Remediation of cadmium contaminated montane forest soils (Horton Plains): Will rubber (Hevea brasiliensis) nut shell and rice husk bio char be effective remediating agents?

Abstract

Horton Plains, one of the two montane forests in Sri Lanka, which provides habitats to many endemic species of plants and animals, has been severely affected by forest dieback. Previous research has identified a direct link between soil pollution with Cadmium (Cd) (through atmospheric pollution) and the phenomenon of forest dieback. As a consequence, forest dieback is increasing proportionately to the soil pollution. As such, it is imperative to protect Horton Plains, and this study focuses on identifying a cost-effective remediation technique to neutralize soil Cd, and thereby reducing forest dieback. This experiment was conducted for a period of three months. Soil samples were collected from Horton Plains, mainly Thotupolakanda site, which shows more than 90% severity of forest die back. Three soil amendments were used in this research as; bio char prepared using (i) rubber nut shells, (ii) rice husks, and (iii) bio char occurring naturally in Horton Plains. Along with the control treatment (only soil), this experiment consisted of four treatments, of 5 replicates. Treatments were added to the soil samples and available Cd was extracted using standard methods in literature, and the Cd concentrations were analysed using the Atomic Absorption Spectrophotometer. Horton Plains forest soil used for the study showed the pH of 5.4 and the Cation Exchange Capacity (CEC) was 210 c moles kg -1 of soil. The soil was highly rich with organic matter and the soil organic matter (SOM) content has reached 13.4%. Immobilization of Cd added to the soil was observed and it was significant (p <0.03). However, the control (only soil) has also shown its ability to immobilize soil Cd. Thus, it appears that the presence of extraordinary level of SOM and active natural sorbents in SOM such as humic acid and fulvic acid have masked the effect of all the biochar treatments. The amount of Cd used for spiking the soil was 0.1 μg and the minute quantity may have been immobilized by the SOM. However, the study proved the possibility of neutralizing Cd toxicity in the soil by using natural sorbents. Other treatments did not show that much of Cd neutralizing