| In recent years, biochar has received increasing interest due to its potential in the environmental remediation. Due to the heterogeneity of biochar and the differences of contaminants, it is hard to achieve a consensus on the removal effect of biochars towards the contaminants. Therefore, more research is needed to be done. In order to systemly demonstrate biochar,s potential in the sorption and transformation of contaminants as well as the underlying mechanisms, this dissertation studied 1) the sorption of heavy metals in aqueous solution by biochars; 2) the sequestration of Hg in the sediment porewater by biochars; and 3) the catalystic oxidation of H2 S in the gas by biochars. The main results are as following:(1) The properties of biochars derived from 3 different kinds of feedstock(plant residues, animal waste and municipal waste) at 3 different temperatures(200 °C, 350 °C and 500 °C) were compared and discussed in order to elucidate the influence of feedstock and temperature. Almost all biochars were alkaline and have certain surface area. Biochars produced at higher temperature have higher surface area. But biochars produced at lower temperature have higher content of functional groups. Biochars derived from animal waste and municipal waste have more ash and minerals, such as Ca, Mg, K, PO43- and CO32-, but biochars derived from plant residues have more C. Overall, the properties of biochars were greatly influenced by the feedstock and pyrolysis temperature, resulting in different performances of biochar toward different contaminants.(2) The biochars derived from dairy manure and rice husk at two temperatures(200 °C and 350 °C) were used to sorb heavy metals(Pb, Cu, Zn, Cd) in the aqueous solution. Biochars derived from dairy manure were effective in sorbing all four metals, with the highest affinity for Pb. The maximum sorption capacities of Pb, Cu, Zn, and Cd by 200 °C dairy manure biochar were 71.6 mg·g-1, 48.4 mg·g-1, 32.8 mg·g-1, and 32.0 mg·g-1, respectively, and those of Cu, Zn, and Cd by 350 °C dairy manure biochar were 51.5 mg·g-1, 31.9 mg·g-1, and 54.4 mg·g-1, respectively. Sorption isotherm of Pb by the 350 °C dairy manure biochar was a vertical line. The maximum sorption capacities of Pb, Cu, Zn, and Cd by rice husk biochars were only 2.9-29.1 mg·g-1. 350 °C rice husk biochar had a higher sorption capacity for the four metals than 200 °C rice husk biochar. The sorption capacities of the four metals by these two rice husk biochars followed Pb>Cd>Zn>Cu. Rice husk biochar showed stronger competition for metal removal than dairy manure biochar when the four metals coexisted, with Pb the least affected and Cd the most inhibited. When each metal was 1 mM in the multi-metal system, the metal removal by rice husk biochar was reduced by 38.4%-100%, much higher than that reduced by 2%-40.9% for dairy manure biochar. The stronger competition for metals removal by rice husk biochar was due to the fact that all metals competed only for the ionized phenolic-O- groups, while the removal of metals by dairy manure biochar resulted not only from the comlexation with ionized hydroxyl-O- groups but also from the precipitation of metals with CO32-and/or PO43- that were rich in dairy manure biochar, resulting in less competition.(3) We have attributed the effective removal of heavy metals to the combined effect of organic functional groups and minerals. Therefore, we took the further attempts to elucidate the mechanisms of Pb(II) removal by biochar via separating the organic and inorganic fractions from biochars and investigating their contributions to the Pb(II) removal. The sorption capacities of organic fractions in dairy manure and rice husk biochars were only around 1 mg g-1, accounting for 0.4%-0.6% of the Pb(II) removal by the whole biochars, while the inorganic fractions showed sorption capacities of over 300 mg g-1, occupying more than 99% of Pb(II) removal. Sorption of Pb(II) by the organic fraction was probably through electrostatic attraction, intrinsic chemical affinity with –COO-, and/or cation-π interactions, while the inorganic fraction sorbed Pb(II) via the chemisorption, primarily through the inner-sphere complexation and precipitation. XRD analysis and MINTEQ modeling evidenced formation of PbCO3, Pb3(CO3)2(OH)2, and Pb5(PO4)3Cl in the inorganic fractions, among which precipitation with phosphate contributed more to the Pb(II) removal than the precipitation with carbonate in the manure biochar(68% vs 32%), while the contribution of the precipitation with carbonate in the husk biochar was more evident(64% vs 36%). The inorganic fractions play the dominant role in the Pb(II) removal by biochar.(4) Activated carbon-like biochars(bagasse and hickory chips biochar) were evaluated for the sequestration ability of Hg(II) at sediment porewater concentration levels(ng·L-1-μg·L-1). Activated carbon which is commonly used for sediment remediation was included for comparison. Both biochars showed higher sorption capacities than AC, following the trend of bagasse biochar > hickory chips biochar > activated carbon. Sequestration of Hg(II) by hickory chips biochar was mainly attributed to the Hg-π binding via Hg(II) bond with C=C and C=O, while formation of(-COO)2HgII and(-O)2HgII were mainly responsible for the removal of Hg(II) by bagasse biochar and activated carbon. As a result, the partition efficiency of Hg(II) in bagasse biochar decreased 17.6% and 37.6% after-COOH and-OH were blocked, respectively. The sorption capacity of Hg(II) by activated carbon decreased 6.63% and 62.2% for-COOH and-OH hindered, respectively. However, blocking the function groups had little effect on the Hg removal by hickory chips biochar. Application of biochar in real sediment porewater also showed the higher Hg removal effectiveness than activated carbon, i.e., about 49.0% and 36.9% of Hg was removed by hickory chips biochar and bagasse biochar, respectively, while activated carbon only removed 21.1% Hg. Although activated carbon has a higher efficiency than biochars at a higher concentration, biochars still have considerable sorption ability for Hg. Taking cost into account, there exists a high potential that biochar can be a substitute of activated carbon for remediation of Hg-contaminated sediments.(5) We have found that biochar is alkaline and rich in minerals, so it is possible to remove H2 S. Biochars were evaluated for their abilities to remove hydrogen sulfide(H2S) from the gas phase via dynamic and static tests. Compared to biochars derived from plant residues, manure biochar and municiple waste biochar were more effective in removing H2 S. The pig manure biochar had higher capacities for H2 S sorption than sewage sludge biochar in both dynamic and static systems, and moisture improves H2 S removal. Increasing the biochar moisture to 25% and 100% in the static system increased the pig manure biochar removal capacities by 15.9% and 58.9%, respectively, compared to the dry biochar(0% moisture). The sewage sludge biochar similarly increased by 1.04 and 3.30 times, respectively. The catalytic conversion to elemental S0 and SO42- was the main route of H2 S removal. The complete oxidation of H2 S into SO42- mainly occurred on the biochar surface, while H2 S underwent incomplete oxidation into elemental S0 in the biochar pores. The SO42- was the dominant form in both biochars, especially for the pig manure biochar which contained 53.9% of the total sulfur under 100% moisture. The SO42- was partly present as CaSO4 precipitate in the sewage sludge biochar, while SO42- in the pig manure biochar was mostly soluble(K, Na)2SO4. The results indicated the waste biomass can be converted into value-added biochar as a sorbent for H2 S, especially at high moisture that promotes complete oxidation of H2 S into SO42-. Strong alkalinity and rich inorganic minerals originated in the biochar play an important role in its high H2 S sorption ability and the final sulfur forms.In conclusion, biochar could be used as a new functional material in the remediation of contaminants from different environmental media. And the immoblization mechanism varies with the different feedstock and temperature for biochar production as well as the species of the contaminants. Therefore, we should choose a proper biochar based on the the contaminants as well as the research purpose. |
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