Soil Amelioration Potential of Legumes for Mine Tailings - Philippine

Philippine Journal of Science 143 (1): 1-8, June 2014 ISSN 0031 - 7683 Date Received: 24 September 2013

Soil Amelioration Potential of Legumes for Mine Tailings Justine Perry T. Domingo* and Carlos Primo C. David Environment Monitoring Laboratory, National Institute of Geological Sciences, University of the Philippines, Diliman, Quezon City, Philippines Substrate fertility is an important constraint in the revegetation of active mining and mined out areas. In particular, the physical and chemical properties of tailings materials preclude any sustainable vegetation cover; more so if the usual practice of planting tree seedlings are used. Focus should first be given to transforming the tailings material into a more viable material for plant growth. This research tested the potential of two legume species, Centrosema molle and Calopogonium mucunoides, in the transformation of the tailings material and in the establishment of vegetation. Improvement in the levels of phosphorus and organic matter was observed in the legume-planted material after 4 months, while heavy metals including copper, arsenic, and cadmium have undergone significant reduction. Results suggest that these species could be effectively used to improve the soil conditions in abandoned mine areas and tailings dumps. Key Words: fertility, legumes, mine rehabilitation, revegetation, soil amelioration, tailings

INTRODUCTION Mining, an important global industry, is often associated with the destruction of the environment (Ye et al. 2001). Impact of mining on the overlying ecosystem is caused by processes involved, such as the extraction of resources, removal of unwanted materials, and disposal of waste products. In particular, waste material from mine and ore processing operations, referred to as mine tailings, have a major impact on the environment (Bleeker et al. 2002) due to its high heavy metal concentration (Shu et al. 2001), nutrient deficiency (Wong 2003), and low water retention capacity (Ernst 1996). These factors inhibit vegetation establishment in tailings, thus leaving their surfaces bare and completely exposed to erosion agents (Conesa et al. 2007). There is a further risk of polluting soil and water due to erosion and leaching of contaminated material (Bleeker et al. 2002). Additionally, these sites are also very unpleasant aesthetically to the landscape (Tordoff *Corresponding author: [email protected]

et al. 2000). Such environmental consequences remain beyond the lifetime of a mining operation, particularly in the case of abandoned mines (Maramba et al. 2006). In the Philippines, mining has raised a large number of issues, especially on matters concerning the environment. It has been related to massive problems and disastrous occurrences in the past, such as mercury poisoning in Mount Diwalwal and the Marcopper Spill in Marinduque (David 2002). To protect the environment and adjacent communities to such disasters, mining companies in the country are obliged by the Republic Act No. 7942, known as the Philippine Mining Act of 1995, to use appropriate technology and facilities that minimize pollution. A revision of the act, known as the DENR AO 96-40, also requires companies to prepare and implement environmental programs which would include rehabilitation, regeneration, revegetation and reforestation of mineralized areas, slope stabilization of mined-out and tailings covered areas, aquaculture, watershed development and water conservation, and socioeconomic development (MGB 2010). 1

Philippine Journal of Science Vol. 143 No. 1, June 2014

Various strategies have been applied for the rehabilitation of abandoned mines, but no single approach is universally applicable due to the variation of physical, chemical, and biological factors that exist among the mine sites (Tordoff et al. 2000). Some rehabilitation techniques involve high cost procedures, or with greater risk of being environmentally invasive due to increase in heavy metal bioavailability and dispersion through changes in physical and chemical properties of the soil (Verkleij et al. 1999; Simon 2005). Among other rehabilitation strategies, the use of vegetation is widely acknowledged as one of the best methods (Tordoff et al. 2000). However, the natural colonization of plants of mine tailings is usually slow (Conesa et al. 2007); therefore, more efficient schemes in establishing vegetation should be utilized.

Domingo & David: Soil Amelioration Potential of Legumes for Mine Tailings

MATERIALS AND METHODS Site description and sampling The Philex Copper-Gold Mine is located in Padcal, Benguet Province, Philippines (Figure 1). It is located 15 km southeast of Baguio City with an elevation of approximately 1500 m above sea level. Average annual rainfall in the region is approximately 4500 mm, experiencing rainy season from May to October . The mine started its underground block cave operations in 1958, and has since produced copper concentrates containing copper, gold, and silver.

