The effect of carrot pulp derived biochar on the adsorption of cadmium and lead in an acidic soil

Soil contamination with heavy metals has become a global concern because of its adverse effects on ecosystem health and food security. Soil amendments including biochar can reduce the bioavailability of heavy metals in contaminated soils and reduce their risk of entering the food chain. Biochar is a carbon-rich material obtained by pyrolysis of biomass, such as agricultural residues and manures under conditions with little or no oxygen. Recent studies have shown that biochar also has great potential for immobilizing heavy metals in soil, because of its highly porous structure, active functional groups, and generally high pH and cation exchange capacity (CEC). In the country, most of the studies on biochar are related to calcareous soils or aqueous solutions and biochar has received little attention as an efficient adsorbent for the adsorption of heavy metals in acidic soils. The goal of this study was to investigate the effect of biochar derived from carrot pulp on properties of acidic soil and adsorption of lead and cadmium in soil.

In this study, biochar derived from carrot pulp was produced through pyrolysis at 550 °C with a heating rate of 25 °C min-1. Carrot pulp biochar was added to acidic soil at 0, 4 and 8% application rates and was incubated for 60 days. After the incubation period, to investigate the effects of time on Cd and Pb adsorption, 25 mL of 400 mg L−1 of Cd and Pb were added to 1gr of soil samples, and then samples were shaken for different times (4, 8, 12, 16, 20, 24, 28, 32, 36 and 40 h). Then, the concentration of Cd and Pb were measured. The results were fitted to pseudo-first-order, pseudo-second-order, and Elovich kinetic models. To determine the adsorption isotherms of Cd and Pb, 25 mL of heavy metals solution with concentrations ranging from 0 to 400 mg L−1 were added to 1gr of soil samples, and then samples were shaken for 24 h, and finally the adsorption data were fitted to Langmuir, Freundlich, and Temkin isotherm models.

The application of biochar significantly increased the pH, cation exchange capacity and electrical conductivity of the soil, although the 8% application rate more effective than the 4% application rate in increasing the pH, cation exchange capacity and electrical conductivity of the soil. The results of fitting the data to the Langmuir, Freundlich and Temkin isotherm models showed that Cd and Pb adsorption on the control soil and soil treated with biochar matches with the Langmuir isotherm model. The pseudo-second-order kinetic model was introduced as the best kinetic model of adsorption of Cd and Pb. The maximum adsorption capacity of Cd and Pb increased from 647.25 mg kg-1 and 855.71 mg kg-1 (in control soil) to 2078.29 mg kg-1 and 3182.72 mg kg-1 (in soil treated with 8% biochar), respectively.

This study showed that carrot pulp biochar increased the adsorption of Cd and Pb in acidic soil by increasing the pH and cationic exchange capacity of the soil. Therefore, carrot pulp biochar can be used as an effective and inexpensive adsorbent to improve acidic soil properties and increase the adsorption of Cd and Pb in soil. Keywords: Biochar, Adsorption, Kinetic, Cadmium, Lead