Document Type : Research Paper
The study of Pb (II) immobility in contaminated soils using phosphate, heat and lime (CaCO3)
Introduction: The nature of heavy metals and their solubility in soil is important in the transition of these hazardous materials into plants, water resources and such compounds pose a serious threat to human health. Water, soil and air pollution are important component of industrial activities and directly exposed to the environment. Lead is one of the most important environmental pollutants and can from different ways contaminate soil and water resources. Since phosphate ions for stable complexes with cations such as Pb and Cd, so cause decrease the solubility and mobility of heavy metal in soil. Liming is the most widely used treatment, and leads to the precipitation of metal as metal–carbonates and significantly decreases the exchangeable fraction of metals in contaminated soil. Temperature is also an important factor in stabilization of heavy metal. Heat causes loss of water and hydrated around cations move them to empty spaces clay part of soil. The aim of this study was to investigate the effect of heat and lime, phosphate application on the immobilization of Pb in contaminated soil.
Materials and methods: Soil sample was collected from waste mining of Angorane in Zanjan region and their physicochemical properties of soil were determined using standard methods. For stabilization mean, two levels of lime 0, 5% and 0, 2.5% phosphate fertilizer and mixed of lime (0, 5%), phosphate fertilizer (0, 2.5%) were added and mixed with 300 gr soil. Treatments as six different temperatures 25, 200, 400, 600, 700 and 800C were heated with electrical oven (Shimiran f.47) and loaded in distilled water for a different times (7, 30, 60 days), so samples were shaken every day 15 minutes. Samples were centrifuged at 2500 rpm for 5 minutes and passed through a filter paper. Pb concentration was measured in the supernatant using atomic absorption model (Shimadzu 6600). All analysis of variance and mean comparison were performed using SPSS and MSTATC software.
Discussion of Result: Physicochemical properties of soil sample showed in table 1. The soil was acidic and saline due to the existence of salts Cd and Pb. Ionic strength of soil increases as a high amount of salts and affected adsorption and desorption processing. Also due to the absence of carbonate in the soil (0%), these salts are available as sulfate of Cd and Pb probably. The soil sample contains 23% clay and has a silty loam texture class. The XRD result showed that the illite was the dominant mineral of soil. Total Pb concentration was 18000 mgkg-1 and indicates the severity of soil contamination. Based on the America's environmental protection agency (EPA) standards the allowable limit for the existence of Pb in soil is 100 (mg kg-1). Therefore, concentration of the metal in the soil was too standard and introduced as a contaminated soil.
Table 1. Some physicochemical properties of soil sample
Soil Texture Clay CCE Ec
(dSm-1) pH CEC
(meq100-1g) Total Pb
Salty loam 23 0 20.4 5.5 8.6 18000
Analysis of variance showed independent effects of heat and treatments during the total periods were significant. The interactions of factors at 7 and 30 days were significant (Table 2).
Table 2. Analysis of variance for effected by heat and treatment (phosphate, lime, and time) on desorption of Pb
Source df Mean Sqaure
7day 30day 60 day
Heat 5 1299.5*** 429.6*** 302.2***
Treatment 3 116.8*** 168.7** 79.7**
Heat*treatment 15 92.8*** 60.2** 31.58ns
Error 24 9.77 19.83 13.68
CV (%) 3.1 4.3 2.3
-no significant (ns), significant on the 0.1 (***), significant on the 0.05 (*)
The results showed that lead release was a function of temperature variation and increased Pb release in all treatments except for 200 °C. High temperatures break the crystalline silicate minerals of the soil and Pb release was significant at a temperature up to 600 ° C, but no significant difference was found between 600 and 800 ° C. Also, the temperature increase reduces the soil's specific surface and increases the release of Pb (Figure 1). It seems that the heat treatment causes structural changes in the structure of clay minerals and different minerals. The increase of phosphate causes the change direction effect of heat at stabilized Pb.
Figure 1. The main effect of heat and treatment in desorption of pb
The lime application was significantly on lead stabilization compared to the blank on first stage and the highest desorption was observed at 700 ° C. The soil pH and EC are main parameters controlling the solubility and mobility of heavy metals in soils. Therefore soil sample was acidic (pH<7) and high EC. The acidity of the soil-studied cause increases the solubility of phosphate anions and Pb solution was precipitated by phosphates anion so reduces the concentration of soluble Pb. The combined effects of phosphate and lime synergistic mode (Synergism) and cause increased of soil pH.
The results showed that lime and ion phosphate behavior were differed in non-removing Pb of contaminated soils, so that phosphate ion reduces the solubility and transfer of lead in contaminated soils. But limestone application was not suitable for the stabilization of Pb in highly polluted soils. Heat changed the structure of clay minerals and increased the release of elements. Also, the heat increase led to a change in the direction of the effect of phosphate on the stabilization of lead. So , the use of phosphate sources was found to be suitable for the stabilization of lead in acidic soils and saltines contaminated with elements.
Keyword: Clay minerals, Heavy metals, Phosphate, Lime.