Introduction

The unprecedented rise in land use area for oil palm plantations in Malaysia and other palm-producing nations in the world have led to the immense quantities of wastes generated annually (Liew et al. 2015). This has lead to a big nuisance with regards to ground water pollution, infestation by rodents, spread of disease and, not to mention, the obvious odour issues. Various methods of re-utilizing the residues of the palm oil production process have been reported by a number of researchers. Almost all of the wastes have found use in one way or the other in different industries. However, with the exception of Embrandiri et al. (2012, 2013, 2016), Haron and Mohammed (2008), Razak et al. (2012), Gafar et al. (2013) and Adam et al. (2014) there is insufficient literature to show the utilization of decanter cake in agriculture, most especially with respect to plant growth. Notwithstanding, several studies have been reported to utilize organic wastes for the enhancement of plant growth. (Kelepsei et al. 2009; Ndaeyo et al. 2013; Adjei-Nsiah and Obeng 2013; Danaher and Pickens 2016; Kumar and Chopra 2016). Therefore, this work contributes to knowledge on the employment of decanter cake for the growth of vegetable plants in palm-producing countries. Although the wastes from the oil mills are expected to be organic in composition, there is unforeseen contamination of metals. This is as a result of leaks in the machinery or during washing process (Singh 2008). Due to the expected increase in oil palm acreage in the years ahead, it is eminent that serious measures have to be taken to ensure the proper management of decanter cake to address the growing environmental concerns. The aim of the present research was to investigate the uptake and accumulation of metal ions in the different plant parts from the soil and decanter cake amendments via the bioconcentration and translocation factors. The focus was on the transfer of nutrients and which parts have accumulated more metals than others and not on the potential of the plant to remediate the soil. It is generally found that when the BCF value is ≤1, the plant can only absorb but not accumulate metals, but BCF value of >1 denotes that plant can accumulate metals.

Methodology

Decanter cake (DC) samples sourced from a mill in Penang, Malaysia, were dried and powdered and then sieved with a 2 mm mesh sieve. Soil samples used in the experiment were collected from a depth of 10 cm also from the vicinity of the mill. It was dried, homogenized and sieved (2 mm) according to standard protocol. The experiments consisted of 3 kg of DC-amended soils in planter pots (15 cm × 15 cm) arranged in completely, randomized block design (CRBD). Amendments (0% (control), 10, 20 and 30% w/w DC) were mixed homogenously, filled into pots and left for 15 days stabilization (Singh 2008). The seeds of lady’s finger, tomato and brinjal/eggplant were immersed in distilled water for 3 min prior to sowing to enhance germination. Ten seeds were sown manually at equidistant positions and seedlings were thinned to three plants per pot and kept in the nursery throughout the experiment.

For the analysis of metals in plant parts, all the samples were dried in the oven at 80 °C. A fixed amount of the ground sample (1 g) was digested in 20 mL tri-acid mixture (HNO3:H2SO4:HClO4 in the ratio 5:1:1) till a clear solution was obtained (Allen et al. 1986). The concentrations of metal ions were determined from the digested solution using the atomic absorption spectrophotometer (PerkinElmer Analyst 200 AAS 2007).

The uptake and distribution of metals in different plant parts were calculated from the translocation and bioconcentration factor values. To evaluate the plants’ ability in transferring metals from the roots to the aerial parts, the translocation factor (TF) was calculated as shown in the formula below according to Mattina et al. (2003):

$${\text{TF}} = \frac{\text{Concentration of metals in aerial parts at different treatments}}{\text{Concentration of metal in root}} \times 100.$$
(1)

The bioconcentration factor (BCF) was used to calculate the metal uptake capacity from soil to plant tissues, using the equation below (Zhao et al. 2003):

$${\text{BCF}} = \frac{\text{Concentration of metals in plant parts}}{\text{Concentration of metal in test soil}}.$$
(2)

Statistical analysis

The obtained data were subjected to one-way analysis of variance (ANOVA) using SPSS 16. Duncan’s multiple range test (DMRT) was performed to test the significance of difference between the treatments at p = 0.05.

