Study On Preparation And Properties Of Electrolytic Manganese Residue-based Slow-release Fertilizers

Electrolytic manganese residue（EMR）as acid leaching residue in the production of electrolytic manganese metal could contain soluble pollutants such as NH₄⁺-N,and Mn,etc.Although building materials have become the main approach for EMR large-scale consumption in China,EMR cannot be completely consumed in the building materials industry owing to the huge output and inventory,limited market demand and other cost problems.Therefore,more efforts should be made to seek diversified strategies to reduce the pressure on building materials industry.To solve the problems about difficult granulation of EMR,rapid loss of nutrients and readily leaching of heavy metals,EMR and geopolymer gel were regarded as Mn source and binders,respectively,to prepare EMR-based SRFs.Mn release behavior and mechanism in EMR-metakaolin（MK）based SRFs prepared by phosphoric acid/potassium silicate activator were comparative study.Furthermore,we explored the possibility for preparing SRFs by utilizing laterite instead of MK as active silicon-aluminum source,and revealed Mn slow-release mechanism in EMR-laterite based SRFs to provide theoretical basis for pot experiment of tea seedlings.The results showed that Mn release concentration in SRFs prepared by alkali-activated MK（SRFs-alGo）is significantly lower than that of SRFs prepared by acid-activated MK（SRFs-acGo）.Mn diffusion kinetics in SRFs-acGo and SRFs-alGo conform to the first-order kinetic model and Ritger-Peppas model,respectively.MnSO₄·H₂O in EMR could transform into MnO₂,Mn₄Si₈O₂₀ and Ca₄Mn₄Si₈O₂₄ in SRFs-alGo,indicating that most of Mn fractions transform into oxidation fraciton or residual fraction.However,the formation of NH₄Mn PO₄·H₂O crystal could not significantly change Mn fractions in SRFs-acGo.Mn in SRFs-alGo uniformly distribute on the particle surface and bulk phase,while a higher proportion of Mn in SRFs-acGo distribute on the particle surface.The K-（A）-S-H gel in SRFs-alGo restrict Mn release by physical encapsulation and ion exchange,while the phosphate-activated geopolymer gel（Si-O-Si-O-P-O）mainly depend on the physical encapsulation/adsorption of Mn.The above concludes determined Mn in SRFs-acGo easier to escape from SRFs particles than SRFs-alGo.Guizhou laterite as slow-release material and active Si-Al source was selected to replace MK to further solve the problems of difficult acquisition and high price of MK.EMR-laterite based SRFs（SRFs-LA）were prepared by laterite and potassium water glass.Laterite-based geopolymers as binder could realize the agglomeration of primary particles of EMR and meet the requirements of national SRFs standard（GB/T 23348-2009）.SRFs-LA alleviates Mn release rate and concentration,and the Mn diffusion could dominated by non-Fick diffusion mechanism and skeleton dissolution.The leaching concentrations of As,Cd,Cr,Hg and Pb in SRFs-LA are extremely low,meeting the standard（GB 15618-2018）.The slow-release mechanism showed that MnSO₄·H₂O in EMR convert to CaMnSi₂O₆、MnO₂ and（NH₄）₂Mn（SO₄）₂·6H₂O after the formation of SRFs-LA.Geopolymer gels could not only imprison Mn（II）through the charge balance effect of[AlO₄]tetrahedron structure in Si-Al system,but also physically encapsulate Mn-containing minerals in the three-dimensional network structure of geopolymer.Meanwhile,geopolymer gels might fill the pores inside the SRFs and reduce the proportion of large pores,reducing the pores effect on the Mn release rate.Pot experiments showed that the high iron content in tea seedlings roots under EMR treatment could be ascribed to the long-term accumulation.The contents of heavy metals in leaves cultured under SRFs-alGo,SRFs-acGo and SRFs-LA treatment all meet the standards（NY659-2003 and GB 2762-2022）,which could promote the growth,protein and photosynthetic pigment synthesis of tea seedlings.The contents of soluble sugar and proline increase significantly after the application of SRFs-acGo and EMR,which could be attributed to the plant-induced metal tolerance.Therefore,it was speculated that white tea seedlings might respond to metabolic disorder induced by Mn and heavy metals via producing a large amount of osmotic pressure,and tea seedlings have reached the limit of manganese tolerance marked as the growth reduction.The excessive accumulation of ROS in leaves caused by EMR leads to the failure of antioxidant system in leaves.Antioxidant enzyme activity results showed that Mn provided by SRFs-acGo might have reached the threshold of Mn required for the growth of white tea seedlings.The order of promoting effects of the different types SRFs prepared in this study on white tea seedlings is SRFs-LA>SRFs-alGo>SRFs-acGo.