Removal of Copper(II) and Lead(II) from Soils by Poly(Amidoamine) Dendrimers and Reductive Immobilization of Chromium(VI) by Stabilized Zero-Valent Iron Nanoparticles
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This research investigated the feasibility of using poly(amidoamine) PAMAM dendrimers for removal of copper(II) and lead(II) from soils, and using a new class of stabilized zero-valent iron (ZVI) nanoparticles for reductive immobilization of chromium (VI) in contaminated soils. PAMAM dendrimers ranging from generation (G) 1.0 to 4.5 and with –NH2, –COO-, and –OH- terminal groups were tested for extraction of copper(II) and lead(II) from a sandy loam soil, a clay soil, and a sandy clay loam. A series of fixed-bed column experiments were conducted to study the effects of dendrimer dosage, generation number, functional groups, pH, ionic strength, and soil type on the removal efficiency. It was found that more than 90% of the preloaded copper (II) was removed by ~66-bed volumes a dendrimer solution containing 0.10% (w/w) of a generation 4.5 dendrimer with –COOH terminal groups and at pH 6.0. Approximately 92% of the initially sorbed lead (II) was removed by ~ 120 bed volumes containing 0.3% (w/w) of a generation 1.5 dendrimer with –COONa terminal groups at pH 4.0. The spent dendrimers were recovered through nanofiltration and then regenerated with 2 N hydrochloric acid. The recovered dendrimers were then reused and showed comparable metal removal effectiveness to that of fresh dendrimers. An ion-exchange based approach was developed to determine the apparent stability constants of the Cu(II)- or Pb(II)-dendrimer complexes. The method was derived by modifying the traditional Schubert ion exchange method, but offered a number of advantages, including the application of a non-linear reference isotherm and extension of the classical approach from mono-nuclear to poly-nuclear complexes. To simulate the dynamic metal removal process by dendrimers, a two-site model was formulated. The model envisions the soil sorption sites as two distinguished fractions: one with a fast desorption rate and the other with a slow desorption rate. The model was able to not only simulate the elution histories of lead and copper by various dendrimers, but also prove promising to predict the metal elution histories under various conditions such as initial metal concentration in soil, dendrimer dosage, and solution pH. An innovative in situ technology for reductive immobilization of Cr(VI) was tested. A new class of stabilized zero-valent iron (ZVI) nanoparticles was prepared using sodium carboxymethyl cellulose (CMC) as a stabilizer. Batch and column experimental results revealed that the ZVI nanoparticles could effectively reduce Cr(VI) to Cr(III), and reduce the Cr leachability by ~90%.