of Al in the leaf, stem, and root sections for day 2 to 10 of treatment by C. asiatica. Readings recorded for the leaf section were 3205, 690, 125, 111, and 72 mg/kg. In total, 4202 mg/kg of heavy metals was accumulated at 10 days of treatment, with an average of 420 mg/kg per day. The highest content of heavy metals was recorded on the second day, which was~76%. On day 10, the percentage of heavy metals in leaves was the lowest, which was 0.02% of the total amount of heavy metals.
The contents of heavy metals recorded for the stem section were 2661, 1364, 303, 122, and 138 mg/kg. About 4588 mg/kg of heavy metals was recorded for 10 days of treatment, with an average amount of 459 mg/kg per day. The highest content of heavy metals was recorded on the second day, which was 58% of the total amount of heavy metals, while the percentage of heavy metals was the lowest on the eighth day, which was 0.03%. The contents of heavy metals in the root section were 1497, 210, 2, −338, and −440 mg/kg. About 931 mg/kg of heavy metals was recorded for 10 days of treatment, with an average amount of 93 mg/kg per day. The highest content of heavy metals was recorded on the second day, which was >160%. On the final day of treatment, the percentage of heavy metals in leaves was the lowest, which was 47% of the total amount of heavy metals. Figure 4 also shows the contents of Fe in the leaf, stem, and roots sections for day 2 to 10 of treatment. The contents of heavy metals accumulated in leaves were 667, 242, 169, 177, and 217 mg/kg. About 1472 mg/kg of heavy metals was accumulated at 10 days of treatment, with an average of 147 mg/kg per day. The highest amount of heavy metals was recorded on the second day of treatment, which was 45% of the total amount of heavy metals, while the percentage of heavy metals was the lowest on the sixth day, which was 11%. The contents of heavy metals accumulated in the stem section were 268, 357, 55, 65, and 204 mg/kg. At 10 days of treatment, 948 mg/kg of heavy metals was recorded, with an average amount of~95 mg/kg per day. The highest amount of heavy metals was recorded on the fourth day, which was 38%. On the sixth day of treatment, the percentage of heavy metals in leaves was the lowest, which was 0.06% of the total amount of heavy metals. Readings recorded for the root section were −214, −245, −451, −484, and −557 mg/kg. Overall,~1950 mg/kg of heavy metals was accumulated at 10 days of treatment, with an average of 195 mg/kg per day. The highest amount of heavy metals was recorded on the 10th day, which was~29%, while on the second day of treatment, the percentage of heavy metals in leaves was the lowest, which was 11% of the total amount of heavy metals. Table 2 shows the TF of aluminum and iron, reflecting the ability of I. aquatica to extract heavy metals from roots to shoots. Results revealed that TF < 1 for I. aquatica, indicating that heavy metals are accumulated in the roots. For Al accumulation, the highest TF was observed on the fourth day of treatment, which was 0.54, and the lowest TF was observed on the final day of treatment, which was 0.09. For Fe accumulation, the highest TF of 0.61 was observed on the sixth day of treatment, and the lowest TF of 0.09 was observed on the eighth day.  Table 2 also shows the TF of aluminum and iron, reflecting the ability of C. asiatica to extract heavy metals from roots to shoots. Results indicated for Al, TF > 1, and the highest TF of 55 is observed on the sixth day of treatment. The lowest TF of 0.16 was observed on the 10th day of treatment. For Fe accumulation, TF < 1 was observed for C. asiatica, implying that heavy metals are accumulated in the roots or other parts of the plant. The highest TF of 3.11 was observed on the second day of treatment, and the lowest TF of 0.37 was observed on the eighth day.

