Study Of The Environmental Behavior And Effects Of Modified Black Carbon In Soils

Soil is the foundation of human existence and sustainable development of society. According the reports publicated by Ministry of Environmental Protection in 2014, soil is polluted seriously in China and soil pollution is getting worse. Heavy metal pollution of soil is one of the important environmental issues and the contaminated area was widely expanding. In situ immobilization is one of the economical and practical techniques on remediation of heavy metal contaminated soils, and the selection of perfect performance passivating materials is the key of this technique. Black Carbon (BC), which is produced from incomplete combustion of fossil fuel and biomass, is widely exist in terrestrial and aquatic environments. BC has the porous structure and contains a good deal of aromatic compounds. It has been reported that BC affords 2% -13% of total organic carbon in soils. But BC is the adsorbent of organic pollutants, it needs to be modified when used as the adsorbent of heavy metals. It is called Modified Nano Black Carbon(MBC). The environmental behavior and effects of MBC must be studied because its oxygen-containing acidic functional groups and surface negative charge. Related research will provide a theoretical basis for the development of practical, economical and safe heavy metal contaminated soil remediation technology.In this paper, nano black carbon was used as soil heavy metal deactivating agent and modified BC to improve its passivation ability. In adsorption thermodynamics and kinetics experiments, the mechanism of adsorption of heavy metals was studied. And the mechanism of adsorption of heavy metals by means of modern instrumental analysis characterizing its morphology, electric properties, molecular groups etc were discussed. We also studied environmental behavior and effect of modified nano black carbon in soils, determined its environmental risk, provided the theoretical basis for development of practical, economical and safe heavy metal contaminated soil remediation technology via leaching experiment, culturing experiment, pot experiment and so forth. We achieved following research results:(1) We selected acidic KMnO4 to make nano carbon black modified agent, modified nano carbon black had the strongest adsorption capacity to Cu2+and Cd2+. The optimal oxidized preparation conditions are:the concentration of KMnO4 is 0.1 mol·L-1, the acidity of modified agent is 3.0 mol·L-1, oxidized modification time is 1.0 h, oxidized modification temperature water bath is 90℃.(2) Compared with the unmodified nano black carbon, the specific surface area increased 0.28 time, pore volume enlarged 7.89 times, the average pole size increased 1.88 times and the permeability of particles inside increased in modified nano black carbon. The analysis of infrared spectroscopy showed that there were more acidic oxygen-containing functional groups on the surface of the modified nano black carbon, leading to increased negative charge on MBC and isoelectric point shifting left. SEM analysis made a further validation that modified nano black carbon was more loose, larger gap appeared between particles, pore volume increased, which improved the adsorption properties to the Cu+ in the aqueous solution and passivation ability of the Cu in the soil. Possible mechanisms included electrostatic adsorption mechanism, complexation mechanism, the adsorption mechanism of the cationic exchange and microporous capture mechanism.(3) Langmuir and Freundlich models were used to fit the thermodynamics of adsorption process of Cu2+ on black carbon. The results showed that the process of Cu2+ adsorption on the nano black carbon accorded with Langmuir adsorption model. Cu2+ adsorption on the BC was single molecule layer adsorption, and Langmuir and Freundlich models both gave good descriptions of Cu+ adsorption process on the modified nano black carbon. Being modified, the adsorption capacity of nano black carbon to Cu had been greatly improved and saturated adsorption capacity was 12.33 mg·g-1 in BC,48.92 mg·g-1 in MBC(increasing about 3 times compared with before modification). The adsorption kinetic studies showed that Cu2+ adsorption on the black carbon and modified black carbon was controlled by many processes and complex factors, when temperature was below 60℃, a higher temperature favors the adsorption of Cu2+ on modified nano black carbon. Improving the pH value of the solution can also increase the amount of adsorption of Cu+ on modified nano carbon black:71.91%-79.88% Cu2+ adsorption to the modified nano black carbon were acid difficult desorption state, not easy to release to the environment again and had certain adsorption stability. After applying modified nano black carbon into soil, the effective content of copper in soil reduced significantly, the migration and transformation of Cu2+ in soil-ryegrass reduced further. So it had certain feasibility in the passivation remediation of heavy metal contaminated soil.(4) After leaching experiments, the results of content in three kinds of soil different profiles and distribution of modified nano black carbon and effective Cu in three kinds of soil indicated that modified nano black carbon did not migrate with water leaching in the vertical direction in cinnamon soil, fluvo aquic soil and acid brown soil. But when the modified nano black carbon addition proportion was 1.5%, it appeared aggregation to the surface soil in all three kinds of soil. After adding modified nano black carbon, the total carbon and dissolved organic carbon in the leaching solution of three kinds of soils were lower than control groups which did not have modified nano black carbon, so modified nano black carbon decreased the loss of soil organic carbon. The 56 days’ culturing experiments indicated that at ordinary temperatures and pressures without added microbes, the content of modified nano black carbon in cinnamon did not have remarkable change over time, proving that modified nano black carbon in soil was comparatively stable in the short term. However, it needs further study about long term stability of modified nano black carbon in soil.(5) Study on effect of modified nano black carbon on the soil enzyme activity showed that modified nano black carbon had activation effect of varying degrees on catalase in cinnamon and fluvo aquic soil, had insignificant effect on activity of catalase in brown soil. Modified nano black carbon had activation effect on activity of urease of varying degrees in three kinds of soil, but had a certain degree of inhibition effect of dehydrogenase activity. Study on effect of soil micro ecosystem indicated that the application of modified nano black carbon improved the respiratory intensity of brown soil and fluvo aquic soil evidently, did not improve respiratory intensity of cinnamon significantly. The number of bacteria and actinomycetes in soils increased and the number of fungi decreased after adding MBC. Modified nano black carbon enhanced the adsorption capacity of soil to phosphorus, NH4+-N, NH3-N and potassium, reduced loss of soil available nutrient.As a consequence of the above, modified nano black carbon has a better performance adsorption and passivation to heavy metals. It does not pollute ground water by leaching and movement, it turned out to be stimulative to activity of soil enzyme, soil microbial biomass, plant growth et al. It also can enhance the adsorption capacity of the soil to the available phosphorus, ammonium, potassium, tends to reduce the loss of soil available nutrients. Consequently, Modified Nano Carbon black is an economic, efficient and environmentally friendly soil passivating materials to heavy metals.