Heavy Metal Accumulation in Sediment and Removal Efficiency in the Stabilization Ponds with the Hydrocotyle ranunculoides Filter

The heavy metal accumulation in the sediment and removal efficiency in stabilization ponds with Hydrocotyle ranunculoides filter were evaluated. Sediment and water were sampled in June, July and August 2018. The sediment sampling for each lagoon was conducted at three sites forming a composite sample. The water samples were collected in the tributary and effluent pipelines to determine the heavy metal concentration and removal efficiency by Hydrocotyle ranunculoides. The determination of heavy metals was performed with the method of atomic flame absorption spectrophotometry. The mean concentration of heavy metals in the sediment, in a descending order, was: Fe > Zn > Pb > Cu > Cd. The concentrations of these metals ranged from 998.0 to 1365.02, from 488.01 to 600.30, from 88.23 to 95.01, from 1.47 to 1.53 and from 0.01 to 0.13 mg/Kg, respectively. In the four stabilization ponds, the pollution factor values for Cd, Cu and Fe qualified as low pollution factor. While for Zn and Pb, they qualified as moderate pollution factors. The heavy metal removal rates from the water varied by metal


INTRODUCTION
The alteration of water quality is of great concern worldwide, due to the progressive deterioration of water bodies. The anthropogenic water pollution is a problem that has been aggravated by the generation of large amounts of wastewater and the excessive use of water in industry due to rapid population growth [Bhatia and Goyal, 2013]. The main pollutants entering water bodies are heavy metals from multiple anthropogenic sources. The contamination by heavy metals is one of the most dangerous problems for the human health and ecosystems because of their toxicity [Goncharuk, 2014;Gleason et al., 2016]. Heavy metals, unlike organic compounds, do not degrade and can be deposited in sediments, bioaccumulating and biomagnifing through the food web [Miretzky et al., 2004].
Heavy metals are of particular concern, because their removal is ineffective in the water treatment [de Kwaadsteniet et al., 2013]. Hence, many methods have been developed to remove the heavy metals from water (ion exchange, reverse osmosis, electrolysis, precipitation, adsorption among others). However, these methods can be very expensive for large volumes, low concentrations of heavy metals and high levels of cleanliness required. At present, in order to treat these contaminants, inexpensive and effective solutions are proposed, such as adsorbents. However, their use needs to be tested in the field in order to visualize their true viability [Joseph et al., 2019].
Another current solution that has been repeatedly validated and continues to give convincing results, is the phytoremediation with artificial wetlands, since its effect for decontamination, combined with microorganisms, is effective under varying conditions due to its adaptability to different environments and easy acclimatization [Salamanca et al., 2015]. In addition, these wetlands allow for the recovery of nutrients from wastewater; this makes them a promising technology for the nutrient recovery useful for agriculture [dhikari et al., 2015].
The biological or bioabsorption methods are recommended as the most economical and effective techniques for the removal of metal ions from aqueous solutions, phytoremediation being an alternative to reduce, remove, degrade or immobilize organic and inorganic chemicals [Módenes et al., 2009;Bahaa et al., 2019]. To date, several studies have examined the ability of some plants to capture metals from water and accumulate them in their biomass [Weiss et al., 2006].
Phytoremediation is an economical, nonintrusive and safe alternative to the conventional treatment techniques. This technique is increasingly accepted to remove heavy metals and reduce the water pollution through different mechanisms, such as phytoaccumulation, phytostabilization, phytodegradation, phytotransformation, phytovolatization and phytofiltration [Ali et al., 2013]. The removal of contaminants from water by phytofiltration can be performed by the absorption or adsorption processes through the roots of the plants (rhizofiltration), seedlings (blastofiltration) or shoots of the removed plants (caulofiltration) [Carolin et al., 2017]. In this context, the objective of the study was to evaluate the heavy metal accumulation in the sediment and the removal efficiency in stabilization ponds with the Hydrocotyle ranunculoides filter.

Description of study area and installation
The stabilization ponds built for the treatment of domestic wastewater are located in the district of Sicaya at an altitude of 3232 m.a.s.l., with an area of 12 600.00 m 2 . The average flow that the wetland receives is 21.6 L /s. This system is made up of a pre-treatment, a Parshall canal, a flow distribution system, two primary facultative lagoons, occupying a total area of 0.77 Ha and two secondary facultative lagoons occupying a total area of 0.602 Ha, effluent collection channels and a perimeter fence. This system was implemented with the intention of complying with the maximum permissible limits (MPL) established by the Peruvian state, since all treated water flows into the Alca stream, a tributary of the Mantaro river.
The seeding of the H. ranunculoides macrophytes, was done after being collected from the Alcas stream, which is located northeast of the stabilization ponds. The results of the H. ranunculoides population growth were published in a preliminary study [Quispe et al., 2019]. In order to monitor the accumulation of heavy metals in the sediment, the composite samples were collected from the four stabilization ponds and to evaluate the suitability of the water for reuse, the samples were collected from 2 sites. The first site is located at the distribution box and the second at the confluence of the effluents before they are mixed with the waters of the receiving body. Monitoring was carried out in June, July and August 2018, twelve months after the installation of the macrophytes.