One key factor in the success of mine rehabilitation through revegetation is the selection of plant species (Bradshaw 1997; Bradshaw and Huttl 2001). Suitable species should have the capability to overcome heavy metal toxicity and lack of major nutrients, which are considered to be the major limiting factors for plant establishment on mine tailings (Bradshaw 1987). Using common plants is not viable on these conditions (Tordoff et al. 2000), hence suitable plants should be selected. Pioneer plants species have been recommended as they are capable of stabilizing the site and modifying soil properties suitable for the colonization of other plants (Lei and Duan 2008). Leguminous species, in particular, are highly desirable since they are major contributors of biologically fixed Nitrogen (N) (Herridge et al. 2008). Since nitrogen is a limiting nutrient for plant growth, the N-fixing capability of legumes gives them significant advantage over other species when colonizing bare soils (Rodriguez-Echeverria and Perez-Fernandez 2005). Previous rehabilitation studies have focused on heavy metal uptake and translocation of pioneer plants to evaluate their efficiency in the restoration of mined areas (Shu et al. 2002; Freitas et al. 2004). However, focus must also be given to soil fertility improvement, due to its significance in vegetation development (Lei and Duan 2008). Different strategies have been developed to improve soil quality of tailings, mainly through the use of organic amendments or plants adapted to the tailings conditions (Asensio et al. 2013). In this study, the impact of legume species in the soil fertility of mine tailings was tested. The effect of the selected species was assessed based on the changes in the heavy metal concentrations, organic matter content, and nutrient levels of the tailings material. The potential of these plants to be used in the rehabilitation of mined out areas were evaluated, in the purpose of developing an alternative revegetation method that will enhance the propagation of other plants in mine tailings. 2

Figure 1. Location Map of Philex Padcal Mine, Benguet Province, Philippines.

Three tailings storage facilities (TSFs) were constructed to accommodate the tailings generated from the processing operation. The first two TSFs have been decommissioned for several years and are mostly revegetated by grasses, while the third TSF remains operational. A portion of TSF3 have been planted with grasses as part of the company’s rehabilitation program. Samples were collected at three random points on the upper 30cm layer of the tailings pond, and were homogenized prior to the pot experiment.



Plant species Leguminous species have been previously identified to naturally occur on the decommissioned tailings ponds of Philex Padcal Mine, particularly Calopogonium mucunoides Desv. (Rillorta and Lagunzad 2004). Seeds of this species, along with another legume, Centrosema molle Mart. ex Benth were obtained from the Bureau of Animal Industry, Department of Agriculture, Philippines. The seeds were germinated on garden soil for 10 days prior to the pot experiment. This was done to obtain 100% survival rate, since preliminary trials using direct seeding method resulted to mortality rates greater than 90%. Experimental design A greenhouse experiment was designed to test the effect of C. molle and C. mucunoides on the properties of mine tailings. The control setup consisted of 1kg of pure mine tailings only. Two experimental setups were prepared, one setup with 1kg of tailings and 10 germinated seedlings of C. molle, and the other setup has 1kg of tailings with 10 germinated seedlings of C. mucunoides. All setups were made in triplicates. The pots were watered daily with approximately 200 mL of tap water and placed in a fenced 5 m x 5 m outdoor area in a randomized block design. The plants were allowed to grow for a period of 4 months. At harvest, the plants were carefully removed before collecting the tailings material on each pot and dish. The samples were subjected to different analyses to examine the changes in the soil properties. Tailings properties were analysed from three control pots on the start of the pot experiment to establish the baseline values in the samples. Soil Analysis Major Nutrients and Organic Matter Samples were air-dried and sieved through a 2-mm sieve before submitting to the Bureau of Soils and Water Management, Department of Agriculture, Philippines for soil characterization. The pH of each sample was obtained by electrometric method using a standard pH meter. Total nitrogen levels were determined using the Kjeldahl Method, through titration. Available phosphorus was analyzed using Olsen Method and measured through Ultraviolet-Visible Spectroscopy. Potassium concentrations were determined through leaching method using ammonium acetate, and measured using Atomic Absorption Spectrophotmeter. Organic carbon analysis was carried out through modified Walkley-Black Method (1934) and using Ultraviolet-Visible Spectroscopy, from which organic matter content was derived. All analytical procedures conform with standard procedures (Soil and Water Resources Research Division 1988).

