Investigation of heavy metals concentration in vermicomposting of soft peel of pistachio, municipal activated sludge and spent mushroom compost

Introduction:
Solid waste management is one of the biggest challenges faced by the whole world. Increase in the world population which itself leads to the increase in solid waste production has made this problem more and more complex(Singh, et al., 2011). Vermicomposting technology using earthworms for recycling organic wastes has been recognized as an eco-friendly and suitable approach for recycling and managing biological wastes (Padmavathiamma, et al., 2008). but the presence of heavy metals in the vermicompost and the danger of the entry of these metals into the food chain is a genuine threat as far as the living beings are concerned )Azizi, et al. 2013.( Heavy metals such as nickel, cadmium, copper, zinc, manganese, chromium, and lead in a limited amount are required by the plants for a normal growth on the other hand high concentrations of these elements have devastating effects on plant growth)Nayak and Kalamdhad, 2013(. One of the ways for the removal of these elements from the contaminated soil is using tissues of organisms such as plants or invertebrates such as earthworms (Edwards and Arancon, 2004). Due to the accumulation of heavy metals in the intestines of earthworms or due to deformation of metal complexes in the process, these heavy metals change from an accessible to an inaccessible form and the danger of these metals in the agricultural wastes is neutralized (Song, et al., 2014). In this study, soft peel of pistachios have been used for vermicomposting. According to the report submitted by Food and Agriculture Organization, Iran is the world’s largest producer of pistachios. As has been witnessed in the last few years, the total by-products obtained from pistachio cultivation have increased by a rate of 310,000 tons which in itself has changed into an environmental problem(Ghasemi, et al., 2012). As no practical solution is available for this issue, in this study we have tried to make an optimal use of soft peel of pistachios for vermicomposting thus changing it into a useful material and we have also studied the change in heavy metal concentration in vermicomposting of these wastes.

Materials and Methods:
Pistachio waste was collected at the time of pistachio harvest. So as to remove any present phenolic compounds, the waste was mixed with coal powder and then was subjected to pre-composting. Municipal sludge and spent mushroom compost (SMC) were used in order to adjust the C/N ratio. In the next step, 2.5 kilos of the dry weight of the main raw material which comprised of different percentages of soft peel of pistachios, municipal activated sludge and SMC were filled in 12 treatments were analyzed and studied for a period of 70 days. Composition of various treatments shows in table 1.

Table 1. Composition of various treatments for the vermicomposting experiment
Soft peel of pistachios (%) Municipal activated sludge (%) SMC (%) Treatments
100 0 0 A
0 100 0 B
0 0 100 C
40 30 30 D
60 20 20 E
80 10 10 F
40 0 60 G
40 60 0 H
60 0 40 I
60 40 0 J
80 0 20 K
80 20 0 L


Analysis of heavy metals:
At the end of the process, one gram of dry and powdered vermicompost was gathered from all beds and was reduced to ashes in a furnace at a temperature of 550 C. 10 ml 2 N HCL was added to the sample, after the absorption of the acid, the quantity of the acid was increased to 50 ml. Atomic absorption spectrometer was used to calculate the amount of heavy metals such as copper, cadmium, zinc, nickel and lead etc. (Salimi, et al., 2016)

Statistical Analysis:
Statistical Analysis of the data collected was performed using the software “Minitab version 17” and the average data analysis was done using one-way ANOVA and the Fisher’s method.

Discussion and results:
In this study, a significant reduction was observed in the final treatments as far as Nickel is concerned. The maximum proportion of nickel during the final phase of the process was observed to be 0.021 ppm which is related to the treatment B and in the final phase rest of the treatments lacked nickel in them. This study illustrates the fact that the reduction of nickel in the final treatments was because of the earthworm and micro organism activity present in the vermicompost heap which had led to the decomposition of the organic matter. During the saturation process of organic matter, nickel gets absorbed and accumulates inside the bodies of earthworms and other microorganisms. Also it is seen that due to rinsing of the beds with latex an amount of heavy metals get removed through the poles of the bed thus reducing the overall heavy metal concentration. Singh reports that the concentration of heavy metals in the vermicompost decreases with the increase in time of the decomposition process (Singh, et al., 2011). In a study about the comparison of heavy metal concentration in composting and vermicomposting of organic waste, it was reported that the risk of heavy metals accumulation in the body of the earthworms is reduced in the case of vermicompost that in the case of compost (Mohee and Soobhany, 2014). In this study, at the end of the process, the concentration of copper was observed to increase in all the treatments. Highest concentration of copper was found out to be 2.36 ppm in treatment H and the lowest was about 0.377 ppm in treatment C. The concentration of heavy metals in the vermicompost is related to the concentration these metals in the raw materials present in the bed. The increase in the concentration of copper in the vermicompost is due decrease in volume and mineralization of the organic matter by the earthworms. As per the studies of Nayak and Kalamdhad, the increase in heavy metal concentration can be a result of loss of initial weight due to the decomposition of organic matter and emission of CO2 gas during vermicomposting. As per the studies of Yadav and Garg, contents of metals like iron, copper, lead and zinc in vermicomposting of mixed raw waste was higher than the initial level(Yadav and Garg, 2013). The concentration of cadmium, zinc and lead decreased in some treatments and increased in some others at the end of the process. Maximum concentration of cadmium at the end of the process was observed to be in the treatment “L” and was about 0.024 ppm whereas the minimum concentration of cadmium was 0 ppm in treatments “A”,”E” and “H”. The maximum concentrations of lead at the end of the process were about 0.67 ppm in treatment “B” and 0.65 ppm in treatment “H”. No concentration of lead was found in treatments “A”, “C”,”E”,”F”,”I”,”K” and “L”. The concentration of zinc was also observed to be different in different treatments. The maximum concentration of zinc at the end of vermicomposting was found out to be 14 ppm in treatment “B” and varied drastically from other treatments. The minimum concentration of zinc was observed in treatment “A” and was about 1.26 ppm. The concentration of heavy metals in the first bed, decomposition of organic matter, rate of adsorption and microbial activity directly affect the concentration of heavy metals in the final bed.


Conclusion:
In this research, reduction of heavy metal concentration due to leachate leakage, absorption of the metals in the substrate material by earthworms and accumulation of these metals in their tissues has been reported. On the other hand it quite possible to say that during vermicomposting, heavy metal concentration increases due to volume and weight reduction of the materials in the bed due to organic decomposition. Finally, it was found that the concentrations of these metals are within the limits of the national standards of Iran and the use of these fertilizers in agriculture does not have any harmful effect.