Evaluation of the effects of mandarin (Citrus reticulate) by-products containing citric acid on immobilization of cadmium in arable soils

Abstract

In a pilot study, we observed cadmium (Cd) immobilization with citric acid (CA) and suggested that mandarin by-products (MB), which contain CA at ca. 1.65%, can be used as soil amendments that reduce Cd bioavailability. In the present study, we (1) elucidated mechanisms of Cd immobilization by CA and (2) evaluated the use of MB as a soil amendment for Cd immobilization. In Experiment 1, CA was mixed with Cd contaminated soil at 0 and 3.5 mmol kg−1. We then added MB to Cd-spiked soil at 10, 20, and 40 g kg−1. Addition of CA decreased F2 (surface adsorbed Cd fraction) contents by 2.64 mg kg−1 compared with the control but was associated with increases in Cd fractions F1 (bioavailable Cd fraction) and F5 (residual Cd fraction) of 1.04 and 1.49 mg kg−1, respectively. Addition of CA enhanced the concentration of fraction F5, likely reflecting Cd precipitation from soil solutions with increased HCO3− concentrations. However, although this treatment immobilized Cd, it also led to increasing residual and bioavailable Cd fractions. Unlike CA treatments, MB increased non-bioavailable Cd fractions without increasing the bioavailable Cd fraction. Moreover, at 40 g kg−1, MB decreased F1 contents by 8% compared with the control, but increased F2, F3, and F5 contents by 3.6%, 0.7%, and 4.5%, respectively. Cd may be immobilized by MB through HxCO3− mediated precipitation as CdCO3 following decomposition of CA and concomitant increases in the negative charge of soil due to the organic matter in MB. MB also improved the chemical properties of soils, with increased nutrient concentrations and cation exchange capacities.