Potential of Sunflower to Extract Heavy Metals from Leachate

The effectiveness of sunflower to extract harmful heavy metals from landfill leachate using phytoremediation technique was studied in this paper. A case study of Phursungi garbage dump yard located in Phursungi village in Pune, India was taken for this study as the residents have to deal with contaminated water throughout the year which induces diseases like dysentery, cholera, he-patitis, and heavy metal poisoning related diseases. There are various methods in phytoremediation, among which phytoextraction was used for this study. An experiment was carried out to test the removal efficiency of BOD, COD, TS, and heavy metals namely As, Hg and Pb from the soil by sunflowers with and without the addition of Vesicular Arbuscular Mycorrhizal (VAM) fungi. These fungi were used in soil to accelerate the process of extraction. Leachate diluted with 60% water was supplied such that the Hydraulic Retention Time (HRT) was more than 15 days (19 days achieved). The values of contaminants were checked for permissible limits according to standards for land disposal of treated leachates given in Municipal Solid Waste Management (MSW) rules, 2016 and Central Pollution Control Board (CPCB), India. The removal efficiency of Pb from soil was almost the same for plants supplied with VAM (67.92%) and plants without VAM (66.67%). Arsenic (As) removal efficiency was 100% for plants with and without VAM. For mercury (Hg), it was more for plants without VAM (96.29%) than plants with VAM (77.78). Thus, it was concluded that VAM was ineffective. Cd and Cr concentrations in leachate samples were observed to be within the permissible limits. Hence, they were discarded for calculation of removal efficiency.


Introduction
About 900 -1200 tons of solid waste is dumped every day at the Phursungi-Uruli Devachi area in Pune for past 20 years which has been causing health issues to the residents due to contamination of the air and water. The groundwater gets polluted due to percolation of leachate into the ground [1]. It can cause skin irritation, nausea, and headache, while chronic exposure can lead to anemia, cancer, and coma [2]. The primary purpose of this study was to test the extraction potential of sunflower (Helianthus annuus L.) for mercury (Hg), lead (Pb), chromium (Cr), cadmium (Cd) and arsenic (As) from the leachate as they are some of the most toxic heavy metals for the human body. The sunflower plant was considered for this study because it has shown high extraction potential of Pb, Cr, and Cd as it is a hyperaccumulator [3] and has potential use as bioenergy crop [4]. Proper selection of tolerant cultivars of sunflowers along with agronomic practices may be an effective strategy for the phytomanagement of soils contaminated with heavy metals [5]. It was found that, due to greater density, Pb uptake in sunflowers was less than Hg and Cr; but its phytotoxicity was found to be more than Hg and Cr [6]. Another study showed that the metal accumulation efficiency of the sunflower plant increased with increasing metal concentration and exposure period. Sunflower has been found to have high biomass at varying exposure concentrations which resulted in increased uptake of heavy metals. Maximum levels of Zn, Cd, and Pb were observed in roots followed by shoots [7]. There are many factors affecting the extraction of heavy metals by plants, and hence it is a challenge to find such species. It was found that the transfer of arsenic (As) from soil to plant is low for most plant species because of several reasons like low bioavailability of As in soil, restricted uptake by plant roots, limited translocation of As from roots to shoots, and As phytotoxicity at relatively low concentrations in plant tissues [8]. Vesicular Arbuscular Mycorrhizal (VAM) was used to accelerate the extraction of heavy metals. It was found that Arbuscular Mycorrhizal Fungi (AMF) may enhance phytoremediation, especially phytoextraction and phytostabilization, by reducing heavy metal stresses on plants, increasing heavy metal uptake and affecting translocation of metals within plants [9]. A VAM fungus enhances the nutrient and water uptake capacity of the plant and helps store it. It also makes the contaminants bioavailable for plant uptake. These fungi are extensions of the roots and work better than the roots themselves and stimulate the growth of roots. VAM is an Endo type of fungus which stores the contaminants intrinsically and transports them to the plant shoots (phytoextraction) [10].
In a past research conducted, landfill leachate was treated using sunflower by horizontal and vertical flow with or without recirculation in a 130 liter tank. The experiment showed removal efficiency greater than 50% for COD, greater than 60% for nitrogen and greater than 90% for phosphorous. Leachate was successfully tested as an alternative fertilizer for plants and did not inhibit biomass development. The vertical flow tank proved the most efficient in treating the lea- used for the experiment are discussed below. Figure 1 shows the arrangement of materials in the reactor along with the location of the perforated pipe with a drain.

