The Effect of Biochars Produced at Different Temperatures on Adsorption and Desorption of Pb2+ in a Calcareous Clay Soil

Background and goal:

 Soil contamination with heavy metals (HMs) has caused great concern with the development of industry and the rapid increase in human activities. Lead (Pb2+) accumulates easily in the environment due to its non-degradable nature and poses a serious threat to the lives of plants, animals, and especially humans. More dangerously, HMs are usually present simultaneously with different compounds and concentrations, complicating the management of these pollutants. Reducing the bioavailability of HMs by adding stabilizing agents to the soil is a reliable and low-cost way to control HMs pollution.

in this study, Walnut leaves biochars were produced at three pyrolysis temperatures of 200, 400 and 600 °C (B200, B400, and B600). For this purpose, 3 g (1% w/w) of the above treatments were added to 300 g of soil sample put in plastic jars, and incubated for 90 days at 21 ± °C and a humidity of 80% of the field capacity. Then, the soil was sampled at 30 and 90 days after incubation. To measure the adsorption of Pb2+ in the soil, first 2 g of each treated soil into 50 ml centrifuge tubes and 20 ml of PbNO3 solution containing concentrations of 0.5, 1, 2, 3, 4, 6, 7 and 8 ml of Pb2+ (single system) and Pb2+ + Zn2+ (mole ratio Pb2+ / Zn2+ = 1; competitive system) were added to the tubes in the 10 mM CaCl2 as the background electrolyte. The linear form of the Longmuir equation was used to determine the individual and competitive adsorption characteristics of Pb2+. To determine the desorption of adsorbed Pb2+, 20 ml of 10 mM CaCl2 was added to the residual soils in centrifuge tubes from the adsorption study.

The results showed that maximum adsorption capacity (qm) of Pb2+ increased with increasing pyrolysis temperature, so the B600 treatment had the highest maximum adsorption capacity among the treatments. The value of this coefficient decreased in the presence of Zn2+ (P <0.05). The qm of Pb2+ adsorption in biochar treatments decreased significantly after 90 days of incubation. The highest amount of Pb2+ strength of adsorption (KL) was in the biochar prepared at 600 °C treatment. The value of this coefficient in the competitive system decreased compared to the individual system. After 90 days of incubation in both adsorption systems, KL decreased compared to 30 days of incubation (P <0.05). The concentration of Pb2+ desorption in 10 mM CaCl2 solution (less than 1% of adsorbed Pb2+) showed that the exchange mechanism of Pb2+ adsorption by biochars doesn’t play importance role and probably the main mechanism of Pb2+ adsorption is formation Pb-phosphate. Overall, the results of this study showed that the application of 1% biochar prepared at 600 °C can affect the Pb2+ adsorption properties in clay calcareous soils in individual and competitive systems.

Walnut leaves biochars produced at different temperatures changed the Pb2+ adsorption and desorption process in loamy clay soil in the presence of Zn2+ and the incubation time. Although calcareous clay soils have a high capacity to Pb2+ adsorption, but biochar was able to significantly increase the strength of adsorption and Pb2+ maximum buffering capacity (MBC) in the soil and reduce the desorption of this metal. Therefore, the use of the biochar can be considered as a low-cost and effective adsorbent for stabilizing and reducing Pb2+ mobility in the soil and increasing the productivity and health of agricultural soils.