For the past twenty-five years or so, studies have shown that plants growing in contaminated soils can tolerate otherwise toxic levels of certain pollutants. Plant communities respond differently to the presence of metals in the soil. Some plants have developed sensitivity to some metals even at very low concentrations while others have developed resistance to the toxic metal stress, which can be achieved either through avoidance or tolerance (Baker 1987). Tolerant plants can accumulate the contaminant in their tissue and this ability to take up pollutants including a variety of metals differs among the plant species (Freitas et al. 2004, Szarek-Lukaszewska et al. 2004, Fischerova et al. 2006). On the other hand, excluder plants restrict contaminant uptake into their biomass (Chaudhry et al. 1998). Unlike organic compounds, the metals released from the decomposition of the waste are not degraded and can stay in the soil by sorption and precipitation for a long time (Jensen et al. 2000). During landfill stabilization, the metals present may be solubilized and become bioavailable (Cecen and Gursoy 2000). When oxygen and oxygen-rich rain enters the landfill, metals will gradually be released from where they are bound (Ostman et al. 2006).
Plant species that have colonized an area in which other plants cannot survive have a potential role in remediating the area (Kim and Lee 2007). Studying the behavior of these plants growing on contaminated soils may be effective for selecting plants that can be used in remediating the soil. In the Philippines, studies on the type of vegetation found in landfills and the role they would play in future rehabilitation efforts seem to be limited or have not been conducted at all.
This study aimed to determine the level of cadmium (Cd) and chromium (Cr) in plant root-zone soil and in the roots, stems, and leaves of plants collected from a landfill; Cd and Cr are part of eight metals identified in polluted environments. Being highly urbanized, it is perceived that a big bulk of the waste thrown to the Cebu City landfill is composed of metal-containing household waste, and industrial and laboratory wastes. A study on the heavy metal content of combustible municipal solid waste in Denmark showed that Cd and Cr were present in the waste (Riber et al. 2005). Cd enters the environment through several activities including disposal of municipal solid waste; the metal is toxic and has a long biological half-life (Sankaran and Ebbs 2007). Cr is considered an essential element in trace amounts (Prasad and de Oliveira Freitas 2003), but the hexavalent form of Cr is relatively toxic (Shanker et al. 2005); in a study on the composition of a municipal solid waste deposition cell, it was found that 30% of the total amount of heavy metals including Cd and Cr in the wastes were in an available reactive form (Flyhammar et al. 1998).
The metal concentration values obtained were used to calculate the Bioconcentration Factor (BCF) and Translocation or Transfer Factor (TF), which were used to quantitatively express metal tolerance or avoidance.
The Cebu City Landfill is located in Barangay Inayawan, Cebu City (barangay is the smallest administrative division in the Philippines) in the Philippines (Fig. 1). The city is known to remain wet and humid throughout the year due to the rainfalls and the humidity at almost 80% all year; the rainy season is seen from June to December (Weather and climate: Cebu Philippines). The city-owned landfill covered a reclaimed area equivalent to approximately 160,000 m2 . It became operational in 1998 with a projected lifespan of seven years, but stopped operating only in late 2011. Although the landfill was designed as a sanitary landfill, it did not incorporate landfill and leachate treatment systems, and thus was considered a poorly controlled landfill. The soil cover was mainly limestone mixed with the soil from the garbage and drainage. Kim and Lee (2007) described a poorly controlled landfill as one with no landfill gas and leachate treatment system and having a final soil covering of minimal thickness and low quality.
A reconnaissance survey was conducted prior to the vegetation survey conducted in February 2010 to determine the sampling sites. Plant and soil samples were collected during the vegetation survey conducted late 2009 and early 2010, prior to the closure of the landfill. Patches of vegetation on top of the wastepile were identified based on the homogeneity of the existing plant communities, with each 1 m2 patch considered as a sampling site (Banaticla and Buot 2005). Thirty-two plant species were collected from areas where dumping activities were no longer done (Fig. 2). The collection of plants was similar to the one conducted by Dogan et al. (2014). At each patch, 3 to 4 individuals were randomly collected (Freitas et al. 2004) and were mixed to form a composite of the plant species.
All the reagents used were of analytical grade, and deionized distilled water was used for all dilutions and for the washing of the plant samples. The glasswares used were soaked overnight in 5% nitric acid (HNO3) solution and rinsed thrice with deionized water prior to use.
Soils were collected from the same sites as the plant samples, from a 0-25 cm depth plant root zone (Baker et al. 1994, Gonzalez and Gonzalez-Chavez 2006); the roots of some of the plants were able to penetrate the garbage under the soil cover. The root-zone soil samples were mixed to form a composite (Sankaran and Ebbs 2007). Soil and debris attached to each individual plant were removed manually and added to the composite soil samples. The soil samples were air-dried for 5-7 days. Residues including tiny roots of plants were removed before the soil was ground and sieved (Wang et al. 2003). The soil samples were microwave-digested prior to the determination of the concentration of Cd and Cr using the Atomic Absorption Spectrophotometer (AAS) (AA-6300; Shimadzu, Kyoto, Japan) flame technique.
Soil pH in the water and in the calcium chloride (CaCl2) solution (Rajakaruna and Bohm 2002) was measured using a pH meter (Basic pH Meter PB-20; Sartorius, Göttingen, Germany). The latter was done in order to mask the variability in salt contents of soils and to maintain the soil in a flocculated condition (Jones 2001). About 2 g of each soil sample was placed in a 125 mL Erlenmeyer flask and was added with deionized water using a 1:5 proportion. After the soil pH in the water was determined, 1-2 drops of CaCl2 were added to the soil-water suspension. The suspension was stirred several times within a period of thirty minutes and was then allowed to stand before taking the pH reading (Rajakaruna and Bohm 2002).
