Effect Of Pectin On Adsorption Of Copper And Cadmium By Variable Charge Soils And Its Mechanism

Large areas of acidic variable-charge soils distribute in the tropics and the subtropics.These soils,because of their low cation exchange capacity and low pH,have relatively low adsorption capacity for heavy metals.Plants normally take up available metals from the soil solution through their roots,which makes the concentration of heavy metals in soil solution particularly important to the ecosystem.Adsorption and desorption on the surfaces of soil particles and of sesquioxides are two key processes controlling the concentration of heavy metals in soil solution.Mucilage from plant roots can enter the rhizosphere,and the pectin in that mucilage adsorbed by soils,especially variable-charge soils.Just as organic acids can increase the adsorption of heavy metals by variable-charge soils and thus decrease the activity of heavy metals in the rhizosphere,so can pectin in the mucilage.This thesis study on the effect of pectin on adsorption of copper and cadmium by variable charge soils,and investigate the influence factors,including the dosage of pectin,the initial concentration of heavy metal ions and the system pH.Using of amorphous iron and aluminum hydroxides to elucidate the mechanisms for pectin-enhanced adsorption of heavy metal on variable-charge minerals.Study on the effect of the surface properties of pectin on aluminum hydrolysis products,provides variable-charge soil heavy metal adsorption enhanced by pectin indirectly.The integrated use of a variety of analysis techniques(infrared spectroscopy,Zeta potential analyzer,surface charge measurement,electron microscope,X ray diffraction)were used to investigate the mechanism of effect of pectin adsorption.The main results are as follows:The influence of pectin on Cu(II)adsorption by two variable-charge soils(an Oxisol and an Ultisol)was investigated.Pectin increased the adsorption,and the extent of adsorption increased linearly with the dose of pectin,being greater in the Oxisol than that in the Ultisol because the adsorption of pectin by the Oxisol was greater.Both Langmuir and Freundlich equations fitted the adsorption isotherms of Cu(II)for both soils well.The fitting parameters of both equations indicated that pectin increased not only the adsorption capacity of the soils for Cu(II),but also the adsorption strength of Cu(II).The effect of pectin decreased with rising pH in the pH range 3.0–6.0 although the extent of electrostatic adsorption of Cu(II)by both soils was markedly greater over the pH range.Fourier-transformed infrared spectroscopy analysis and zeta potential measurement of soil colloids indicated that adsorption of pectin by the soils made the negative charge on both soils more negative,which was responsible for the increase in the electrostatic adsorption of Cu(II)induced by the addition of pectin.In conclusion,pectin-enhanced adsorption of Cu(II)especially at low pH would be beneficial to the soils as it would decrease the activity and mobility of Cu(II)in acidic variable-charge soils.Batch experiments were conducted to investigate cadmium(II)(Cd(II))adsorption by two variable-charge soils(an Oxisol and an Ultisol)as influenced by the presence of pectin.When pectin dosage was less than 30 g kg–1,the increase in Cd(II)adsorption with the increasing dose of pectin was greater than that when the pectin dosage was > 30 g kg–1.Although both Langmuir and Freundlich equations fitted the adsorption isotherms of Cd(II)and electrostatic adsorption data of Cd(II)by the two soils well,the Langmuir equation showed a better fit.The increase in the maximum total adsorption of Cd(II)induced by pectin was almost equal in both the soils.However the increase in the maximum electrostatic adsorption of Cd(II)was greater in the Oxisol than in the Ultisol,because the former contained greater amounts of free Fe/Al oxides than the latter,which,in turn,led to a greater increase in the negative charge on the Oxisol.Therefore,the presence of pectin induced the increase in Cd(II)adsorption by the variable-charge soils mainly through the electrostatic mechanism.Pectin increased the adsorption of Cd(II)by the variable-charge soils and thus decreased the activity and mobility of Cd(II)in these soils.Batch experiments were conducted to investigate the adsorption of pectin and Cu(II)by amorphous Fe/Al hydroxides,to elucidate the mechanisms for pectin-enhanced adsorption of heavy metal cations on variable-charge minerals.The morphology,mineralogy and functional groups of pectin-Fe/Al hydroxides were examined using X-ray diffraction,scanning electron microscopy,and Fourier transform infrared spectroscopy analysis.The amount of pectin adsorbed by amorphous Al(OH)3 was much greater than that by amorphous Fe(OH)3 at pH values between 3.5 and 6.5 due to the higher positive charge density on Al(OH)3 and greater electrostatic attraction between the hydroxide and pectin compared with Fe(OH)3.The addition of pectin decreased the positive surface charge on amorphous Fe and Al hydroxides.The presence of pectin enhanced the adsorption of Cu(II)by the Fe and Al hydroxides.The increase in Cu(II)adsorption on amorphous Fe hydroxide was more obvious at low pH values than at higher pH values,while an opposite changing trend was observed for amorphous Al hydroxide.The greater adsorption of pectin by Al hydroxide was mainly responsible for the larger enhancement of pectin on Cu(II)adsorption on Al hydroxide at higher pH values compared with Fe hydroxide.The adsorption of pectin on Fe and Al hydroxides decreased the positive charge on the hydroxides and thus enhanced the adsorption of Cu(II)by the hydroxides.Study on the effect of the surface properties of pectin on aluminum hydrolysis products,provides variable-charge soil heavy metal adsorption pectin promote indirect evidence.By means of X diffraction,the surface morphology and surface structure of the infrared spectra.The hydrolysate of aluminum in pectin were characterized and confirmed that the crystal structure of pectin hydrolysis aluminum more disordered,and then forming defects in the structure of pectin hydrolysis aluminum reduces the isoelectric point and the surface positive charge.With the participation of pectin increased,the effect is more obvious.In summary,this thesis selects two variable charge soils(an Oxisol and an Ultisol)as the research object to study found that pectin can enhance the adsorption of copper and cadmium invariable charge soil.There are probably two aspects of the mechanism.First,variable charge soil contain amount of free iron and aluminum oxides.Adsorption of pectin by Fe/Al oxide surface reduced the positive charge of the surface of variable charge soil and increased the adsorption of heavy metals,such as Copper and Cadmium.This is a direct mechanism for effect of pectin on adsorption of heavy metal on variable charge soils.Secondly,in southern China,aluminum hydrolysis can influenced by pectin,reducing the surface positive charge of aluminum hydrolysis has indirect effect on the variable charge soils.Pectin enhance the adsorption of heavy metals on variable charge soil,result in reduce the activity of toxic heavy metals in variable charge soils.