Translocation Factor of Heavy Metals
The results showed that there were significant differences in iron translocation factors in the stems by Ipomoea aquatica compared to leaves. For Centella asiatica, a significant value was obtained for the amount of iron in the leaves and stems. For aluminum, Ipomoea aquatica recorded a significant value for metal translocation from roots to leaves and from roots to stems. However, no significant value was recorded for the aluminum by Centella asiatica.
Basically, most of the heavy metals accumulate in plant roots because the surface area of root hairs is greater than those of stems and leaves for the absorption of heavy metals [14]. Besides, roots are the first part of a plant that absorb heavy metals from soil or a waterbody before their distribution to the other parts. In addition, compared to stems and leaves, roots are the only covered part, and they are in contact with the growth medium. Hence, roots demonstrate more potential to absorb nutrients and pollutants due to their plant physiology.
Ismail et al. [3] found that the Scirpus grossus accumulated Al and Fe from mining wastewater, implying the potential of Scirpus grossus as a hyperaccumulator in the phytoremediation of Al and Fe. Syabani et al. [44] have reported bioconcentration factor (BCF) < 1 for C. asiatica. However, TF > 1 was observed for the accumulation of Pb, Cu, and Zn, indicating that even though C. asiatica is not a potential hyper accumulator or a phytostabilization agent, this species is tolerant to Pb, Cu, and Zn. A BCF range from 0.01 to 0.10 for Zn and Pb revealed that C. asiatica is a weak accumulator for these metals. Based on the study conducted by Yoon et al. [45], the highest ratio of lead (Pb) concentration reflects the accumulation from roots to stems, while the lowest ratio reflects accumulation from roots to leaves. The study determined the concentrations of Pb and Cd accumulated in I. aquatica at Laguna de Bay Filipina. Results indicated that the Pb concentration of the plant section follows the order of roots ≥ vertical stems ≥ horizontal stems ≥ leaves. The ratio is 0.82 stems/roots and 0.36 leaves/roots.
A graph of the heavy-metal concentration (mg/L) over treatment days revealed that the regression (R2) value is positive and close to R2 of 1: R2 for Al is 0.7607, and R2 for Fe is 0.8239 for I. aquatica, and R2 for Al is 0.8182 and R2 for Fe is 0.8862 for C. asiatica. This result indicated that the heavy-metal concentration exhibits a good relationship for reflecting the decrease in the heavy-metal concentration over the treatment day. If the regression was similar to R2 = 1, it verified the relationship between the variables and vice versa, if R2 < 1.
In addition, the TF of roots to stems was calculated to determine the location of heavy metal accumulation as an alternative for the accumulation of heavy metals from roots to leaves. Table 3 summarizes the TF of aluminum and iron, reflecting the ability of I. aquatica to extract heavy metals from roots to stems. The results indicated that TF < 1 for I. aquatica, indicating that most of the heavy metals are still accumulated in the roots. For Al accumulation, the highest TF was observed on the second day of treatment, which was 1.89, while the lowest TF was observed on the final day, which was 0.10. For Fe accumulation, the highest TF was observed on the second day of treatment, which was 0.73, and the lowest TF was observed on the eighth day, which was 0.01. Table 3. Translocation factor for Ipomoea aquatica and Centella asiatica from root to stem (R/S).  Table 3 also shows the TF of aluminum and iron, reflecting the ability of C. asiatica to extract heavy metals from roots to stems. Results indicated that TF > 1 for Al accumulation, and the highest TF is observed on the sixth day of treatment, which is 133. The lowest TF was obtained on the final day, which was 0.31. For Fe accumulation, TF < 1, and it is negative, indicating that most of the heavy metals are not accumulated in the roots due to the amount of heavy metals in the roots was negative.

Ipomoea aquatica
Results obtained for the TF of heavy metals via their accumulation in leaves and stems by I. aquatica revealed that a significant TF variation is observed for the stem section compared to the leaf section. For Al accumulation, I. aquatica exhibited significant TF of heavy metals from roots to leaves (R/L) and from roots to stems (R/S), while for C. asiatica, a significant value was obtained for the amount of Fe in leaves and stems. No significant value for Al accumulation was observed, even though TF for stems (TF = 132) and leaves (TF = 54) was different.

Conclusions
In this study, Ipomoea aquatica and Centella asiatica were selected as phytoremediation plants based on their rapid growth rate and lower maintenance costs. Results revealed that I. aquatica exhibits higher Al content than C. asiatica. It was also found that the average content of Fe accumulated by I. aquatica was greater than that accumulated by C. asiatica. Besides, with respect to the wastewater treatment by I. aquatica, a significant variation between the TF of Fe in the stem compared to the leaf section was observed, indicating that most of the Fe accumulate in stems. For C. asiatica, TF < 1 for Fe in leaves and stems, and it was negative, indicating that heavy metals are not accumulated in roots because the amount of heavy metals in the roots was negative. TF > 1 for Al accumulated in leaves and stems of both plant species, indicating that both plants can accumulate and extract heavy metals from wastewater.