Heavy Metals Each sample was filtered through a 63 um sieve to obtain the clay fraction, and oven dried at 60 °C overnight. Digestion of samples was carried out in Teflon digestion tubes using concentrated HF and HNO3 sample, in a 2:1 volume ratio, with 1.5mL mixed acid for every 0.01g sample. River Sediment (GBW08301) Certified Reference Material was used, having the following certified values: As 56µg/g, Ba 375µg/g, Cd 2.45µg/g, Co 16.5µg/g, Cr 90µg/g, Cu 53µg/g, Fe 3.94%, Mn 975µg/g, Hg 0.22µg/g, Pb 79µg/g, Se 0.39µg/g, and indicative values for Be, Ni, V, Zn (Measurement Standards Laboratory of New Zealand 2009). The vessels were put on a hot plate at 80°C temperature overnight. Addition of HNO3 was done repeatedly upon drying of the samples to ensure complete digestion. Digests were transferred into 15mL centrifuge tubes before filtering through 0.45 um sieve onto 50mL centrifuge tubes. Samples were diluted to 50mL with ultrapure water, from which 5mL aliquots were collected and used for the scanning the following heavy metals: chromium, nickel, copper, zinc, arsenic, cadmium, and lead, using the Inductively Coupled Plasma – Mass Spectrophotometer (Agilent 7500CX) in the National Institute of Geological Sciences, University of the Philippines, Quezon City, Philippines. Statistical Analysis Data for the tailings with plant species were analysed by one-way ANOVA to examine the effect of the plants on the nutrient content and heavy metal levels of the samples. Specifically, results were compared with the control to see any significant changes in the tailings properties.

RESULTS Physico-chemical characteristics of the tailings prior to the experiment indicate neutral to slightly basic pH and poor soil nutrients (Table 1). Total nitrogen in the sample was minimal at 0.01% while phosphorus was not detected, i.e. less than 0.1 mg kg-1. The potassium content of the sample was 78.0 mg kg-1. The organic matter content of the tailings material was likewise very low at 0.19%. After 4 months, the pH of the control setup became more neutral most probably owing to the breakdown of sulfide minerals and generating acidity. The phosphorus content have increased from less than 0.1 mg kg-1 to 0.45±0.01 mg kg-1 which may be attributed to residual phosphorus in water. Total nitrogen and organic matter remained at the baseline level (Table 1). In comparison with the legumeplanted samples, the pH of the material in the C. molle setup had become slightly acidic while the C. mucunoides setup maintained its neutral-slight alkalinity (Figure 2). Organic matter levels in the material have significantly increased in 3



Table 1. General physico-chemical characteristics of pure mine tailings from Philex Padcal Mine before and after the experiment. Control

pH

Total N%

P (mg kg-1)

K (mg kg-1)

OM%

t = 0 months

7.63±0.13

0.01±0.00

ND

78.0±0.0

0.19±0.03

t = 4 months

7.04±0.09

0.01±0.00

0.45±0.01

78.0±0.0

0.22±0.03

Values indicate Mean ± SE; n = 3. ND - Not detected.

Phosphorus 3.5 3

P(ppm)

2.5 2

Control C. molle

1.5

C. mucunoides

1 0.5 0

0

1

Months

2

4

0

1

Months

2

4

Figure 2. Changes observed in the tailings parameters after 4 months. Values indicate Mean ± SE; n = 3.

both setups, accumulating more than twice the amount of organic matter in the control. Phosphorus had undergone the greatest improvement in both setups, with the C. mucunoides setup having a higher phosphorus increase (2.93±0.39 mg kg-1) compared with the C. molle setup (2.17±0.52 mg kg-1). There is a significant difference observed in most parameters among control pots and pots with legumes (P < 0.05) except in nitrogen and potassium contents, which remained at values of 0.01% and 78.0 mg kg-1, respectively.

DISCUSSION

The initial concentration of most heavy metals in the tailings were below the Effects Range Median (ERM) sediment quality guidelines set by the National Oceanic and Atmospheric Administration (NOAA) and the Australian and New Zealand Environment and Conservation Council (ANZECC). Only copper (ppm) exceeded the ANZECC and NOAA sediment quality guideline limit (Table 2). Meanwhile, heavy metal concentration had been substantially reduced in legume-planted tailings. Results of the ANOVA test indicate significant differences in the concentrations of copper, arsenic, and cadmium among the control pots and the experimental pots (P < 0.05). Moreover, there is also a striking difference in the copper

The pH of the tailings material was found to be surprisingly near neutral after four months, indicating that any acidity produced by the breakdown of sulfide minerals is being drained out. Normal plant growth could be attained at this pH (Shu et al. 2001). On the other hand, total nitrogen content in most soils usually ranges from 0.05 to 2.0%, and lower values indicate nitrogen deficiency (Tian et al. 2010). Nitrogen levels for all setups remained at 0.01%, implying that the amount of nitrogen in the material was still unfavorable for plant growth. The limited nitrogen content in the material may be related to the high heavy metal concentration in the tailings since these adversely affect the activity of soil organisms. In particular, the

4

concentrations between the two legume setups (Figure 3). In the C. molle setup, copper concentration decreased to 2847±301 ppm, compared with 703.3±124.8 ppm in the C. mucunoides setup. Both concentrations, however, still exceeded the NOAA and ANZECC limit for copper.



