Preliminary Study On Oyster Shell As Slow-release Alkalinity Source Of Bio-floc System And Phosphorus Removal By Magnetic Modified Oyster Shell Powder

At present,bio-floc technology（BFT）has the problems of complicated alkalinity control and high accumulation of phosphorus in water.In order to optimize the bio-floc technology,oyster shell（OS）and oyster shell powder（OSP）was selected as the main object in this paper to study the effect of slow-release alkalinity and static release alkalinity of oyster shell powder as the source of bio-floc culture system;meanwhile,the effect of different conditions on the adsorption and removal of phosphorus in water and the mechanism of phosphorus removal were studied.1.The effect of oyster shell as slow-release alkalinity source of bio-floc culture systemThe feasibility of adding 0.00 g/L oyster shell（control group,group A）,0.36 g/L oyster shell（group B）and 0.72 g/L oyster shell（Group C）to supplement alkalinity was evaluated at the start-up stage of bio-floc system.The results showed that the alkalinity,p H and calcium level in group C were significantly higher than those in group A（P<0.05）,but there was no significant difference in water quality between group B and group C（P>0.05）.The alkalinity of the oyster shell group was higher than that of the control group.On the basis of the start-up stage,the effects of two forms of oyster shell（shell powder,group E）and（shell,group F）to supplement alkalinity for bio-floc tilapia（Oreochromis niloticus）culture system were compared.There was no significant difference in water quality parameters,enzyme activity,non-specific immune parameters and bacterial community composition among all groups（P>0.05）.Oyster shell and shell powder had no significant negative effect on tilapia growth,so they could be used in bio-floc culture system.However,under the current experimental conditions,the oyster shell can not completely replace sodium bicarbonate in the bio-floc culture system.2.Effect of different conditions on the static release alkalinity of oyster shell powder In order to explore the effect of adding alkalinity to oyster shell powder in ideal environment,the effects of different initial p H（5.50,6.50,7.50,8.50）,initial calcium ions（0,10,20,30 mg/L）and initial alkalinity（0,70,130,200 mg/L）on the dissolution and release alkalinity of oyster shell powder were investigated.The results showed that p H=8.50 inhibited the dissolution of calcium in oyster shell powder.The alkalinity of p H=8.50 was significantly lower than p H=6.50,7.50（P<0.05）,lower than p H=5.50,but the difference was not significant（P>0.05）.When p H=8.50,the amount of calcium added to the oyster shell powder was significantly lower than that of p H=5.50 and 6.50（P<0.05）.SEM showed that the dissolution of oyster shell powder was not complete when p H=7.50.In the initial calcium ion effect experiment,the initial calcium ion in the solution would affect the dissolution of oyster shell powder.The lower the initial calcium ion,the higher the dissolution alkalinity and calcium ion level of oyster shell powder.The increase of alkalinity and alkalinity of initial calcium ion=0 mg/L and 10 mg/L was greater than that of initial calcium ion=20 mg/L and 30 mg/L,and the difference was significant（P<0.05）.The increase of calcium ion of initial calcium ion=0 mg/L was the largest,which was（17.24±1.05）mg/L was higher than that of the other three levels,but the difference was not significant（P>0.05）.With the decrease of the initial alkalinity,the dissolution alkalinity and calcium ion level of oyster shell powder increased.The increase of alkalinity with initial alkalinity=0 mg/L was significantly higher than the other three levels（P<0.05）.There was no significant difference between the initial alkalinity=130mg/L and the initial alkalinity=200 mg/L（P<0.05）.3.Adsorption performance and mechanism of magnetic modified oyster shell powder on phosphorus in waterThe amount of the adsorbent（2.00,4.00,6.00,8.00,10.00,12.00,14.00,16.00,18.00,20.00 g/L）,TP were studied concentration（10,20,30,40,50,60,70,80,90,100 