| Desulfurization gypsum, whose main ingredient is calcium sulfate dihydrate, is asgood as natural gypsum in quality and can be used as raw material for the preparation ofwidely used calcium sulfate whisker (CSW). This study was carried out to prepare CSWthrough a method of cooling recrystallization with calcium sulfate dihydrate reagents anddesulfurization gypsum, aiming at extending the application of CSW in environmentalfield and realizing the goal of resource utilization. Moreover, this paper also studied thepurifying effect of arsenic in the simulated water sample by the preparaed CSW.Calcium sulfate dihydrate of analytical grade chemical reagents and desulfurizationgypsum were respectively used to prepare CSW, and several operation conditions ofsulfuric acid concentration, supersaturation, aging time/temperature, cooling rate, andcalcination were conducted to disscuss their effect on the character of CSW product. Theresults indicated that when the sulfuric acid concentration was3.75mol/L, supersaturationwas0.5, cooling interval was80~20°C and the cooling process was performed in theambient air under20°C, CSW with length/diameter (L/D) of20~200could be obtained,mostly with hexagonal prisms morphology and the smooth surface and infarctate/hollowstructures. In addition, all the studied operation conditions could affect the morphologyand size, but among which the key to this preparation method was the control of coolingrate and the selection of sulfuric acid concentration. It was also indicated that the sulfuricacid could be repeatedly used and the method of cooling recrystallization had theadvantages of moderate reaction condition, simple operational procedure, and ease ofcontrol. The experimental results of CSW preparation with desulfurization gypsum wereidentical to that using calcium sulfate dihydrate of analytical grade as material, achievingnearly100%conversion. Moreover, the formation of CSW was through a classic“dissolution-recrystallization” process with advancing steps at crystal surface.The study also explored the removal efficiency of arsenic by the CSW productsthrough batch tests, and the effect of pH, reaction time, whisker dosage and initial Asconcentration were also studied. Meantime, the mechanism of arsenic uptake by CSW wasdiscussed through the simulation of adsorption isotherm and analysis of XRD/SEM. It wasconcluded that pH of the aqueous solution was an important parameter for theAs(III)/As(V) removal, and an excellent removal efficiency could be achieved understrongly alkaline condition for both calcium sulfate dihydrate whisker（CSDW）andcalcium sulfate anhydrous whiske（rCSAW）prepared through calcium sulfate dihydrate ofanalytical grade chemical reagents. Both of the whiskers exhibited a removal rate lower than60%for As(III) while higher than97%for As(V) under the solution pH of12.5.When the solution pH was11, As(V) uptake on the two types of whiskers wasaccomplished in6~8hours, and As(V) adsorption experimental data on both CSDW andCSAW were better described with Extended Langmuir EXT1Model under a wide As(V)concentration range of1~300mg/L. It could be speculated that the interaction mechanismbetween As(V) and CSW was a complex adsorption process combined with the surfacedissolution and chemical precipitation due to the fact of morphology change of reactedwhiskers by SEM and detection of calcium arsenate compounds by XRD. The CSWprepared with desulfurization gypsum also exhibited a good performance under thestrongly alkaline condition, and when the solution pH was12, the residual arsenicconcentration could be decreased to lower than the standard of surface water (0.05mg/L)as the initial arsenic concentration was lower than1mg/L. This could also decrease thecost of waste water removal combined with the two stage method of arsenic removal usinglime-ferrisulfas.What this paper has studied exhibited a certain value to the development of new typeenvironmental material of CSW and achievement of effective utilization of desulfurizationgypsum. |
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