Reactors
For the experiment, two plastic reactors, 70 liters each (R1 and R2) were used. A

Aggregate
At the bottom of reactors, two layers of gravel were filled with size of 1.

Soil & Net Filter
For each reactor, 50 kg of soil was used. Coco peat, cow dung fertilizer, and Lindane (BHC) powder were mixed with the soil. The ratio of coco peat and cow dung fertilizer in soil by volume was 1:0.25:0.25. Four tablespoons of the Lindane powder mixed in 50 kg of soil. Coco peat was used to make the soil light and aerated whereas BCG powder was used as a pesticide. Soil layer was filled up to 20 cm above the layer of aggregate and dried leaves, stem and roots. This was done for both the reactors. The grain size distribution of the soil was carried out using sieve analysis. Figure 2 shows the grain size distribution curve and Table   1 shows the classification of soil type according to grain size distribution. A tightly stitched net was used to wrap the perforated pipe to prevent the intrusion of soil in the pipe.

Vesicular Arbuscular Mycorrhizal Fungi
It is a type of fungi that enhances the uptake of water and nutrients and helps to store them in the plants. It acts in the rhizosphere and is differentiated accordingly into Endo and Ecto AMF. Endo AMF was used for this experiment as it acts intracellularly with the roots. It also enhances the growth of roots and breaks down the contaminants into soluble forms for plant uptake which are not otherwise broken down.

Parameters to Be Considered
The parameters tested were Biochemical Oxygen Demand (BOD), Chemical Oxygen Demand (COD), Total Solids (TS) and heavy metals namely Cd, Hg, Cr, Pb, and As. Standard procedures for measuring BOD, COD, and TS were adopted. Potassium dichromate and Ferrous ammonium sulfate were used as oxidizing and reducing agents, respectively in the COD test. BOD of three days incubation period at 27-degree celsius was measured. The extraction of contaminants by plants was tested by putting the plants in a muffle furnace and tested for heavy metal concentration using Atomic Absorption Spectroscopy (AAS). Figure 3 shows the arrangement made before the experiment. Figure 4 shows the growth stages of sunflowers.

Calculation of Hydraulic Retention Time (HRT)
It is the average length of time that a compound (in this case, leachate) remains in the storage unit (soil). The HRT has a significant effect on phytoremediation processes. It depends on the plant species used, type of soil, temperature, and wastewater. Scientific literature recommends a minimum HRT of 7 days for landfill leachate phytoremediation applications; but a minimum value of 15 days, resulting from previous experiences with sunflowers, is strongly recommended [6]. The HRT has been calculated by Equation (1).    Table 4 show the concentration of contaminants before supplying  leachate and after collecting the same sample from the outlet of the reactors, respectively; Table 5 shows the initial concentration of heavy metals in soil and Table 6 shows the heavy metal concentrations in soil and plants after application of leachate. The values of contaminants were checked for permissible limits according to Central Pollution Control Board (CPCB) norms for land disposal, India and standards for land disposal of treated leachates given in Municipal Solid Waste Management rules (MSW), 2016, Schedule II, which enlists standards for processing and treatment of solid waste under Part A. The initial soil results serve as the control. It can be seen from Table 5 & Table 6 that the heavy metal concentrations in 60% diluted and raw leachate are not differing much from the concentrations in the initial soil results. In fact, the concentration of Pb has increased drastically which shows that there was a high concentration of Pb in the soil originally. Therefore, a control plant for testing the extraction of heavy metals supplied with normal tap water was not considered in the study. The soil on the roots of the plants was collected to represent the soil samples for all cases. For calculating the heavy metal removal efficiency according to extraction by plants from the soil, the average concentration of heavy metals from soil sample 1 and 2 and concentration in plants with and without VAM from Table 6   The BOD, COD, and TS were calculated according to outlet sample results and the removal efficiency was more for plant samples without VAM than with VAM. Cr and Cd were discarded because they were in permissible limits. The removal of BOD, COD, and TS from the system (reactor) based on the samples collected from the outlet is shown in Figure 5(a). Figure 5(b) & Figure 5(c) shows the removal efficiency of heavy metals in terms of extraction by plants and outlet samples, respectively.

