Removal of Heavy Metals from Landfill Leachate Using Horizontal and Vertical Subsurface Flow Constructed Wetland Planted with Limnocharis flava

Transcription

1 International Journal of Civil & Environmental Engineering IJCEE-IJENS Vol: 11 No: Removal of Heavy Metals from Landfill Leachate Using Horizontal and Vertical Subsurface Flow Constructed Wetland Planted with Limnocharis flava Ain Nihla Kamarudzaman, Roslaili Abdul Aziz, and Mohd Faizal Ab Jalil Abstract Heavy metals were present at relatively high concentrations in the landfill leachate. Therefore, the exposure of heavy metals into the environment is great concern due to their serious effects on food chain and furthermore on animal and human health. This study focussed on comparing the efficiency of horizontal and vertical subsurface flow (SSF) constructed wetland in the removal of heavy metals (Fe and Mn) in landfill leachate. Where, it also determines the amount of heavy metals uptake by Limnocharis flava and the amount of heavy metals retained in the soil media. A laboratory-scale study was conducted on SSF constructed wetland systems operated in vertical and horizontal mode. Each system comprises of one planted and one control system. The planted systems namely HP and VP were planted with Limnocharis flava, while the control systems namely HC and VC were left unplanted. The systems operated identically at a flow rate of m 3 /d and HRT of 24.1 hours and 19.7 hours in HSSF and VSSF systems, respectively. The results shows both system performed well in the removal of heavy metals from landfill leachate with the overall removal efficiency ranging from % and % for Fe and Mn, respectively. This research also publicized the suitability of Limnocharis flava to be used in constructed wetland to treat landfill leachate. Index Term Landfill leachate, Constructed wetland, Heavy metals removal, Plant uptake, Soil media I. INTRODUCTION Alandfill, also known as a dump, is a site for the disposal of waste materials by burial and is the oldest form of waste treatment. The main purpose of landfill is to stabilize the waste and to make it hygienic through the use of natural metabolic pathways [1]. Landfill leachate produced from these areas are toxicity, classified as problematic wastewaters and represent a This work was supported by Ministry of Higher Education, Malaysia under Fundamental Research Grant Scheme (FRGS). K. Ain Nihla is with School of Environmental Engineering, Universiti Malaysia Perlis, Jejawi, Perlis, Malaysia (Phone: ; Fax: ; ainnihla@unimap.edu.my). A. A. Roslaili is with School of Environmental Engineering, Universiti Malaysia Perlis, Jejawi, Perlis, Malaysia. A. J. Mohd. Faizal is with Perlis State Department of Environment, 2 nd Floor, KWSP Building, Jalan Bukit Lagi, Kangar, Perlis, Malaysia. dangerous source of pollution for the environment due to its fertilizing and toxic effects [2]. Landfill leachate mainly consists of heavy metals, organics with different biodegradation and inorganic matters such as ammonia, sulphate and cationic metals [3]. However, landfill leachate characteristics were varying depending on the operation type and the age of the landfill. Health problems and environmental pollution are often related to inadequate landfill leachate treatment. Proper collection, treatment and disposal of landfill leachate are necessary to promote better environment and healthful condition. Therefore, the treatment of landfill leachate by natural systems seems to be environmentally sustainable for treatment of many constituents. Constructed wetlands have proven very effective method for the treatment of variety of wastewaters. The environmental benefit treatment of landfill leachate in a constructed wetland includes; decreased energy consumption by using natural processes rather than conventional; efficiently removed many pollutants from wastewater and also enhance the environment by providing a habitat for vegetation, fish and other wildlife [4]. Studies of the long-term use of wetlands for landfill leachate treatment have demonstrated significant economic advantages, mainly through lowered construction, transportation and operation costs [5]. Reference [5] also reported the removal of several metals in treatment wetlands, including aluminium, arsenic, cadmium, copper, iron, manganese, mercury, nickel, silver and zinc. Metals are removed in treatment wetlands by three major mechanisms; binding to soils, sediments, particulates, and soluble organics by cation exchange and chelation; precipitation as insoluble salts, principally sulfides and oxyhydroxides and uptake by plants, including algae and by bacteria. Removals of heavy metal occur mainly through adsorption and precipitation and to a minor extent through plant uptake for some metals. Metals are retained in the soil profile or the sediments or substratum. Metals can precipitate out as sulfides and carbonates, or get taken up by plants [6]. Several studies have demonstrated that constructed wetland systems were very effective to remove and immobilize metals

