Department of Agriculture and Environmental Sciences

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 USEPA TLCP 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 10% addition 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 immobili-zation need to be evaluated in the long-term perspectives.

The aptness of the ∑SEM/AVS (Simultaneously Extracted Metals/Acid Volatile Sulfides) index to ascertain environmental risk from potentially toxic elements in calcareous saltmarsh soils was tested using Structural Equation Modelling. This technique allows to detect both direct and indirect relationships among AVS, SEM and other soil variables, representing results in a graphical view. The dataset included 90 soil samples from 21 different sites belonging to six different saltmarshes and featured a wide range of soil chemico-physical properties. Variables included in the a priori model were hydroperiod, pH, soil redox potential, labile organic carbon, carbonates, total Fe and total amount of potentially toxic elements (PTE). The best optimised model pointed out the main soil properties which affect AVS accumulation and SEM speciation in these soils. Effect plots of AVS and SEM calculated with the partial linear mixed-effects models included in the piecewise Structural Equation Modelling showed a significant and positive influence of pH and of carbonates on AVS and a highly significant effect of carbonates and labile organic carbon on SEM. Single SEM components were also considered separately in order to define potential contributions of labile organic carbon or carbonates as alternative binding phases. Simultaneously extracted Cu, Ni and Zn resulted preferentially bound to carbonates followed by labile organic carbon, whereas Pb and Cd were easily bound to labile organic carbon.