The effects of chitosan composites on the immobilization of chromium in soil and marigold (Calendula officinalis) growth

To study the effectiveness of various chitosan composites on the stabilization of Cr 6+ in contaminated soils and growth characteristics of the marigold plant, a factorial pot trial with completely randomized design and three replications was performed in controlled conditions. The experimental treatments were: the contamination levels of soil to Cr (0, 20, 60 and 180 mg/kg) and adsorbent types (pure chitosan, biochar, zeolite and manganese dioxide, composites of chitosan–biochar, chitosan–zeolite, chitosan–manganese dioxide and control). The results showed that increasing the concentration of Cr in the soil interfered with the processes of nutrient uptake by the marigold and led to a significant reduction in growth characteristics. But the addition of adsorbents to the soil by immobilizing some of the available Cr reduced marigold uptake of Cr and thus reduced its toxic effects. By adding adsorbents to the soil, marigold uptake of nutrients such as phosphorus, potassium and iron was improved, and growth indices increased. Among the adsorbents used in this experiment, the chitosan–manganese dioxide composite had a higher efficiency in the immobilization of Cr in the soil. By applying the chitosan–manganese dioxide composite to the soil at the rate of 0.5% W/W, the concentration of Cr in the shoot of the marigold plant decreased about 87.61% when compared to the control treatment. But the shoot height and shoot dry weight increased by 48.38 and 59.28%, and the concentration of phosphorus, potassium and iron in plant tissues increased by 100.00, 129.62 and 320.33%, respectively.