2 International Journal of Civil & Environmental Engineering IJCEE-IJENS Vol: 11 No: contained in landfill leachate. Reference [7] reported the percentage of Mn removal using free water surface (FWS) constructed wetland planted with Eichhornia crassipes was achieved more than 60% removal. In this study, reference [7] was also remarked that Eichhornia crassipes had shown capability to absorb heavy metals in leachate. It shows that wetland plant plays an outstanding role as a heavy metal decontaminator. Reference [8] reported high heavy metals (Fe and Mn) removal by using vertical subsurface flow (VSSF) planted with S.sumatrensis and S.mucronatus. The reason might be because the effect on using two types of plants such as S.sumatrensis and S.mucronatus in one constructed wetland system which is can increased the removal of heavy metals in landfill leachate. The main objective of this study is to compare the efficiency of horizontal subsurface flow (HSSF) with vertical subsurface flow (VSSF) constructed wetland systems in the removal of heavy metals (Iron (Fe) and Manganese (Mn)) from landfill leachate planted with Limnocharis flava. This study also examines the accumulation of heavy metals in plant tissues and the amount of heavy metals retained in the soil sediments. II. METHODOLOGY A. Leachate Collection and Preparation In this study, the leachate used as feeding substrate was taken from the municipal solid waste landfill (MSWLF) site located at Kampung Padang Siding, Ulu Pauh, Perlis, Malaysia ( N, E). The Padang Siding MSWLF area is about 20 hectares, where its received abundance amount of municipal solid waste from the whole Perlis state with a loading approximately 300 tonnes/day. The landfill leachate was collected at the leachate collection pond and stored in a high density polyethylene (HDPE) bottle. The collected landfill leachate was later diluted with tap water to achieve 25% concentrations, in order to provide an acceptable condition for plant growth. B. Experimental Set up In this study, four laboratory scale constructed wetland systems have been constructed, which consist of two vertical subsurface flows (VSSF) and two horizontal subsurface flows (HSSF) constructed wetland systems. Each system comprises of one planted system and one control system. The planted systems namely VP and HP were planted with Limnocharis flava, while the control systems namely VC and HC were left unplanted. Each of the VSSF and HSSF system consists of a feeding tank, a wetland reactor and settling tank. The wetland reactor and operation characteristics are summarized in Table I. The wetland reactors were constructed using acrylic with the dimension of 0.58 m length, 0.31 m wide, and 0.33 m depth. Both HP and VP reactors were planted with Limnocharis flava with density of 15 peduncles (stem) per reactor, which transferred from a ditch near paddy field in Kampung Sungai TABLE I REACTOR CHARACTERISTICS Total reactor height 0.33 m Total surface area m 2 Total planting area m 2 Weight of gravel used per reactor 35.6 kg Weight of soil per reactor kg Average gravel size mm Average void volume per reactor m 3 Flow rate m 3 /d Hydraulic Retention Time (HRT) per cycle HRT HSSF HRT VSSF Bakau, Perlis, Malaysia. The Limnocharis flava was chosen in this study because of its availability, where it can be commonly found throughout the state of Perlis, Malaysia. It was also chosen due to the fact that it has long fibrous roots that can provide oxygen supply to the media and promote uptake of contaminants. After the transplantation, the wetland reactors (HP and VP) were loaded with tap water to establish the emergent plant. The duration takes 7 days for the acclimatization process, where the readiness of the plant for the actual experimental procedure was illustrated by the healthy leaves and stem and also by the growth of new leaves and inflorescence. The influent flow across the wetland reactors and effluent was collected in