Effect Of Biochar On The Fate Of Heavy Metals In Soil Environment

Biochar was considered as an ideal soil conditioner due to its special properties.But the biochars obtained from animal manure and municipal sewage sludge contained high concentration of heavy metals. There were few researches about the macroscopic as well as microscopic change process and reaction mechanisms of these heavy metals in biochars and soil. In this study, poultry and sludge biochars were pyrolysed at 500 ? in a muffle furnace without oxygen. Then the biochars were ozonized with O₃ to study the influence of ozone oxidation on characteristics and heavy metal adsorptive performance of biochars. The resulting biochars were designated as PBO and SBO for the poultry manure and drying sludge derived biochars, respectively. The mechanism of adsorption on poultry biochar and sludge biochar were also studied. Aiming at increasing application of biochar to soil, we investigated and analyzed macro/micro changes and mechanism of carbon, sulfur,phosphorus as well as heavy metals concentration within biochar in red and mining area soil. The main results of this thesis were derived as under:?1? Changes in structural characteristics for poultry manure biochar ?PB? and sludge biochar ?SB? before and after oxidation were investigated by FTIR, XPS, XRD,BET and elemental analyzed. The results showed that the oxidation of O₃ increased oxygen content of PB to increase the type and amount of its oxygen functional groups?which added the carboxyl group?, while reducing its aromatic and specific surface area, but the ozone oxidation was not useful for SB. Sulfur in the biochars were predominantly inorganic S, among which the inorganic sulfur account of PB and PBO was 91.83% and 83.46% respectively. Ozone oxidation could not change the mineral composition of biochars. PB was mainly composed by SiO2 and CaCO₃. The mainly minerals in SB were CaSO₄, SiO2 and CaCO₃.?2? Adsorption capacity and mechanisms of lead, copper and zinc by the PB and SB were studied by batch adsorption experiments, FTIR and XRD. It was found that the adsorption process of lead, copper and zinc were consistent with the Langmuir isothermal adsorption equation. The maximum adsorption capacity Qm of PB was Pb?903.36 mmol/kg?> Cu ?196.08 mmol/kg?> Zn ?64.10 mmol/kg?. The maximum adsorption capacity Qm of SB was Pb ?2927.44 mmol/kg?> Cu ?158.73 mol/kg?> Zn?45.45 mmol/kg?. And the adsorption mechanisms for lead, copper and zinc included the oxygen-containing functional groups complexation, Pb2+? interaction with ?electrons of C=C and co-precipitation with minerals.?3? The carbon as well as sulfur functional groups and chemical characteristics of the biochars before and after incubation in typic udic ferri soils were determined using spectroscopic techniques, such as FTIR, XPS, combined with chemical extractions.Metal species were characterized using Cu K-edge X-ray absorption near-edge structure and chemical sequential fractionation schemes. The results showed that the potential changes in structural characteristics and metal species of biochars in soil were highly dependent on the composition of the biochars. PBO comprised highly aromatic and chemically stable C, whereas SBO contained a substantial amount of easily degradable C. Oxygen-containing groups slightly increased after incubation in the soil with either 60% water holding capacity ?WHC? or flooding for 3 months.Sulfur in the biochars was predominantly inorganic S. Minerals such as K, Na, Mg,and S were mobilized from the biochars, accounting for 5-55% depending on the properties and sources of the element. Both PBO and SBO contained high concentrations of Cu and Zn. CuO in PBO dissolved within 3 months, whereas CuS in both PBO and SBO was partly oxidized in the soil with 60% WHC for 9 months and adsorbed to the organic phase. Zn had relatively high mobility in both biochars,resulting in its vertical migration into soils.?4? The carbon, phosphorus and sulfur functional groups of PBO before and after incubation in mine soil were investigated using C ls NEXAFS, P K-edge XANES,S K-edge XANES. The results showed that oxygen-containing groups slightly decreased after incubation in mine soil with maize ?60% water holding capacity?and rice ?flooding?. But the proportion of carboxyl in PBO after aging showed an increase. Results of P K-edge XANES showed that the most abundant inorganic P species in PBO was hydroxyapatite ?70.9%?. Hydroxyapatite was gradually transformed into AlPO₄ and Ca?H2PO₄?₂ after incubation in mine soil with maize ?60%water holding capacity? and rice ?flooding?. The results of S K-edge XANES showed that the most abundant inorganic S species in PBO were sulphates ?Na2SO₄: 39.1%,Al2?SO₄?3: 19.2%, CaSO₄: 17.0%?. Sulphates were gradually transformed into Kjarosite after incubation in the mine soil with maize ?60% water holding capacity?and rice ?flooding?.?5? CaCl₂, TCLP, EDTA and SBET extraction methods were used to study the effects of the conditions of culture and addition of biochars on the activities of arsenic,lead and zinc in contaminated soil. The results showed that arsenic activity increased during cultivation. The conditions of culture had no significant effect on the bioavailability and bioavailability of lead, but the leaching toxicity of lead was significantly decreased by flooding cultivation. Culture conditions had no effect on zinc activity. The addition of PBO and SBO reduced bioavailability, bioaccessibility,plant-availability and leaching toxicity of lead but increased these activities of arsenic in soil. Furthermore, it had no significant effect on the bioavailability and bioaccessibility of zinc, but reduced its leaching toxicity and plant-availability.Therefore, PBO and SBO could be used as remediation agents for lead contaminated soil, but they are not suitable for the remediation of arsenic-contaminated soil.?6? The main ingredients of Pb in mine soil were composed of chemical precipitates,such as Pb5?PO₄?3Cl,Pb?OH?₂,Pb?OH?Cl and PbFe4O7,which were dominated by their solubility product. It was observed that contents of PbS in soil were increased with the reduction of Pb?OH?₂ after incubation with flooding.Furthermore, an addition of PBO in soil sample resulted in the presence of Pb-CO₃2-in the sample. And the content of Pb?OH?Cl also increased with 60% water holding capacity ?WHC?. The addition of SBO yielded in increased percentages of PbS and Pb?OH?Cl upto 4.9-8.4% and 30-37%, respectively. The phosphorus-rich biochars had no obviously effect on soil lead because of its low activities. But the pH of soil pore water and the content of Pb?OH?Cl could control the concentration of lead in soil pore water.?7? Moreover, the main ingredients of Zn in mine soil sample were composed of Zn₃?PO₄?₂,Zn-LDH, aqueous Zn, Kaolinite-Zn and Zn-sorbed gibbsite. It was discerned that the addition of PBO in soil sample resulted in increased percentage of Kaolinite-Zn,Zn₃?PO₄?,Zn-LDH as well as decreased percentage of aqueous-Zn. The results shown that PBO could immobilize Zn by releasing PO₄3-?Al3+?OH- in soil pore water.