Experimental characterisation and evaluation of perlite as a sorbent for heavy metal ions in single and quaternary solutions

Due to its unique properties such as low bulk density, high water retention capacity and high air-filled porosity, perlite has been recently employed as an important substrate additive to support vegetation in green roofs, vertical gardens and biofilters. Considering that the runoff quality of the above systems often comes under debate, it is important to explore the sorption capacity of substrate additives. Thus, the present study explores the potential of perlite to sorb important metal ions such as Cd2+, Cu2+, Ni2+ and Pb2+ in single and multi-component solutions. Characterisation of perlite indicated that the material possesses low bulk density (148 kg/m3), high water holding capacity (27.5%) and high air-filled porosity (31.1%). Experimental results according to the Langmuir model revealed that perlite sorbed 0.130, 0.089, 0.057 and 0.025 mmol/g of Pb2+, Cu2+, Ni2+ and Cd2+ respectively, at an optimum pH of 5. The kinetics data were well described by pseudo-first order model, followed by the pseudo-second order and Elovich models. Quaternary sorption experiments showed that the presence of multiple ions had an adverse effect on the sorption capacity of perlite towards individual ions. Application of Sheindorf–Rebhun–Sheintuch equation to experimental quaternary isotherm data confirmed that sorption of Cd2+ ions was severely affected whereas Pb2+ ions were favoured.