Immobilization of Lead in a Calcareous Contaminated Soil using Organic and Inorganic Amendments

Soil pollution with heavy metals have become a global concern because of its damaging effects on the environment, including human health, toxicity in plants and long-term effects on soil fertility. Heavy metals stress in plants is characterized by decrease in photosynthesis, nutrient uptake, damaging of roots and finally plant death. Lead (Pb) is found to be the most dangerous heavy metal, responsible for reduced soil fertility and elevated environmental pollution. Lead toxicity causes the inhibition of seed germination and exerts adverse effects on growth and metabolic processes of plants, which retards plant and crop production. The overproduction of reactive oxygen species (ROS) is the best indicator for secondary stress, which results in a number of toxic effects on biochemical processes in many plant cells. The overproduction of ROS due to Pb stress brings about changes in cellular membrane permeability, which in turn damages organelles such as nuclei, mitochondria, and chloroplasts in plant cells which decreased plant growth and yield. Chemical stabilization is an in situ remediation method that uses inexpensive amendments to reduce contaminant availability in polluted soil. The aim of this study was to investigate the immobilization of lead in a calcareous contaminated soil using two types of biochar as organic and Pumice, Leca, Zeolite and Bentonite as inorganic amendments.
In order to investigate the effect of organic amendments (biochar 640°C, and biochar 420°C) and inorganic amendments (Pumice, Leca, Zeolite and Bentonite) on Pb stabilization in a contaminated soil (1500 mg/kg), a greenhouse experiment using maize plant was carried out. This experiment was conducted in a completely randomized design consisting of 6 types of amendments (Pumice, Leca, Zeolite, Bentonite, Biochar 420°C, and Biochar 640°C) and at 1% and 5% levels of each amendment (12 amendments plus 1 control). The experimental treatments were incubated for 3 months. At the end of incubation time, the potential bioavailability of Pb in non-amended and amended soils was assessed by chemical extractions, as: extraction with DTPA, with ammonium acetate and with ethylenediaminetetraacetic acid (EDTA). After the end of incubation time, the pots were transferred to a greenhouse and in each pot five maize seeds were planted and then reduced to three seedlings in each pot after germination. After 3 months, all the plants were harvested. The Pb concentration in each plant, its biomass, its chlorophyll and its antioxidant enzyme activities levels were analyzed. All statistical analyses were performed using SAS software. Means of different treatments were compared using LSD (P ≤0.05) test.
The results indicated that the addition of amendments to soils reduced the concentration of Pb extracted with DTPA and EDTA. The 5% biochar 640 had the greatest reduction effect on DTPA-extractable Pb. The smallest concentration of Pb in the leaves and root of maize plant was observed in treated soil with organic amendments (biochar 640°C, and biochar 420°C) and treated with 5% zeolite, respectively. The highest increase in plant growth parameters like SPAD value, leaf area, plant height, number of leaves per plant, dry biomass yield and dry matter of roots were observed in organic amendments compared to the control. The application of 5% amendments in soil caused a significant increase in plant height and number of leaves as compared to control. The increase in growth and biomass of zea mays L. under various amendments might be due to decreased bioavailable Pb concentrations in soil amended which may be attributed to reduced Pb toxicity through improvement of soil fertility. Also, the application of amendments resulted in a significant increase in antioxidant enzyme activities such as superoxide dismutase (SOD), catalase (CAT), peroxidase (PX), and ascorbate peroxidase (ASP) in maize plants compared to the control. The increase of leaves enzyme activities with addition amendments may be due to a lower Pb accumulation in leaves because excess Pb generates free radicals and reactive oxygen species (ROS) those causes oxidative stress in plants.
The results indicated that the application of amendments were successful in lowering the potential bioavailability of Pb in the soils. The 5% biochar 640 treatment had the greatest reduction effect on extractable Pb. The application of amendments decreased the uptake and accumulation of Pb in maize plants, via the reduction of DTPA- extractable Pb. The amendments also significantly increased leaves antioxidant enzyme activities and photosynthetic pigments compared to the control.