Effects of natural zeolites on ryegrass growth and bioavailability of Cd, Ni, Pb, and Zn in an Albanian contaminated soil

Purpose The use of eco-friendly and cost-effective adsorbent materials in the remediation of soils contaminated by potentially toxic elements (PTE) is a sustainable way of reducing the transfer of these elements into the food chain. However, an evaluation of the potential of natural zeolites to immobilize toxic elements in contaminated soils was required to enable their efficient use. Materials and methods The effect of natural zeolite (Stilbite-Stellerite) from the Munella area (Northern Albania), added at rates ranging from 1.25 to 10 % w/w on a contaminated soil was investigated in a greenhouse pot experiment with ryegrass (Lolium multiflorum L.) and by selective extractions. PTE availability for plants was assessed either as their accumulation in plant tissue or by DTPA-extraction. Oral bio-accessibility was estimated by the in vitro PBET method and the mobility and consequent potential risk of leaching by the USEPATLCP method. The effect of zeolites on soil properties (pH, electrical conductivity-EC, organic C, and total N) was also investigated. A five steps sequential extraction procedure (SEP) was applied to investigate the immobilization mechanism. Results and discussion The addition of 2.5% w/w of natural zeolites caused a significant decrease of PTE mobility, but to observe a significant reduction of DTPA-extractable metals, it was necessary to reach 10­dition rate. In contrast, plant growth showed a gradual increase with addition rate and a corresponding decrease of concentration of PTE in plant tissue. Correlation between DTPA-extractable PTE and their concentration in both root and shoot plant tissue was rather poor. Human hazard due to soil ingestion (PBET method) changed only for Cu and Zn in the gastric phase with 1.25 and 5% addition rate respectively, whereas decreased for Cu and Zn at 5% rate in the Intestinal phase. The results of SEP support the hypothesis that the main mechanism involved in metals fixation are as follows: (1) insolubilization by pH rise, (2) adsorption on Fe/Mn oxides (3) increase of cation exchange retention, (4) organic complexation. Conclusions The results of this work suggest that the addition of natural zeolites from the Munella area (AL) is a sustainable practice to reduce the environmental impact of PTE contaminated soils, but an assessment on the longevity of their immobilization need to be evaluated in the long-term perspectives.