Abstract
The present study investigated the adsorption of Cd2+ by nonmagnetic and magnetic biochars (CMB and M-CMB) derived from chicken manure, respectively. The adsorption characteristics were investigated as a function of initial pH, contact time, initial Cd2+ concentration and magnetic separation. Adsorption process of both biochars were better described by Pseudo-second-order kinetic equation and Freundlich isotherm model, which were spontaneous and endothermic in nature. It was found that maximum capacities were 60.69 and 41.07 mg/g obtained at the initial Cd2+ concentration of 180 mg/L for CMB and M-CMB, and the turbidity of adsorption-treated solution was reduced from 244.3 to 11.3 NTU after magnetic separation of 0.5 min. These indicated that M-CMB had lower adsorption capacity of Cd2+ than CMB, though it was successfully separated from the treated solutions. Furthermore, both biochars before and after adsorption were analyzed by SEM-EDS, XRD and FTIR. Adsorption mechanisms mainly included precipitation, ion-exchange, complexation and Cπ-coordination, in which precipitation and ion-exchange dominated the adsorption process by CMB, while in M-CMB, precipitation was always predominant mechanism, followed by ion-exchange. The two other mechanisms of complexation and Cπ-coordination were trivial in both biochars, jointly contributing 7.21% for CMB and 5.05% for M-CMB to total adsorption. The findings deepen our understanding of the mechanisms governing the adsorption process, which are also important for future practical applications in the removal of heavy metals from wastewater by the biochars.
Highlights
Cadmium (Cd) is one of the most toxic heavy metal pollutants in wastewater, since it accumulates in living organisms and passes through the food chain into human organs, causing serious toxicity even at low concentration of 0.001–0.1 mg/L [1]
Both biochars were alkaline with high pH values (Table 1, 11.43 for CMB and 11.55 for M-CMB), suggesting that it would potentially remove heavy metals from acid mine drainage [24]
The ash contained a large amount of alkaline mineral elements such as K, Ca, Na and Mg, as demonstrated by EDS and XRD results, which might begin to separate from the organic matrix during the pyrolysis process at high temperature (>300 ◦ C), resulting in the increase of pH to strong basicity [25]
Summary
Cadmium (Cd) is one of the most toxic heavy metal pollutants in wastewater, since it accumulates in living organisms and passes through the food chain into human organs, causing serious toxicity even at low concentration of 0.001–0.1 mg/L [1]. Significant quantities of Cd are introduced into the environment by anthropogenic activities, including manufacturing of nickel-cadmium batteries, synthetic pigment production, metal coatings, stabilizers in plastic products, and incineration of solid wastes, in which the worldwide production of Cd in 2005 was estimated to be 20,000 metric tons [2]. The adsorption using agricultural waste materials is one of the most popular and effective. Res. Public Health 2020, 17, 1602; doi:10.3390/ijerph17051602 www.mdpi.com/journal/ijerph
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