Adsorption Of SO₄^2- Onto Light Sierozem And Migration Modeling Of SO₄^2- In Soil Column In The Presence Of Biochar

The content,migration and transformation of nutrients in soils influence plant growth.Effective utilization of nutrients in soils is very important to food security and non-point source pollution control.On the other hand,biochar has a porous structure,surface charge and surface functional groups of features.Biochar application to the soil is not only affects the soil the structure and properties but also influences the interface process and migration of soil’s nutrient.The scientific theoretical basis to minimize the nutrients leaching and losses is by using diluted rich biochar.The effect of diluted biochar on the migration,transformation of nutrient Sulfur in soils was examined.In the experiments,biochar produced by pyrolysis rape（Brassica campestris L.）straw by biomass hypoxia.The effect biochar on light sizerozem（LS）adsorption SO₄^2-and its mechanism and SO₄^2-migration in soil columns were studied.Analysis of inorganic sulfur form has changed in soil column leaching.CXTFIT2.1 sofware was used to fit the breakthrough curves（BTCs）of SO₄^2-transport in mixed media.The results are of great significance for the application of biochar,the control of sulfur leaching in soil and the application of sulfur fertilizer.Main results are:（1）In the static adsorption experiments Sulfate(SO₄^2-)adsorption on rape straw biochar produced at 300°C and 600 °C（BS300 and BS600）,LS and a 1:9（w/w）biochar: soil mixture（BS600/BS300-LS）was investigated by batch adsorption experiments.The effects of the contact time,initial SO₄^2-concentration,temperature and solution pH on the adsorption were studied.The results showed that SO₄^2-adsorption on the three adsorbents was well described by a pseudo-second-order kinetic model.The amount of time needed to achieve equilibrium SO₄^2-adsorption was less on LS and BS600-LS than on BS600 and BS300.The adsorption isotherms could be fitted by the Langmuir and Freundlich models.For initial SO₄^2-concentrations of up to 500 mg·L^-1,the SO₄^2-adsorption capacities of the absorbents decreased in the following order: LS > BS600-LS > BS300-LS > BS600(28.63,26.81 23.01,19.57 and 16.61 mg·g^-1,respectively).These results indicated that BS600 amendment did not increase the SO₄^2-adsorption capacity of LS.Based on the calculated thermodynamic parameters,SO₄^2-adsorption on all three adsorbents was spontaneous and endothermic.The adsorption capacities decreased slightly as the initial pH of the SO₄^2-solution increased.SO₄^2-mainly adsorbed on BS600 and BS300 through electrostatic interactions,whereas SO₄^2-adsorption on LS occurred via electrostatic interactions and the formation of poorly soluble CaSO4.The results of this study provide a reference for the effects of biochar amendment on sulfate migration in soils.（2）The sorption of SO₄^2-onto natural soil was studied in column experiments.BS600 and BS300 were employed for the adsorption of SO₄^2-using fix-bed columns,and the effect of columns（5.2,10.3,15.3 and 19.7 cm）,biochar additions amount（0%,5%,10% and 15%）on the BTCs for adsorption SO₄^2-was investigated.The results show that the migration BTCs of Br-in mixed media is symmetrical and no obvious tailing phenomenon.By addition same proportion BS600 and BS300 in LS,breakthrough the balance of pore volume have no significant difference.The results showed that the breakthrough time would be increased by increasing soil columns height and increasing additions biochar amounts.When these factors were observed,the BTCs of the solute SO₄^2-in mixed media showed asymmetry and tailing phenomenon.（3）Using CXTFIT 2.1 software,the one–dimensional transport of nonreactive solutes Br-by equilibrium convection dispersion equation（CDEeq）fits dispersion coefficient（D）values from 0.47 to 1.11 by addition BS600 different content and from 0.48 to 0.79 by addition BS300 different content.The leaching data for SO₄^2-was well described by CXTFIT 2.1.The parameters of bath obtains retardation coefficient（R1,R2）and CXTFIT 2.1 fitting retardation coefficient（Rmod）was well estimating SO₄^2-under non-equilibrium condition transport,indicating that batch experimental obtains parameters is well represent SO₄^2-in soil leaching.The parameter dimensionless mass transport coefficient（w）<<100,indicting the non-equilibrium of adsorption sites plays a dominant role in the transport of SO₄^2-in the mixed medium.（3）In addition,in BS600-LS mixed system,H₂O-S,adsorbed-S and HCl-S of inorganic sulfur increased with columns height and initial solutions pH increasing,respectively.The total inorganic sulfur content 115.42,106.52,131.34 and 89.47 mg·kg^-1 when columns were height with biochar amount 19.7cm,15%,the value 9.01 of initial solutions pH and concentration 50mg·L^-1,respectively.In long term leaching BS300-LS mixed system,there was no significant relationship between H2O-S,adsorbed-S and HCl-S and the content of BS300.When the initial solution concentration increasing,the content of HCl-S in BS300-LS mixed medium was reduced.