Removal Of Cu(Ⅱ) From Aqueous Solutions And Its Fixation In Red Soil By Biochars From Crop Straws

There are large amounts of annual crop residues such as crop straw resulting from agricultural practices in china, although part of them has been used as feed, fuels or straw returning, there is still about2×1011kg of them has been burned in field which is not only a waste of resources but also causing pollution to the environment, bringing a range of social problems. On the other hand, with the development of China’s copper smelting,copper electroplating wastewater excessive emissions, copper pesticide spraying and sewage irrigation, copper was accumulate in the soil and water. Copper in the water and soil can not be broken down by microorganisms to use, but through the enrichment found and endanger human health through the food chain.In the partial or total absence of oxygen, thermal decomposition of plant-derived biomass (oxygen-limited pyrolysis) is manipulated to yield a solid carbon-rich residue generally referred to as biochar. There are large numbers of variable negative charge and active oxygen-containing functional groups on biochar surface which can absorb heavy mentals in water and soil effectively. But the mechanism of biochar absorb the heavy mentals is unclear.Biochars were prepared from four corp residues including legume straws of soybean and peanut, non-legumes as rice and canola using an oxygen-limited pyrolysis method. The removal of Cu (II) from aqueous solutions and its fixation in red soil by biochars from crop straws are investigated. The major sesults obtained were summarized as follows:(1) Three crop straws, including canola straw, soybean straw and peanut straw were used to prepare biochars using a low temperature (400℃) oxygen-limited pyrolysis method. The researches indicate that in the range of solution pH from3.5to6.0the adsorption increased with increase in solution pH. The adsorption capacity followed the order:peanut straw char> soybean straw char> canola straw char, while the desorption of pre-adsorbed Cu (Ⅱ) had a reverse trend. Fourier transform midinfrared photoacoustic spectroscopy data revealed that adsorption of Cu(Ⅱ) caused an apparent shift of the vibrational bands assigned to the carboxyl and phenolic hydroxyl groups along with less negative zeta potential of the biochars that adsorbed Cu(Ⅱ), which suggested that the Cu(Ⅱ) was adsorbed specifically through the formation of surface complexes. The Langmuir equation was used to predict adsorption capacity of Cu (Ⅱ) by the biochars, with maximum predicted adsorption in the ranges0.58-1.40and0.48-0.79mol·kg-1at pH5.0and4.5, respectively. The adsorption of Cu (Ⅱ) by these biochars was greater than that by a commercial activated carbon in the pH ranged from3.5to5.0with a maximum adsorption of0.18mol/kg at pH5.0.(2) The biochars were derived from the straws of canola, rice, soybean and peanut at different temperatures (300℃,400℃,500℃) by means of oxygen-limited pyrolysis. The effects of material source, pyrolysis temperature, the amount of addition biochars, mixed reaction time and initial pH of wastewater were studied so that the optimum reaction condition can be determined. The results showed that biochar produced from peanut straws and pyrolysis at400℃, Cu (Ⅱ) removal rate up to98%in the condition of mixed reaction time for8h and the waste water by adding biochar per liter of processed20g.(3) The effects of four biochars derived from straws of peanut, soybean, canola and rice and pyrolysis at400℃on Cu (Ⅱ) adsorption by the red soils from Jiangxi and Guangxi were investigated. The results indicate that the incorporation of the biochars increased the adsorption of Cu (Ⅱ) by the soils. The enhancement of the Cu (Ⅱ) adsorption induced by the biochars increased with the increase in the amount of biochars added and more enhancements was observed at relatively low pH. At pH4.0and the biochar addition level of2%, adsorption of Cu(Ⅱ) by the red soil from Jiangxi increased by97%,79%,51%and54%as a result of the incorporation of biochars derived from the straws of canola, peanut, soybean and rice, respectively. The adsorption of Cu (Ⅱ) by the red soil from Guangxi increased by61%and44%as a result of the incorporation of peanut straw char and soybean straw char, respectively, and corresponding increases were97%and165%, respectively, when the addition level of the biochars was increased to4%. Biochars normally carry negative charge and have ample amounts of oxygen-containing functional groups on their surfaces, thus they can increase the electrostatic and specific adsorption of Cu(Ⅱ) by red soils simultaneously and the biochars enhanced the specific adsorption more than the electrostatic adsorption as observed. Therefore, incorporation of biochars in red soils can decrease mobility and bioavailability of Cu (Ⅱ).