Figure 3. Heavy metal content of pure mine tailings from Philex Padcal Mine before and after the experiment. Values indicate Mean ± SE; n = 3; in parts per million (ppm).

5



proliferation of nitrogen-fixing bacteria found in legumes and other biological processes such as soil organic matter decomposition and nitrogen mineralization are hindered (Liu et al. 2012). A longer time period may be necessary to observe a higher increase in the major plant nutrients in the tailings material. Phosphorus and potassium are both major nutrients for plant growth and are considered limiting factors as well (Dragovich and Patterson 1995). While potassium levels in the material had also remained at 78.0 mg kg-1, the phosphorus content in the material had increased by 480% in the C. molle setup, and 650% in the C. mucunoides setup. Although previous studies have found phosphorus to be the most difficult mine spoil nutrient to correlate to vegetation response (Roberts et al. 1988), the results of the present study suggest that these legume species can significantly improve the amount of phosphorus in mine tailings. Table 2. Heavy metal content of pure mine tailings rom Philex Padcal Mine before and after the experiment. Control (t = 0 months)

Control (t = 4 months)

ANZECC / NOAA ERM limit

Cr

166.1±17.5

140.0±9.2

370

Ni

23.0±2.4

19.2±0.8

51.6

Heavy metal

Cu

3355±608

3174±266

270

Zn

269±27

99.9±16.5

410

As

2.87±0.40

3.90±0.28

70

Cd

0.69±0.04

0.39±0.01

9.6

Pb

15.5±0.6

15.1±0.9

218

2008). From the experiments conducted, copper, arsenic, and cadmium has undergone significant reduction in tailings planted with legumes. This result suggests the ability of the legume species used to hyperaccumulate heavy metals, particularly C. mucunoides, which have been previously suggested (Hidayati et al. 2006; Nwaichi and Onyeike 2010). As mentioned, this capability should indirectly improve the soil organism activity occurring in the tailings material.

CONCLUSIONS AND RECOMMENDATIONS Overall, the results of the study indicate that both legume species can significantly augment the phosphorus levels and amount of organic matter, as well as decrease heavy metal concentrations in mine tailings. Using these species to pioneer vegetation establishment on tailings dumps is feasible, even without the addition of organic/inorganic ameliorants, which is believed to be necessary for this purpose (Yang et al. 2003). Moreover, using these plants in revegetating mined out areas will be advantageous as these plants have not only exhibited metal tolerance but also contributed to biological soil development. Further experiments should be conducted to assess and improve the establishment of these plants in natural field conditions. The ultimate goal of stabilizing the site and improving soil properties suitable for the colonization of other plants can be achieved through these plants, and hence will be beneficial to sustainable mine rehabilitation plans.

Values indicate Mean ± SE; n = 3; in parts per million (ppm). ERM-Effects Range Median.

Organic matter is vital to vegetation establishment and sustainability (Dragovich and Patterson 1995). High levels of organic matter improve aggregation and infiltration capacities, consequently increasing the availability of nutrients (Singh et al. 2004), and determining the amount of available heavy metals to the plant (Vega et al. 2004). In the study, the organic matter in the material had increased in both legume species. Organic matter levels in the C. molle and C. mucunoides setups have been enhanced by 260% and 280%, respectively. Previous studies have suggested that pioneer plant species modify soil fertility of mine tailings by initially supplying more organic matter (Jacob and Marinus 2004; Archer and Caldwell 2004; Singh et al. 2004). In this sense, both legume species have exhibited positive contribution in the fertility of the tailings material. The removal of heavy metals from the soil is essential in the phytostabilization of mine tailings (Lei and Duan 6

ACKNOWLEDGEMENT The authors would like to thank the Office of the Chancellor of the University of the Philippines Diliman, through the Office of the Vice Chancellor for Research and Development, for funding support through the Outright Research Grants; and to Philex Mining Corporation, especially to VP for Exploration Redempta Baluda, VP for Environment and Community Relations Victor Francisco, Padcal Environment Manager Rodolfo Saguid, and Padcal Environment Staff Dante Molina, for providing tailings material; and to the Bureau of Soils of Water Management, especially to Director Dr. Silvino Tejada and Senior Agriculturist Beatriz Magno. Contributions from Pamela Louise Tolentino, Joan de Vera, Eliza Laudencia, Josephine Rafols, Menandro Loresco, Joven Marbella, and Dr. Antonio Alcantara have been very helpful in the conduct of the study and are greatly appreciated.


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Soil Amelioration Potential of Legumes for Mine Tailings - Philippine

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