Effect of pH
The initial pH of the leachate remained almost neutral as the leachate had reached in its methanogenic stage while collecting it from the site. It was observed to be almost the same after the experiment. According to MSW rules, 2016, the pH was observed to be in permissible limit, i.e., 5.5 to 9.

Effect of BOD
The BOD of leachate reduced considerably when diluted. According to MSW rules 2016, it was observed to be in a permissible limit (<100 mg/l) in the inlet sample itself. BOD removal efficiency for R1 (43.75%) was more than R2 (12.5%) which implies that VAM was ineffective.

Effect of COD
Removal efficiency for R1 (47.5%) was more than R2 (12.5%) which shows that VAM did not work effectively. Also, the overall removal efficiency was poor.
There is no specification for permissible limit for land disposal in the MSW rules, 2016 but the limit for inland surface water (<250 mg/l) was used as it is anticipated to be lesser than the limit for land disposal. Hence, the outlet samples had very high COD concentrations than permitted and removal of COD was unsuccessful.
Effect of TS  Removal efficiency for R1 (13%) was more than R2 (4.84%) which shows that VAM did not work effectively and also the overall removal efficiency was very poor. According to the MSW rules, 2016, the permissible limit for dissolved solids is 2100 mg/l and for suspended solids, it is 200 mg/l. Hence the limit for total solids can be taken as the addition of the suspended and dissolved solids (<2300 mg/l). The outlet samples had very high concentrations of total solids and hence the removal of TS was unsuccessful.

Mercury (Hg) measurement
The mercury concentration in the rainy season was more than the permissible limit for land disposal according to CPCB, India (<0.1 ppm) and that in the summer season it was approximately equal to 0.1 ppm. The Hg concentration in the rainy season should have been lesser as the leachate was observed to be more diluted in this season than summer. Also, the concentration in the inlet sample (60% diluted) was 0.76 ppm, which was more than the sample from which it was

Conclusion
Sunflower is one of the many plants used for phytoremediation because of its capacity to extract and store contaminants. In this study, they grew faster than expected, i.e., in 1.5 months due to leachate application as it acted as a fertilizer, which can be inferred from high BOD content observed in the leachate sample in summer season. The roots of all the plants remained short due to the continuous supply of leachate. There were no signs of damage to the plants. The leachate was odorless and did not cause any nuisance by insects throughout the experiment. Removal efficiencies of BOD, COD and TS from soil based on outlet results for plant samples without VAM (BOD-43.75%, COD-47.5%, TS-13%) were more than samples with VAM (BOD-12.5%, COD-12.5%, International Journal of Geosciences TS-4.84%). The removal efficiency of Hg and As based on outlet results showed a similar trend. This may be least contributed to the action of VAM fungi because, for getting outlet samples for testing, leachate was required to be supplied on the same day, sometime before the collection of outlet samples which may have caused an error because there was not sufficient time for the contaminants to be removed from the system. In fact, the actual removal efficiencies of BOD, COD, and TS would be more than those calculated from the test results because the leachate which was being treated was retained in the system and it was not feasible to obtain it from the outlet. The density of Pb being high and due to prior presence in the soil, its removal efficiency was negative, i.e., it got saturated in the soil. The removal efficiency of heavy metals according to extraction by plants was very high for Hg and As, while for Pb, it was considered good. The VAM was unsuccessful in improving the removal efficiency of heavy metals from the soil because, according to results based on extraction by plants, As removal was the same for both R1 and R2, Hg removal was less for R2 and there was negligible difference between R1 and R2 for Pb removal. The function of VAM was to make the heavy metals bioavailable for the plant so that it can extract the heavy metals easily, which was not observed. Due to calculation constraints for HRT, the concentration of contaminants was high in the soil within a short duration and hence the VAM was not effective. Thus, it is anticipated that a slow rate of application of leachate while practical implementation of the process would solve this problem and also help the plants to sustain longer. Also, there is a need for more evidence in the form of test results for the extraction of these contaminants by sunflowers so that this technique can be implemented practically.