Sample collection and analysis
The sediment and water were sampled in June, July and August 2018. The sediment sampling from each lagoon was conducted at three sites forming a composite sample (left, center, and right with respect to the direction of water flow). The sediment was sampled by a 10 cm diameter, 50 cm long stainless steel cylindrical dredge. The surface sediment was collected based on the criteria of other researchers who found no statistical difference in the sediment metal concentrations from various depth profiles in a HSSF CW treating domestic wastewater [Vymazal an Krása, 2003]. The water samples were collected in the tributary and effluent pipes to determine the concentration of heavy metals and the removal efficiency by H. ranunculoides. The collected samples were transported under refrigerated conditions to the laboratory for analysis.
The sediment samples were digested according to the USEPA 3051A method [USEPA, 2007] with some modifications. In brief, one gram of dry sample was transferred to a 150 ml beaker, 2.5 ml of nitric acid (HNO 3 ) and 10 ml of hydrochloric acid (HCl) were added, the beaker was covered with a clock moon and it was taken to digestion by means of the microwave-assisted method. The established digestion program was: 17 minutes at 120°C, 15 minutes at 210°C and 30 minutes at 210°C. After cooling, the digestion product was transferred to a 100 ml phial and was measured with ultra-pure water. The sample was stored at 4°C and filtered prior to the analysis. The determination of heavy metals was performed with the method of atomic flame absorption spectrophotometry (air-acetylene) using the Perkin Elmer Analyst AA-6800 atomic absorption spectrometer, brand name Shimadzu.

Data analysis
The analysis of the heavy metal concentrations in the sediment and water from the H. ranunculoides filter stabilization ponds of the domestic wastewater treatment plant was performed through descriptive statistics expressed in tables and figures using the R software [Ajah et al., 2015]. As the water and sediment samples were taken at finite entry and exit points, it was necessary to check the concentration of the median of metals with the Mann Whitney method and the differences were considered significant at P <0.05 [Villares et al., 2002]. The percentage removal

Sediment quality indices
The accumulation of heavy metals in H. ranunculoides filter stabilization pond sediment was evaluated with three sediment quality indices, including: pollution factor (CF), pollution load index (PLI) and geoaccumulation index (I geo ). The calculation method, application and contamination levels are shown in Table 1. The CF and the I geo are obtained from the concentration of each element with its geochemical background reference. In turn, the calculation of the PLI involves comparing the concentration of elements with their background reference. Due to the absence of a background concentration of elements considered in the study, the IAEA-SL-1 material was used as reference values [International Atomic Energy Agency, 1999].  (Figure 3), revealing higher enrichment by these metals. However, the concentrations of heavy metals in the sediments of the stabilization ponds evaluated are lower than the concentrations of these metals in the wetlands that treat the water from the mining industry [Leung et al., 2017]. Table 3 shows the values of pollution factor (CF), pollution load index (PLI) and heavy metal geoaccumulation factor in the sediment obtained from stabilization ponds with H. ranunculoides.

Contamination and accumulation of heavy metals in stabilization pond sediments
In all four ponds, the CF values for Cd, Cu and Fe qualified as low pollution factor (CF < 1). In turn, for Zn and Pb, they qualified as moderate pollution factors (1 ≤ CF < 3). The PLI indicated that there is no appreciable contamination by the heavy metals evaluated with the values ranging from 0.1826 to 0.3177. The Igeo values of the metals evaluated were 0> I geo < 1, indicating that the sites show signs of contamination. These results are due to the shorter life span of these stabilization ponds, the type of wastewater they receive and the role of metal retention through the filtration of this macrophyte and adsorption processes [Leung et al., 2017]. Table 4 shows the descriptive statistics of heavy metals in H. ranunculoides filter stabilization pond water and removal efficiency. The    The results obtained are also supported by other researchers who evaluated the potential bioaccumulator Hydrocotyle ranunculoides under laboratory conditions revealing that this macrophyte could be a potential bioaccumulator of heavy metals [Rizzo et al., 2012]. However, the studies on the removal capacity of heavy metals by H. ranunculoides in the systems and under conditions similar to the present study are scarce. Additionally, the studies that address the accumulation of heavy metals in stabilization pond sediments with this macrophyte.

Heavy metal removal efficiency by Hydrocotyle ranunculoides
The heavy metal removal efficiencies of this study are lower than those reported by other researchers on the treatment of municipal and industrial wastewater using horizontal subsurface flow wetlands with vegetation [Galletti et al., 2010;Hadad et al., 2018].

CONCLUSIONS
This preliminary research on the accumulation of heavy metals in the sediment of the stabilization ponds with the Hydrocotyle ranunculoides filter in theee areas above 3000 m.a.s.l., reveals that 40% of the heavy metals (Zn and Pb) detected in the sediment have the concentrations that greatly exceed the average values of the IAEA-SL-1 reference material and the CISQG. Meanwhile, 60% of the metals evaluated did not exceed the average values of the reference material. This result indicates that Hydrocotyle ranunculoides is a good contaminant filter with a high capacity to store and remove Zn, Cu and Fe as biomass. The use of macrophytes to reduce the concentration of heavy metals in the sediment from stabilization ponds is still at an early stage in high Andean cities in central Peru.