Results and discussion

The accumulation of metals by plants generally depends on a number of factors including the physicochemical properties of the soil, the species planted, climatic conditions and chemical speciation of metals (Alloway et al. 1991; Greger 1999). The initial physiochemical characteristics are presented in Table 1. The results of BCF revealed the highest level of Zn in the leaves of brinjal plants, which were grown in control soil (2.37 ± 0.26) as compared to other plants, whereas the BCF values of Mg BCF in all three plants were found to be in very low range (0.01–0.05) (Tables 1, 2, 3). The bioconcentration factor of Cu was highest in the fruits of lady’s finger grown in control (3.58 ± 0.72), while BCF of Ni was maximum in brinjal leaf at 10% DC treatment (0.66 ± 0.02). This varying trend can be attributed to the chelation properties between the metals and amino acids (Odum et al. 2005). Kim et al. (2003) have accredited the difference in metal concentrations in various plant parts to compartmentalization and translocation through the vascular system. Plants could accrue and store metals in root and stem in nontoxic forms (Baker and Brooks 1989). Lower BCF values indicate that the test plants showed difficulties to mobilize metals from the soil. The movement of metal ions from roots to shoots occurred due to speciation of elements, soil pH and soil organic matter (Kabata-Pendias and Pendias 1984). The wide variations in BCF values of heavy metals for sunflower, castor oil, alfalfa and mustard plants grown in hydroponic cultures were observed by Zhi-xin et al. (2007). The BCF values obtained for control and DC-amended plants were less than 1 (Tables 1, 2, 3). In tomato and brinjal plants, some BCF values were found to be above 1, which can also be considered safe for consumption as they do not exceed the recommended WHO limits.

Table 1 Analysis of soil, fresh decanter cake and mixtures (mean ± SD)
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Table 2 Bioconcentration factors of metals in leaves, shoots and fruits of lady’s finger grown in different decanter cake amendments (mean ± SD)
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Table 3 Bioconcentration factors of metals in leaves, shoots and fruits of tomato grown at different decanter cake amendments (mean ± SD)
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The value of TF for all three plants was higher than control plants but with respect to individual metal ions and plant parts there were varying trends. TF value of Zn in the leaves of lady’s finger plants was greater than 1 (Fig. 1), whereas in both shoots and leaves, translocation factor of Ni was higher in only 30% DC soil. In tomato plants, all treatments showed translocation factor of Zn greater than 1, except in control, whereas for translocation factor of Mg (Fig. 2) values were greater than 1 in root and shoot portions. For brinjal plants, Ni TF was higher than 1 in leaves and shoots (Fig. 3). This varying concentration of metals is due to the difference in assimilation rate and plant physiology (Singh et al. 2007, 2009, 2012). Even though tomato and brinjal belong to the same family, brinjal has a higher rate of assimilation of nutrients while tomato and lady’s finger do not. This is could be attributed to the different physiological mechanisms and graft incompatibility in solanaceous plants (Kawaguchi et al. 2008). In a similar study, the translocation factor of Fe for Jatropha curcas clones was higher in plants grown in the control soil than in the treatment plants (Ghavri and Singh 2010). Plants with both bioconcentration factor and translocation factor greater than 1 (TF and BCF > 1) have the potential to be used in phytoextraction (Yoon et al. 2006). Therefore, based on data in this study, all three tested plants did not prove to have phytoextraction potential. On the other hand, Yoon et al. (2006) also noted that plants with bioconcentration factor greater than 1 and translocation factor less than 1 (BCF > 1 and TF < 1) have the potential for phytostabilization (Fig. 3; Table 4). Figure 4  presents some of the pictures taken during the experiment.

Fig. 1
figure 1

Bar graphs showing the translocation factors of Cu, Ni, Mg and, Zn of the leaves (L) and shoots (S) of lady’s finger plants (mean ± SD)

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Fig. 2
figure 2

Bar graphs showing the translocation factors of Cu, Ni, Mg and Zn of the leaf (L) and shoot (S) portions of tomato plants (mean ± SD)

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Fig. 3
figure 3

Bar graphs showing the translocation factors of Cu, Ni, Mg and Zn of the leaf (L) and shoot (S) portions of brinjal plants (mean ± SD)

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Fig. 4
figure 4

Pictures during the experiment. a Lady’s finger fruit in 10% DC in soil. b Brinjal plant at 30% DC in soil. c Tomato plants uprooted at 10% DC. d Fresh decanter cake

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Table 4 Bioconcentration factors of metals in leaves, shoots and fruits of brinjal plants at different decanter cake amendments (mean ± SD)
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Conclusions

This study revealed that even though there was bioconcentration and translocation of metals such as Ni, Cu, Mg and Zn from the soil to plant parts, concentrations did not transfer to the fruit portions or exceed permissible FAO limits for vegetables. There is potential of lady’s finger, tomato and brinjal to be used as phtyostabilizers with thorough investigations. Oil palm decanter cake compost amendments could therefore serve as a potential fertilizer source without the risk of metal accumulation and toxicity. Characterization of decanter cake and soil is necessary prior to pot application as metal mobility increases with lowering of pH and enhancement of organic carbon in the receptor soil. Decanter cake can function as an economical source of soil amendment to farmers particularly in Malaysia and other palm-producing nations. This would, over time, reduce the dependency on synthetic chemicals that have resulted in severe damages to flora and fauna communities and by extension to man.