Sorption of Lead (Pb2+) from aqueous solutions using chars obtained in the pyrolysis of forestry pine, rubber tires and plastics

In the present study it was investigated the characteristics and ability of chars produced in the co-pyrolysis of forestry pine, used tires and plastic wastes for the removal of lead (Pb2+) from aqueous medium through a batch process. The textural properties of the chars were determined and the effects of the following experimental parameters in the adsorption capacity were investigated: pH, contact time, initial Pb concentration and adsorbent mass. To elucidate about the mechanism of lead adsorption, the release of several inorganic species from the chars after the adsorption of lead was determined. The results of the textural characterization indicated that chars from feedstocks with pine biomass in their composition are predominantly of macroporous nature; on the other hand, the introduction of tires in the pyrolysis feedstock enhanced the mesoporous content and surface area of the resulting chars. The kinetic study indicates that Pb2+ adsorption with the pyrolysis chars is a slow and complex process in which the overall sorption rate is actually a balance of adsorption and desorption processes. The study on the effect of solution pH indicates that the optimum pH is around 6.0. The study on the effect of the initial Pb2+ concentration shows that, in general, the adsorption capacity of the chars increases with an increase of lead concentration; inversely, it was observed a decrease in the removal efficiency (%) of Pb2+ with the increase of the initial concentration of lead. Regarding the study on the effect of adsorbent mass, it was found a general increase in the lead uptake for the three chars with the increase of char's mass. For high char dosages, the adsorption capacity decreases due to an aggregation effect of chars particles. The mixture of the three raw materials (forestry pine, used tires and plastic wastes) did not have advantage for the resulting pyrolysis char concerning the adsorption capacity and removal efficiency of Pb2+. The char obtained from the pyrolysis of tires mixed with plastics presented the better performance in lead adsorption with a removal efficiency of 98.9%. The study on the adsorption mechanism of this char revealed that significant amounts of Zn, Ca and K were released after Pb sorption which put in evidence that chemisorption by cation exchange was an important mechanism on Pb2+ removal.