| The intensification of livestock and poultry production has led to increased animal wastes as by-products, which pose serious environmental problems. Currently, aerobic composting and pyrolysis technologies have been used to eliminate or reduce the risk of spreading of pathogens, parasites and weed seeds in animal wastes. These techniques lead to stabilized products, such as composts and biochars. Manure compost can be recyled as a source of nutrients (particularly N, P and K) for plant growth. Because of its special physicochemical properties, biochar has relatively high agricultural values and the potential to remediate polluted environment. However, the presence of non-biodegradable and toxic heavy metals in livestock and poultry manure may lead to their accumulation in soil. Heavy metals uptake by plants from the soil and successive accumulation in human tissues through the food chain can be harmful to human health.The objectives of this work were to:(i) investigate the heavy metal pollution in swine manure on large-scale farms;(ii) study the effects of composting process on heavy metal distributions and phytotoxicity of swine manure;(iii) clarify the effects of pyrolysis process on extractability and bioavailability of heavy metals;(iv) understand the changes in bioavailability of Cu, Zn and Mn after manure and biochar application;(v) evaluate the effects of different manure and biochar treatments on growth of pakchoi as well as the uptake of Cu, Zn and Mn;(vi) explore the Cu(II) adsorptive capacity and mechanisms by swine-manure-derived biochars. The results provide theoretical and technical guidance for the utilization of swine manure and its biochar, the sustainable development of agriculture, and using of biochar for remediation of wastewater polluted with heavy metals. Main results are summarized as follows:1. The results of heavy metal concentrations in swine manures showed that the mean contents of As, Cd, Cu, Hg, Mn, Ni, Pb and Zn in swine manures were18.18, 0.30,825.85,0.06,473.60,5.54,3.40and1218.03mg/kg, respectively. According to the heavy metal limits for Germany compost standard, all manure samples exceeded the limitations for Cu and Zn. It is also need to pay attention to the high contents of As and Mn in swine manure. DTPA extractable contents of Cu and Zn in swine manure were high, indicated that the Cu and Zn were available for plant uptake.2. The distributions of various species of Cu, Mn and Zn in swine manure and their phytotoxicity during the aerobic composting process were investigated. The sequential extractions results showed that Cu was mainly bound to organic fraction, while Mn and Zn were mostly bound to carbonate and reducible iron and manganese oxide fractions. During the composting process, there was a significant decrease of the exchangeable and carbonate fractions of Cu, Mn and Zn, while the organic matter bound and residual fractions of heavy metals gradually increased. The water soluble and DTPA extractable contents of Cu, Mn and Zn decreased with composting time, indicating that the potential bioavailability and mobility of these metals decreased after composting. Furthermore, the germination index of radish and pakchoi increased during the composting time, indicating that phytotoxicity of swine manure diminished with composting time. Multiple regression analysis revealed that the sum of mobile fractions of Cu (F1234-Cu) and DTPA-Cu were two major factors affecting the phytotoxicity of the swine manure.3. Biochars derived from the pyrolysis, at400and700℃, respectively, of fresh and aerobically composted swine manure, were characterized and the effects of pyrolysis process on extractability and bioavailability of heavy metals were investigated. The biochar yield significantly increased following composting, but decreased with increased temperature. Pyrolyzing swine manures increased their pH and produced alkaline biochars. The ash content, surface area, electrical conductivity, mineral nutrients, pore volumes and aromatic structure of biochar produced at700℃were higher than at400℃, whereas the higher heating value, oxygen-containing functional group, pore size and the total C, N, H and O contents were decreased. The concentrations of heavy metals (except Cd at700℃) were higher than those in the raw swine manure. Water soluble and DTPA extractable contents of Cu and Zn increased with increased temperature. During the pyrolysis process, there was a significant decrease of the exchangeable and carbonate fractions of Cu, Mn and Zn, while the residual fraction of heavy metals increased.4. Effects of application of swine manure and its biochar on soil properties and available heavy metal contents were investigated. The results of incubation experiment showed that swine manure or biochar addition to the soil significantly increased soil pH. Soil pH value was higher at the higher rate of manure or biochar addition. pH values were higher in soil treated with biochar, while the pH value in soil treated with compost was higher than soil treated with fresh manure. The Cu, Mn and Zn contents in soils treated with swine manure and biochar were higher than control soil. The Cu contents in soils treated with the highest rate of3%manure and biochar exceeded the permitted value which is stipulated in environmental quality evalution standards for farmland of edible agricultural products. The long-term use of manure and biochar can cause Cu and Zn accumulation in soils. The application of manure or biochar triggered an increase in the DTPA-extractable Cu and Zn in the soils and became more pronounced as the application rate increased. Compared to manure, DTPA extractable contents of Cu and Zn decreased in biochar treated soil. DTPA-Cu and Zn contents in soil were linearly related to pH, indicating pH played an important role in the activation of Cu and Zn.5. Pot experiments revealed that the land application of swine manure or biochar may significantly increase the biomass of pakchoi. The contents of Cu, Mn and Zn in the shoot of pakchoi were higher than those in the roots. The higher addition rate of swine manure could increase the contents of Cu in the shoot, whereas it decreased the contents of Mn and Zn. Lower addition rates of biochar could significantly decrease the Mn and Zn contents in the shoot. However, the content of Cu in pakchoi shoots was below that required by the Chinese Food Hygiene Standard, whereas the Zn content exceeded the Chinese Food Hygiene Standard of20mg Zn/kg based on fresh weight. The uptake of Cu and Mn by pakchoi was significantly correlated with DTPA-Cu and Mn contents in the soil, and a highly statistically significant correlation was obtained between the Zn content in the shoot and soil pH. 6. Batch experiments were conducted to investigate adsorption characteristic of swine-manure-dervied biochars pyrolyzed at400and700℃for the removal of Cu(Ⅱ) ions from aqueous solutions. Adsorption experiments were carried out to investigate the effects of solution pH, adsorbent concentration, contact time, initial concentration, and temperature on Cu(Ⅱ) adsorption process. The optimum adsorption conditions were:solution pH5.0, biochar concentrations5g/L, contact time24h, and reaction temperature25℃when the Cu(Ⅱ) was1mM. The processes of Cu(Ⅱ) adsorption by four kinds of biochars are well described by a pseudo-second-order kintic model, and the adsorption isotherm closely fitted the Sips model. Thermodynamic analysis suggested that the adsorption was endothermic. Only the adsorption of Cu(Ⅱ) onto biochar derived from swine manure compost (TB2-400) was spontaneous. The maximum Cu(Ⅱ) adsorption capacities of biochars derived from fresh and composted swine manure at400℃were17.71and21.94mg/g, respectively, which were higher than those at700℃. XRD patterns indicated that the silicate and phosphate particles within the biochars served as adsorption sites for Cu(Ⅱ). The removal of Cu(Ⅱ) ions from industrial effluent indicated that TB2-400can be considered as an effective adsorbent. |
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