a settling tank and manually transferred back into the feeding tank to be re-circulated to the wetland reactors on a daily basis for the whole treatment period. A 20 mm PVC pipe and a 20 mm valve were used to regulate flow. The inlet feeding pipe and perforated holes in each wetland reactors were installed at 0.08 m below the surface of the substrate. The experiments were continuously monitored throughout the whole treatment period. Fig. 1 shows the experimental set up for horizontal and vertical subsurface flow constructed wetland system. C. Analysis of Plant Tissues 24.1 hours 19.7 hours Analysis of the plant tissue was conducted initially before the treatment procedure begin and after the termination of the experiment. This analysis was conducted to determine the uptake of heavy metals by the plant. The method used for the analysis of the plant tissue was Dry Ashing Method [9], where two replicate samples from the planted (HP and VP) reactors were selected and harvested. The plants were cleaned by washing them with tap water followed by distilled water and sorted into leaf, stem (peduncles) and root component. The plants samples were then placed in a porcelain crucible and ashed by heating it overnight in a muffle furnace at 500 C. The ash residue was then cooled and 1 g of each samples (leaf, stem, and root) were weighted and dissolved in 5 ml of 20% hydrochloric acid (HCl) for digestion. The solutions were then shaken for four hours with orbital shaker. It was later filtered through an acid-washed filter paper into a 50 ml

3 International Journal of Civil & Environmental Engineering IJCEE-IJENS Vol: 11 No: VC, and VP) for the final characterization study. The samples were analysed using X-Ray Fluorescence (XRF) Spectrometer. Prior to the XRF analysis, the soil samples were oven dried at 105 C overnight, grinded and sieved to obtain soil samples size of less than 70 µm and pressed into pellet by using hydraulic Pellet Press Model PP 25. The preparation of the soil samples as shown in Fig. 3. Fig. 1. Initial experimental set up Fig. 3. Preparation for soil analysis. (a) soil sample after oven dried; (b) soil sample after sieved; (c) soil pellet; (d) XRF spectrometer unit III. RESULT AND DISCUSSION Fig. 2. Samples preparation for plant analysis (a) Leaf sample; (b) root sample; (c) stem sample; (d) orbital shaker volumetric flask. The solutions were then diluted to volume with deionised water and mixed well. The solution was analyzed for heavy metals according to United State Environmental Protection Agency (USEPA) approved methods, by using HACH DR 2800 spectrophotometer. Fig. 2 shows plant digestion for analysis of heavy metals in plant tissues. D. Analysis of Soil Composition The analysis on the soil composition was conducted to identify the initial and final composition of the soil used in both HSSF and VSSF constructed wetland system. Only one sample was used for the initial characteristic, while three replicate soil samples were collected at different depth (surface, mid depth, and bottom) of each reactors (HC, HP, A. Heavy metals Removal The initial leachate characterization study was conducted to determine the most significant heavy metals that will be the parameter of interest. The results of initial leachate characterization study are summarized in Table II. By referring to Table II, it can be clearly observed that the leachate sample was exhibited significant value of heavy metals content, among which the highest concentration was recorded for Fe and Mn with 11.6 mg/l and 10.6 mg/l, respectively. TABLE II RESULT OF INITIAL LEACHATE CHARACTERISTICS Parameter Unit Value Manganese (Mn) mg/ L 10.6 Nickel (Ni) mg/ L Calcium (Ca) mg/ L ND Magnesium (Mg) mg/ L Zinc (Zn) mg/ L ND Iron (Fe) mg/ L 11.6 Copper (Cu) mg/ L ND Chromium (Cr) mg/ L ND Cadmium (Cd) mg/ L ND Aluminium (Al) mg/ L Plumbum (Pb) mg/ L Note: ND = Not detected

