Effect Of Black Carbon On Sorption-Desorption And Bioavailability Of Pentachlorophenol (PCP)

Black carbon (BC) dominates the sorption and desorption of hydrophobic organic contaminants (HOCs) in sediment and affects the bioavailability of HOCs to some extent. In this dissertation, rice straw charcoal, fly ash, and soot were chose as precursors to produce BC particles by HCl and HCl/HF treating, and recorded as RC(pyrolyzing under the oxygen-limited conditions for 3h at 600℃), FC and SC, respectively. The characteristics of BC were determined by methods of BET, FTIR, XRD, et al. and the sorption and desorption behaviors of PCP to each BC were studied. Based on this, sediment collected from Qiantang river and pentachlorophenol (PCP) were used for research object and a representative HOCs, respectively. Then, the sorptive ability of sediment amended with RAC (natural burning), FC, SC, and effect of each BC on the toxicity and release of PCP in BC-sediment system were investigated. The information derived from this study will contribute to a better understanding of sorption and isolation of HOCs by BC in sediment and influence of BC on bioavailability of HOCs. Furthermore, it will provide important basis for applying BC to treat HOCs, establishing the sediment standards and assessing risk. The main conclusions of experiments are as follows:(1) The surface topography of RC, FC and SC varied greatly, and RC possessed the largest surface area (234.9m2/g), the highest pore volume (0.439 mL/g) and size (64.3nm) of all, and the largest amount of functional groups (2.995mmol/g).(2) The maximum sorption capacity of PCP to RC, FC and SC were 25140.0, 2962.0 and 10913.9mg/kg (pH=7.0), respectively, and sorption-desorption hysteresis, which was obviously increased by pH value decreasing, was exhibited. No matter the pH value was 9.0 or 7.0, the Hysteresis Index (H) value was FC>RC>SC. The irreversible sorption capacities of RC, FC, SC were 981.1,11104.0,3207.1 mg/kg at pH 9.0, and 1515.2,17386.0,5220.8mg/kg at pH 7.0, respectively. Therefore, it can be concluded that RC is the preferable sorbent for pollution-control measures in alkaline or neutral environment.(3) Sorption capacity of PCP to BC-sediment system increased with content of BC increasing in sediment, and sorption capacity of PCP to RAC-sediment was higher than FC and SC with the same addition. When concentration of PCP in sediment was 50 mg/kg, the PCP extractable rate decreased with the increasing content of BC in sediment and fell to 0% while the content of BC was 10%. With the same addition, the order of extractable rate was as follows:FC>SC>RAC. Moreover, it was demonstrated that BC could inhibit the toxicity of PCP and then lead the germination rate, shoot length and root length increase along with content of BC in sediment decreasing. The reduced toxicity of PCP in sediment amended with RAC and SC was more remarkable than that amended with FC. Noticeably, each BC had no toxic but stimulative effect on root elongation and early seedling growth, and it was found that the stimulative effect on root elongation and early seedling growth caused by RAC and SC were more evident than FC. Thus it can be seen, RAC has the lower toxicity and is effective to reduce biological availability of pollutants in environment.(4) It was found that sorption capacity of PCP to BC-sediment decreased with the aging time of BC-sediment system increasing, and the decrease was relative to the increase in temperature. After aging of BC-sediment system, the reducing rate of sorption capacity was FC>SC>RAC, and closely related to the kinds and contents of BC. Although the aging process could reduce the sorptive property of RAC-sediment the contribution of RAC to the sorption capacity of whole system was still high. After aging 90d at 25℃, the contribution of 2% RAC in sediment to sorption capacity of RAC-sediment was 82.74%.(5) The result of release of PCP in BC-sediment-PCP system showed that:in the range of this experiment, pH value had no effect on the release of PCP in each system; liquid-to-solid ratio, temperature, ionic strength and dissolved organic matter (DOM) were significantly influenced the release of PCP in FC/SC-sediment-PCP system; whereas the release of PCP in R2-sediment-PCP system was not influenced by these factors. Therefore, pollutants locked by sediment amended with RAC are hard to release when the amount of water, pH value, temperature, ionic strength and DOM change. This suggests that RAC may markedly inhibit the release of pollutants in water system of great change, whereas FC and SC were vulnerable to affect by environmental factors, so it is feasible to apply RAC to treat pollution in environment of major change.