Sorption of Trace Elements by Biotic and Abiotic Soil Components and Their Composites
Zhu, Jun (2010) Sorption of Trace Elements by Biotic and Abiotic Soil Components and Their Composites. [Tesi di dottorato] (Inedito)
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With the increasing discharge by excessive application of fertilizer and the development of various industrial productions, the pollution by trace elements in natural environment has received great attention in recent decades. The sorption of trace elements on soil components is considered to be a vital process in controlling their mobility and bioavailability. Extensive studies have investigated the sorption of trace elements on phyllosilicates, metal oxides, organic substances and microorganisms in single sorbate or on single sorbent systems. However, to date, scant attention has been paid to the competitive sorption among heavy metals on organomineral and oxide-bacteria complexes, the effect of low molecular weight organic ligands (LMWOL) on the sorption of heavy metals by bacteria, the competitive sorption of heavy metals on phyllosilicates such as montmorillonite and the effect of cationic macroelement such as Fe, Al, Ca and Mg on their sorption. The influence of foreign ligands such as phosphate on the sorption of arsenate by layered double hydroxides (LDH) has not been studied in detail. In this thesis, we studied the competitive sorption among Cu, Cr(III) and Pb on ferrihydrite, goethite, bacteria (B. thuringiensis), montmorillonite and some of their composites. We also investigated the influence of LMWOL on the sorption of Cu, Cr(III) and Pb by bacteria, the influence of macroelement including Fe, Al, Ca and Mg on the sorption of Cu, Cr(III) and Pb by montmorillonite and the influence of phosphate on the sorption/desorption of arsenate on/from LDH. The main results are outlined as follows: 1. Ferrihydrite sorbed much larger amounts of Cu, Cr and Pb than the organomineral complexes when mmol kg-1 mass scale was considered, whereas the opposite trend was true when mol m-2 surface scale was taken into account. Higher organic carbon content in the precipitates induced larger sorption (mol m-2) of heavy metals. The competition in sorption of the heavy metals was affected by the nature and the initial concentration of the elements, the surface properties of the sorbents, the addition sequence of the trace elements and the residence time. 2. The sorption of Pb, Cu and Cr by B. thuringiensis was greater and stronger on nonliving cells than that on living cells. The competition among three heavy metals in the sorption was more intensive on living cells than that on nonliving counterparts and the competitive ability followed the order of Pb > Cu > Cr on both cells. The sorption of examined heavy metals was depressed by anionic ligands especially on nonliving bacteria, a sequence of citrate > oxalate > tartrate in inhibitive ability was observed on both cells. 3. The sorption of heavy metals was more and faster on goethite-B. thuringiensis complex than that on goethite and was suppressed by their competition. The competitive sorption between Cu and Cr was more intensive on goethite-B. thuringiensis complex than on goethite and the competitive ability of Cr was stronger than that of Cu on both sorbents. Desorption of sorbed heavy metals by Ca(NO3)2 was usually slightly higher from single systems than that from binary systems. The lower amounts (percentages) of heavy metals desorbed from goethite than from goethite-B. thuringiensis complex confirmed the higher affinity of heavy metals for the former than for the latter sorbent. 4. The selectivity of montmorillonite for Cu, Pb and Cr followed the sequence of Cr >> Cu > Pb in simultaneous sorption, the competition among heavy metals was relieved by the increase of pH and intensified by the increase of surface coverage. The sorption of heavy metals on montmorillonite was generally inhibited by Ca and Mg, while Al and Fe could inhibit or promote the sorption of heavy metal, depending on the solution pH and the concentration of macroelements. The sequence of Al > Fe > Ca > Mg was observed in the inhibition of macroelements on heavy metal sorption while higher efficiency of Fe species over Al species was found in their promotion on the sorption of heavy metals. 5. The LDH samples (LDH-Cl and LDH-CO3) sorbed more phosphate than arsenate, but after calcination LDH-CO3 showed a greater capacity to sorb both the oxyanions. Competition in sorption between phosphate and arsenate was affected by pH, reaction time, surface coverage and sequence of addition of the anions. The final arsenate sorbed/phosphate sorbed molar ratio (rf) increased with reaction time or by adding arsenate before phosphate, but decreased by increasing pH and by adding phosphate before arsenate. High percentages of arsenate initially held on LDH minerals were desorbed by phosphate (about 80%), but from LDH-CO3-450 (<40%), probably due to the formation of strong inner-sphere complexes onto the surfaces of mixed Al-Mg oxides formed after calcination. The information found in this study furthered the understanding on the mobility of trace elements in both cationic and anionic forms in polluted environments.
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