4 International Journal of Civil & Environmental Engineering IJCEE-IJENS Vol: 11 No: In this study it can be clearly observed that the influent concentration of Fe in the leachate sample was subsequently reduced to a significantly low concentration throughout the treatment period as shown in Fig. 4. The final effluent concentration of Fe was significantly reduced and varies among all four wetland microcosm, with mg/l, mg/l, mg/l and mg/l for reactor VC, VP, HC, and HP, respectively. The optimum removal percentage of the planted systems (VP and HP) was recorded on the day 3 of the treatment period with 15.5% and 17.2% for VP and HP, respectively. While, for the unplanted system (VC and HC) the optimum removal was only achieved on the day 21 of the treatment period with 32.8% for VC and 24.1% for HC reactor. However, it was observed the overall treatment efficiency for all reactors does not varies greatly between each other, with the most efficient system in the removal of Fe from landfill leachate was reactor HP with 99.2%, while the least efficient system was reactor VC with removal of 91.5%. This finding was higher than those reported by [10]; [11] and [12], which proved the effectiveness of constructed wetland in the removal of Fe from wastewater and concurrent with the findings by [13] and [14]. In this study, both VP and HP reactors were effective in reducing the high level of Mn from landfill leachate. As it been demonstrated in Fig. 5, the final concentration of Mn was reduced to a significant value of mg/l, mg/l, mg/l and mg/l for VC, VP, HC, and HP reactors, respectively. The highest treatment efficiency was recorded for reactor HP with 99.8% removal and the least was reactor VC with 94.7% removal at the end of treatment period. The optimum removal of Mn for the planted systems was recorded on day 3 of the treatment period with 18.9% and 20.8% for VP and HP respectively, while the optimum removal of the unplanted system (VC and HC) was only achieved on day 12 and day 15 of the treatment period with 17.0% for VC and 25.7% for HC reactor. In this study it can be clearly observed that all reactors managed to subsequently reduce the concentration of Fe and Mn to significantly low concentrations after 45 days of treatment period. The control system (unplanted) also demonstrated high reduction of heavy metals which is more than 90% removal. The reduction of heavy metals in the SFF wetland system maybe was due to settling and sedimentation, uptake by algae and bacteria, precipitation as insoluble salts, and binding to soil, sediments and particulate [5]; [15]. However, the reduction of Fe and Mn in control system still showed lower removal if compared to the planted system. Plants species have variety of capacity in accumulating and removing heavy metals. Several processes are envisioned as being effective in pollutant reduction; for example metals are taken up by plants, and in many cases stored preferentially in the roots and rhizomes [16]. B. Heavy Metals in Plant s Tissue The analysis of plant tissues were conducted to study the extents of phytoaccumulation or phytoextraction of heavy metals (Fe and Mn) in the plant tissues which was segregated into three main components which is leaves, stems, and roots. The results of the plant tissue analysis as shown in Fig. 6 shows that there was an accumulation of heavy metals in the tissue of Limnocharis flava planted in both HP and VP subsurface flow system. The accumulations of heavy metals shows that the contribution of macrophytes in the sense of the uptake of pollutants are significant in this study, apart from providing a large surface area for attached microbial growth, supplying reduced carbon through root exudates and microaerobic environment and a via root oxygen release in the rhizosphere, and stabilizing the surface of the bed [17]; [18]; [19]; [20]. The ability of Limnocharis flava to uptake heavy metals was also proven in this study. Where, the highest amount of heavy metals were determined in the root for both VSSF and HSSF, with mg/g (VSSF) and mg/g (HSSF) for Fe and mg/g (VSSF) and mg/g (HSSF) for Mn, respectively. In which it was consistent with the findings by reference [21] and [22]. These roots have been reported to be the most beneficial for phytostabilisation of the metal contaminants. As depicted previously in Fig. 6, the result shows that Mn uptake by plants was less than Fe. Study by references [23], [24] and [25] also reported that the amount of Fe uptake by plants was higher compared to Mn in the plant tissues. Fe 2+ was the micronutrient for plants that was required in higher concentration than Mn 2+ [26]. Additionally, plants require a small amount of Mn, high level of Mn interfere with enzyme structure and nutrient consumption. As it can be noticed in Fig. 6, HSSF systems exhibited a higher uptake of heavy metals as compared to VSSF system due to the higher HRT for HSSF system. These findings have shown the significant and positive effect of macrophytes on pollutants removal [19]. Whereby, the roles of macrophytes as an essential component of constructed wetland have been