Reactor Construction For Production Of Biosynthesized Schwertmannite And Its Application In Arsenic Immobilization In Soil

Schwertmannite,one kind of hydroxyl sulfate iron minerals which are metastable and secondary,widely exists in sulfate-rich environments such as acid mine drainage(AMD).The results showed that biosynthesized schwertmannite had large specific surface area and rich surface-active groups,which was more advantageous than the chemically synthesized schwertmannite in physical and chemical properties.Owing to its poor crystallinity and pore-distributed structure,schwertmannite has been used in the fields of Fenton-like degradation of organic pollutants and treatment of heavy metal pollution in water.Although it is a known fact that schwertmannite has excellent performance in catalysis,adsorption and co-precipitation,the synthesis efficiency of schwertmannite is too low.In fact,only 30%iron in the system can be oxidized and hydrolyzed to form mineral,resulting in a large amount of iron loss.In addition,batch formation using shaking flasks requires much more manpower and material resources.Therefore,it is of great significance for the application of schwertmannite in various fields to find one method which can prepare schwertmannite efficiently to improve the formation rate of iron mineral and put it into large-scale practical production.The remediation of arsenic-contaminated soil has aroused great concern for the reason that arsenic is one kind of highly toxic and carcinogenic elements.In recent years,some remediation technologies for arsenic-contaminated soil have been carried out.However,their drawbacks are hard to ignore.In contrast,the stabilizing remediation of contaminants has easy operating conditions and varied materials,and this technology can effectively immobilize the arsenic in soil to weaken its bioavailability.In consequence,it is very essential to find out the suitable stabilizer for the remediation of arsenic-contaminated soil.Iron minerals have been often used in the stabilizing remediation of contaminants because of the large specific surface areas,abundant active sites and rich ion exchange groups on their surface.The biosynthesized schwertmannite has showed remarkable superiority in specific surface area,pore structure and active groups in the surface.However,there are few studies on biosynthesized schwertmannite as stabilizer to remediate arsenic-contaminated soil.Therefore,the ability of biosynthesized schwertmannite to immobilize the arsenic in soil remains to be explored.In this paper,the efficient method system to prepare biosynthesized schwertmannite and large-scale production process were studied.What’s more,the effect of schwertmannite prepared by the mentioned method on stabilizing arsenic in soil was also studied.The main contents include:(1)Designing an efficient method system to prepare biosynthesized Schwertmannite and constructing a small-scale reactor.Exploring the differences of synthesis efficiency of schwertmannite under different reaction conditions(including agitator speed and HRT),and characterizing the schwertmannite(SEM,XRD)to compare their physical properties in order to optimize the operating conditions of the reactor;(2)Constructing large-scale reactor and applying the optimum conditions of small-scale reactor to the large one’s production;(3)Exploring the differences of the effect of biosynthesized schwertmannite on arsenic immobilization under different reaction conditions(including the dosage ratio of mineral and soil,water condition,the mixture ratio of biochar and mineral).After a series of experiments,the main results were obtained as the following:1.In the process of synthesis,the oxidation rate of Fe2+ was 232.72 mg·L-1·h-1.The reduction rate of Fe3+ increased with the acceleration of mechanical stirring speed,but 300r/min was safer and more stable than 400 r/min so that 300 r/min was the suitable speed.The synthesis efficiency of 3 d system or 4.5 d system was better than that of 1.5 d system.After the same period of time,the formation rate of iron mineral in 3 d system or 4.5 d system was always higher than that in 1.5 d system.The time required to precipitate all iron in 3d system or 4.5 d system was both two days earlier than that in 1.5 d system.The metallogenic amount(g·L-1·d-1)of 3 d system was 1.16 times more than 1.5 d system and 4.5 d system was 1.15 times more than 1.5 d system,respectively.The 3 d system which performed the best in metallogenic amount ran smoothly while the synthesized mineral was actually schwertmannite.Therefore,3 d was the suitable HRT for the 3 L reactor.The optimum conditions for the reactor used to prepare schwertmannite efficiently are 300 r/min of speed,3 d of HRT.2.The oxidation rate of Fe2+ and the formation rate of iron mineral were both stable during the production process of the large-scale reactor although those turned a little lower than before during the last stage due to the cold weather.The reduction efficiency of Fe3+was in the range of 45%～65%rather than 100%.In addition,using tap water to synthesize mineral might influence the mineragraphy.The synthesized mineral produced in the large-scale reactor were the mixture of ihleite minerals.3.The effect of biosynthesized schwertmannite on arsenic immobilization in soil was positively correlated with the addition ratio of mineral and water condition.Excessive dosage could easily lead to undue soil acidification.0.5%of dosage not only ensured the ideal immobilization efficiency of bioavailable arsenic,but also slowed down the degree of soil acidification.Therefore,0.5%of dosage is the suitable condition.With the increase of water,the risk of arsenic desorption increased as well.The dissociation and transformation of schwertmannite could be relatively weakened when the ratio was 1:1(water/soil),so 1:1(water/soil)is suitable.The immobilization effects of extractable arsenic and water-soluble arsenic under different mixture ratio of the composite consisting of biochar and schwertmannite showed some difference.For extractable arsenic,the immobilization efficiency under different mixture ratio(Biochar/Sch)was:1:1>1:2>2:1;for water-soluble arsenic,the rule was:1:2>1:1>2:1.The final immobilization efficiency of 1:1 is only about 1%different from that of 1:2.On one hand,biochar has more economic benefits than schwertmannite.On the other hand,the raw materials to produce biochar are abundant and easy to obtain.Therefore,1:1(Biochar/Sch)is the suitable condition.Therefore,this paper provides some reference value for the efficient production of schwertmannite in large scale and its application in immobilization remediation for arsenic-contaminated soil.