mg/L）,p H（p H=2.00,4.00,6.00,8.00,10.00,12.00）,coexisting ion（50,100,150,200 mg/L）The effect of（NO₃^-,HCO₃^-）on the adsorption and removal of phosphorus from simulated water by magnetic modified oyster shell powder and its characterization were studied.The desorption effect of magnetic modified oyster shell powder was also investigated,and the adsorption mechanism of phosphorus removal was discussed by adsorption thermokinetics.The results showed that the TP removal rate increased from（33.05±5.71）%to（84.61±1.47）%.However,when the amount of TP added increased from 10.00 g/L to20.00 g/L,the TP removal rate of magnetic modified oyster shell powder fluctuated in the range of（81.74±1.47）%～（85.61±6.23）%;when the initial TP concentration increased from 5.00 mg/L to 50.00 mg/L,the TP removal rate of magnetic modified oyster shell powder increased to（87.35±1.06）%,and the q_e increased to（5.45±0.22）mg/g.However,when the initial TP concentration increased from 50.00 mg/L to 100.00 mg/L,the TP removal rate decreased to（74.65±1.38）%and the q_e continued to rise to（9.87±0.06）mg/g.when the p H was in the range of 2.00 to 6.00,the TP removal rate and q_e of oyster shell powder were higher than（80.13±3.27）%and（2.04±0.02）mg/g,respectively,was higher than the TP removal rate and q_e in the p H range of 8.00 to 12.00;HCO₃^-had an obvious inhibitory effect on the adsorption of phosphorus.With the increase of HCO₃^-from 50.00mg/L to 200.00 mg/L,the TP removal rate of oyster shell powder decreased from（76.45±3.75）%to（27.11±5.28）%,and q_e decreased from（4.94±0.24）mg/g to（1.70±0.33）mg/g.With the increase of NO₃^--N from 50.00 mg/L to 200.00 mg/L,the TP removal rate of magnetic modified oyster shell powder decreased from（89.63±0.25）%to（82.62±0.87）%,and the q_e decreased from（5.78±0.02）mg/g to（5.18±0.05）mg/g,both of which had no obvious change The effect of Na OH on the desorption of oyster shell powder was the best.After one desorption and regeneration of the magnetic modified oyster shell powder,the removal rate of TP from the simulated water with the initial TP concentration of 50.00 mg/l was（22.97±1.77）%.The results showed that the specific surface area of modified oyster shell powder decreased and the pore size increased.The surface of shell powder is mainly composed of fine particles with uniform distribution.The surface covering composition of iron bearing oyster shell powder is mainly microcrystalline Fe₂（PO）₅ and Fe₄（PO₄）₂O with small grains,but the structure is not stable.The thermodynamic study shows that the adsorption and dephosphorization of magnetic modified oyster shell powder belong to Freundlich adsorption model when the initial TP concentration is 26.40 mg/L～71.08 mg/L and the temperature is 15℃～35℃.The energy distribution of adsorption site on the surface of oyster shell powder belongs to exponential type,and the phosphorus removal by magnetic modified oyster shell powder belongs to heterogeneous adsorption.The maximum q_e of magnetic modified oyster shell powder was 6.06 mg/g.The E_a of phosphorus adsorbed by magnetic modified oyster shell powder is 22.09 k J/mol,which belongs to chemical adsorption.The kinetic study shows that the adsorption and dephosphorization of magnetic modified oyster shell powder belong to pseudo-second kinetic model in the range of initial TP concentration of 50.00mg/L and temperature of 15℃to 35℃.The process of phosphorus removal by oyster shell powder adsorption is divided into three stages:the first stage is the arrival of phosphorus molecules on the surface of oyster shell powder;the second stage is the diffusion of phosphorus molecules inside oyster shell powder;the third stage is the weakening of diffusion inside oyster shell powder.The adsorption and dephosphorization process of magnetic modified oyster shell powder belongs to spontaneous endothermic reaction.25℃and 35℃are favorable for the adsorption and dephosphorization of oyster shell powder,while 15℃is unfavorable for the adsorption and dephosphorization of magnetic modified oyster shell powder.