Halloysite nanotubes as a fine grained material for heavy metal ions removal in tropical biofiltration systems

Biofiltration systems are landscape depressions or shallow basins used to slow and treat on-site stormwater runoff and are considered as one of the important components of a sustainable drainage system. Biofilters generally consist of two components: a filtration media which is sand-dominant and a top vegetated soil layer. The efficiency of a biofiltration system is normally assessed by two key parameters namely hydraulic conductivity and percentage removal of pollutants. In tropical areas like Malaysia where rainfall intensity is normally high, hydraulic conductivity of the biofiltration systems needs to be high enough to prevent ponding and possible flooding of the system. To date, several studies have been done on development and maintenance of such systems; however, few have studied such systems under tropical climates with heavy and intense rainfall which needs high hydraulic conductivity. The present study aims to identify proper soil filter media that not only can remove heavy metal ions efficiently but also has reasonably high hydraulic conductivity. For this, a soil column experimental set up was developed and the effectiveness of adding different fine grained materials such as fly ash, halloysite nanotubes (Hals) from two different origins (Imerys from NZ and HalloPure from I-Minerals, Idaho), and zeolite in sand-based soil media was assessed. To assure the validity of the results for each proposed filter media three replicates were prepared. The performance in removing heavy metal ions Fe(III), Mn(II), Cu (II), Zn(II), Ni(II), and Pb(II) was then evaluated for each soil composition using Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES) test. Synthesized stormwater was used to provide consistency of pollutant concentration in experiments. The watering dosage was calculated based on hydrological data of a Malaysian catchment. Infiltration rate of each soil composition was also measured for further comparison. Results showed that increasing the percentage of fine materials can improve the heavy metal ions removal; however, the drawback would be significant decrease in infiltration rate. In general, Hals were found to fulfill the requirements for both high percentage removal and high infiltration rate compared to zeolite and fly ash. Moreover, the effect of aspect ratio, surface area, particle size and chemical composition of each fine material on its efficiency in heavy metal ions removal and infiltration rate were compared.