well established [17]; [27]. C. Heavy Metals in Soil Media The wetland media is one of the important components of constructed wetland, as it provides a viable condition for maximum removal of pollutant, since the reduction is said to be accomplished by diverse treatment mechanisms including sedimentation, filtration, chemical precipitation and adsorption, microbial interactions and uptake by vegetation which governed by the accurate selection of media type [28]. Therefore in this study, soil analysis was conducted to determine the suitability of the media beds used, as it is indicated by the accumulation of the heavy metals within the soil media. Fig. 7 and Fig. 8 shows concentration of Fe and Mn in the soil media collected at different depth of four reactors (VC,

5 International Journal of Civil & Environmental Engineering IJCEE-IJENS Vol: 11 No: VP, HC and HP). The unplanted control systems (VC and HC) exhibited a higher concentration of Fe and Mn in the soil samples collected at the bottom of the reactors, with an increase of 5.2% (VC) and 7.7% (HC) for Fe and 0.2% (VC) and 0.3% (HC) for Mn. While, the reactors planted with Limnocharis flava exhibited a higher concentration of Fe and Mn in the soil samples collected at mid-depth of the reactors, with an increase of 3.9% (VC) and 6.6% (HP) for Fe and 0.2% (VP) and 0.3% (HP) for Mn, respectively. Fig. 4. Concentration of Fe in landfill leachate effluent throughout treatment period Fig. 5. Concentration of Mn in landfill leachate effluent throughout treatment period Fig. 6. Accumulation of heavy metals in plant tissues after 45 days of treatment period Fig. 7. Concentration of Fe in soil at different depth Fig. 8. Concentration of Mn in soil at different depth

6 International Journal of Civil & Environmental Engineering IJCEE-IJENS Vol: 11 No: The increased concentration of heavy metals (Fe and Mn) in the soil samples collected at the bottom of unplanted control reactors (VC and HC) indicates that the heavy metals were actually precipitated towards the bottom of the reactors [12]. While, the higher concentration of Fe and Mn at the middle of the planted reactors was due to the rhizofiltrations of these heavy metal in the rhizosphere since precipitation and rhizofiltration are the main mechanism in the removal of heavy metals in constructed wetland [12]. IV. CONCLUSION Based on the above results and discussions, it can be summarized that HSSF system has higher removal efficiency compared to VSSF system for the removal of heavy metals. The higher removal of HSSF system was due to the higher HRT value for this system, which also indicates the importance of HRT that affects the removal efficiency of heavy metals in the constructed wetland system. Also, it s so obvious that by comparing the planted and control system, both systems were achieved high percentage of heavy metals removal at the end of treatment. The greater heavy metals removal in the control system maybe was due to clogging of the substrate in the soil media. So it can be concluded that, reduction of heavy metals concentration in the planted and control system were most likely due to chemical precipitation and sorption on sediment, and aided by the macrophytes. This is also shows the shorter treatment period is required in achieving optimum removal for planted system as compared to unplanted system. However, for a longer treatment period there were only slender differences in the effluent concentration of pollutants between the planted and control system. To further enhance the result obtained in this study, the following areas of investigation are recommended: (1) degradation by microorganism is among the important mechanisms in the removal of pollutants. However, this study does not quantify the development of microorganism within the wetland reactor. If the microorganism formation and development within the reactor could be measured, it surely will enhance the findings in this study and (2) further studies should vary the flow rates, retention time, types of plant and size of constructed wetlands system in order to determine the efficient of pollutants removal. ACKNOWLEDGMENT We are grateful for the university resources provided by Universiti Malaysia Perlis (UniMAP), Malaysia. Special acknowledge to the Ministry of Higher Education (MOHE), Malaysia for granting us financial support under the Fundamental Research Grant Scheme (FRGS) ( ). REFERENCES [1] A. Yalcuk and A. Ugurlu. 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