Mechanisms of cadmium and lead retention by novel phosphorus/magnesium-engineered biochar

Abstract

In this study, novel phosphorus/magnesium-engineered biochars were prepared from co- pyrolysis of poultry litter and phosphate sources and magnesium oxide (MgO) and tested for

their cadmium (Cd2+) and lead (Pb2+) retention capacities, unraveling the adsorption mechanisms. In the first article, which was published at Science of the Total Environment Journal (Elsevier), batch experiments were conducted to evaluate the adsorption ability of Cd2+ by biochars and a wide range of characterization techniques were used. Results showed that, in general, Cd2+ removal did not drastically change with initial pH variation and was

relatively fast (up to 3 h). The surfaces of the biochars contain a rich variety of oxygen- containing functional groups as well as phosphate groups. Since the specific surface areas of

the biochars are considered low (up to 25.19 m2 g -1 ), surface groups contributed more to Cd2+ retention. Complexation and precipitation were the predominant adsorption mechanisms. Thus, P/Mg-engineered biochars are considered effective and eco-friendly adsorbents for Cd2+ removal from aqueous medium. In the second study, it was determined the efficiency of Pb2+ removal by phosphorus/magnesium-engineered biochars through batch assays and the retention mechanisms were elucidated, using advanced characterization methods. Adsorptions of Pb2+ by the P/Mg-engineered biochars (up to 600 mg g-1

) were nearly 10 times

more effective than the non-treated biochar (60 mg g-1

). The functional groups on the surface of the biochars are modified during adsorption, especially phosphorus and oxygen-containing groups. The contents of P and Mg increased after modification, showing high correlation between adsorbed Pb and P and O, which interacted with Pb through chemisorption (especially precipitation and complexation). Treatment with phosphate compounds and MgO, which increased the pH of the biochars, caused the formation of insoluble lead apatites on the surface of the P/Mg-engineered biochars, as well as other Pb inorganic crystalline components, including Pb3(CO3)2(OH)2 and PbCO3, as observed in the X-ray diffraction

(XRD) patterns, and confirmed by X-ray absorption spectroscopy (XAS). Thus, P/Mg- engineered biochars produced from poultry litter are considered effective and eco-friendly

adsorbents for Pb-contaminated aqueous environments, such as wastewater. Since the retention mechanism was deeply investigated, this study provides a basis for a successful practical application of the biochars in environmental remediation regarding Pb and Cd contamination.