In the laboratory, the samples of the thirty-two plant species identified plants were rinsed with tap water to remove loose materials on the surface and then rinsed twice with ethylene diaminetetraacetic acid (EDTA) for about a minute per rinsing to remove any possible adsorbed metals (Gothberg et al. 2002). Final rinsing with deionized distilled water was done thrice. The plants were then separated into roots, stems and leaves, and then airdried (Robinson et al. 2000, Gothberg et al. 2002, Remon et al. 2005, Segura-Muñoz et al. 2006). An agate mortar and pestle was used to ground the air-dried samples and for hard plant parts an electronic food mill was used. The samples were then oven-dried at 80oC to constant weight (Carlosena et al. 1997, Robinson et al. 2000, Sharma et al. 2008).
The wet-digestion of the samples that followed was done under a fumehood using the procedure employed by Tüzen (2003). About 2 grams each of the samples were placed in a 250-mL Erlenmeyer flask. Ten mL of concentrated H2SO4 was cautiously added and the mixture was shaken until no lumps remained. About 5 mL of concentrated HNO3 was added to the mixture and mixed. A glass funnel was placed on the Erlenmeyer flask and the reacting mixture was allowed to stand overnight. The reacting mixture was heated cautiously on a hot plate until the vigorous reaction subsided. Each sample was then allowed to cool. Five mL of concentrated HNO3 was added, replacing the cover and refluxed for about 30 minutes. If brown fumes were still produced, indicating the oxidation of the sample by the HNO3, the addition of 5 mL concentrated HNO3 was repeated over until no brown nitrous fumes were given off by the sample, indicating the complete reaction with HNO3. During this time, a transparent solution was obtained.
The mixture was then heated until white fumes were evolved and the solution was allowed to concentrate to about 5 mL by heating it at 80-85oC without boiling for two hours. A covering of solution was maintained over the bottom of the vessel at all times. The solution was then cooled and 2 mL of deionized water and 3 mL of 3% hydrogen peroxide (H2O2) were added. The glass funnel was put back to cover the vessel and the covered vessel was returned to the heater for warming and to start the peroxide reaction. This was done carefully to ensure no losses due to excessive vigorous effervescence. The mixture was heated until the effervescence subsided and the vessel was then cooled. The H2O2 solutionwas continually added in 1-mL aliquots, with warming, until the effervescence was minimal. When the digest was clear, the funnel was removed and the resulting solution was heated at about 80oC nearly to dryness.
BCF = [M]roots/[M]soil
where [M]roots is the total metal concentration in the roots, and [M]soil is the total metal concentration in the soil, and wherein for this particular study the metal refers to chromium and cadmium.
TF = [M]leaves /[M]roots
where [M]leaves is the total Cr or Cd concentration in the leaves, and [M]roots is the total Cr or Cd concentration in the roots.
TF indicates internal metal transportation (Nouri et al. 2009). According to Yoon et al. (2006), both the BCF and TF can be used to estimate a plant’s potential for phytoremediation; BCF is used to estimate a plant’s ability to accumulate the metal in the roots while TF is used to estimate a plant’s ability to translocate metals from the roots to above-ground parts. Plants exhibiting BCF and TF values less than one are unsuitable for phytoextraction (Yoon et al. 2006).
The results of the determination of the soil pH in both deionized water and in CaCl2 solution, as well as concentration of metals in the plant root-zone soil, are presented in Table 1. Results show that the soil samples were under generally acidic conditions with pHH2O ranging from 6.18-7.24 and pHCaCl2 ranging from 5.99-7.02. The mean pHH2O was 6.69 and mean pHCaCl2 was 6.58, as presented in Table 2, which also shows the range, mean, and standard deviation for root-zone soil Cd and Cr concentrations. The soil chemical properties significantly affect the distribution of the metals among the different soil fractions (Mocko and Waclawek 2004). For instance, pH is an important factor influencing the availability of metals in the soil for plant uptake (Prasad and de Oliveira Freitas 2003, Bradl and Xenidis 2005). Jones (2001) stated that the pHCaCl2 value is lower than pHH20 and this was evident inthis study. Under acidic conditions (pH 4.0-8.5), metal cations are mobile (Saxena and Misra 2010).
Soil pH and level of Cd and Cr in the plant root-zone soil, Cebu City landfill in the Philippines
Summary of results for pH and soil concentrations of Cd and Cr, Cebu City landfill in the Philippines
Bert et al. (2002) considered soil to be contaminated if it contains greater than 2 mg kg-1 Cd, based on the French agricultural approved National Farmers’ Union (NFU) 44,041 norm. The concentration of Cd in soil was relatively low, ranging from “below detection limit” to 1.58 mg kg-1 (for
The concentration of Cr in the soil ranged from 7.93 mg kg-1 (for
The plant species in the landfill differed in their Cd and Cr content, which indicated that they differed in their capacities for metal uptake, as also observed in other studies on metal contaminated sites (Freitas et al. 2004, Nouri et al. 2009).
Cadmium. Of the thirty-plant species analyzed, only 3 showed to have Cd either in the roots, leaves or stems. Cd was detected in the roots of
In a phytosociological analysis of roadside communities in Kerala, India,
The soil in the landfill was generally acidic and under this condition metals are more mobile and become more available vertically upward for plant uptake. Results show that the plant species in the landfill differed in the metal content, which indicated that the plant species differed in their behavior towards the presence of Cd and Cr. Of the 32 plant species sampled,
To confirm the Cd accumulation and internal transfer of