Capturing Lithium From Wastewater Using A Fixed Bed Packed With 3-D MnO₂ Ion Cages For Application

Lithium?Li?is a critical material for energy-related technologies.Accordingly,there has been increasing demand for lithium and Li compounds with the development of science and technology,particularly in the rapid expansion of rechargeable lithium ion batteries?LIBs?.We are now facing a contraction the of the Li supply and increasing demand.Currently,a lot of spent LIBs are produced each year.The Li content in spent LIBs is an important resource of lithium.Effective and efficient recovery of Li could become a very important because it not only can relieve Li shortages,but also can resolve environmental pollution problem caused by spent LIBs.Additionally,research on lithium recovery from LIBs have attracted considerable interest in recent years.Adsorption is one of the most promising methods for lithium recovery from lithium wastewater because it is one of the most cost-effective and environmentally friendly methods.The adsorbent plays a vital role in determining the performance of packed bed adsorption systems;however,the majority of adsorbents have a lower adsorption capacity and poor selectivity for Li.Ionic-cage adsorbents possess a “memory effect” and that makes them ideal for extracting target ions with an extremely high selectivity.In the paper,we have synthesized 3-D Mn O₂ cages?CMO?and investigated the performance of a CMO adsorbent for Li?I?removal.The Dubinin-Ashtakhov?DA?site energy distribution model based on Polanyi theory described the linear increase of Li adsorption capacity?Q₀?with increasing temperature?Q₀=k₃×E_m+d₃=k₃×?a×T?+d₃?.Furthermore,the pore diffusion model?PDM?accurately predicted the lithium breakthrough?R2 ?0.99?.Higher temperatures increased the number of BVs that may be treated,which implies that CMO will be useful in treating industrial Li?I?wastewater in regions with different climates?e.g.,Northern or Southern China?.The specific research results are as follows:1.The 3-D Mn O₂ ion cages?CMO?were prepared using a two-step process by hydrothermal synthesis via a low-temperature solid-phase reaction.The specific surface area?BET?characterizationof CMO indicates that CMO have a high degree of porosity and large specific surface area,whcih can provide abundant adsorption sites.The results of SEM and XRD showed the CMO possess a good crystal shape and surface morphology.The adsorption behavior of Li in ion cages was ion exchange reaction,which was confirmed by XPS.The stability of the Mn-O bond plays an indispensable role in forming Li cages and it has a high charge density forming a strong electrostatic field and a suitable size for adsorbing lithium ions in the CMO nanocrystal cubic phase.Therefore,CMO can well adsorb Li?I?.2.The maximum equilibrium adsorption capacity was 45.25,55.92 and 61.32 mg/g for 20,30 and 40 ?,respectively.CMO showed large adsorption capacity,which is higher than those reported for other lithium ion-cage adsorbents.The adsorption capacity also increased with the increasing temperature and a fast adsorption kinetics on CMO.In the experiment of the effect of p H on adsorption capacity,the maximum adsorption capacity for Li?I?was obtained at a p H of 13.The CMO ion exchange/adsorption site is an-OH group and can exchange the H+ for Li?I?.The CMO exhibit a much higher selectivity separation factor?i ka?and affinity of Li?I?as compared with other competitive ions?including Na+,K+,Mg2+ and Ca2+?.These attributes are the result of 8a-16d-8a channels in a three-dimensional interstitial space provided by the-Mn-O-Mn-O-framework,only allowed Li+ through the channel.The regeneration experiment for Li?I?demonstrated that the CMO has good stability,anti-interference and regeneration performance,which validated a significant potential of CMO apply to wastewater treatmnet.3.The Langmuir,Freundlich and Dubinin-Radushkevich?D-R?isotherm models,pseudo-first-order and pseudo-second-order model,Thermodynamic equation were used to fit the experimental data,respectively.Freundlich model is more suitable the for describing the adsorption process,indicating that the adsorption of Li?I?on CMO is heterogeneity and multilayer adsorption.Simultaneously,1/n are the Freundlich parameters for values in the 0 < n < 1 range,indicated the adsorption is favorable.The Dubinin-Radushkevich?D-R?isotherm model suggested that the adsorption proces is an ion-exchange reaction.The ?H0?11.34 k J/mol?and ?S0?0.08 k J/mol K?by Thermodynamic equation.The ?G0 was calculated to be-12.20,-13.05 and-13.77 k J/mol for 20,30 and 40 ?,respectively.The thermodynamic parameters indicated that the adsorption process on CMO is endothermic,spontaneous and the degree of randomness increases during Li?I?adsorption.4.The Dubinin-Ashtakhov?DA?site energy distribution model based on Polanyi theory described the linear increase of Li adsorption capacity?Q0?with increasing temperature??7??8?33330kd EQad Tkm?10????28??10???28??.We ues the Pore Diffusion Model?PDM?and the Homogeneous Surface Diffusion Model?HSDM?to describe adsorption kinetics.Simultaneously,using the models predict Li breakthrough in short fixed bed column tests,full scale performance,and evaluated the impact of multiple ions on Li breakthrough.We also determine the Li recovery regenerating the fixed bed.The maximum number of bed volumes?BVs?treated for a treatment objective were 1,374,1972 and 2493 for 200 ?g/L at 20,30 and 40 ?,respectively.Higher temperatures increased the number of BVs that may be treated,which implies that CMO will be useful in treating industrial Li?I?wastewater in regions with different climates?e.g.,Northern or Southern China?.5.A simplified version of the homogeneous surface diffusion model?HSDM?was used to determine the surface diffusion coefficient.The calculated the Dp and Ds value that would give the same result of as the PDM using the surface to pore diffusion flux ratio?SPDFR?equal to 1.0.Hence,when competing ions are not present,we can use either the HSDM or the PDM because they give similar results.And the two model could be used for preliminary design if there are no competing ions.when we fit the data with the simplified HSDM,the calculated value of Ds is 4.63×10-10 m2/s and the SPDFR was 2.6.The HSDM did not predict the data well?R2? 0.86?.Consequently,the HSDM prediction has an earlier breakthrough.We calculated the value of the external mass transport coefficient?kf = 4.787 × 10-4 m/s?and the pore diffusion coefficient?Dp =1.60 × 10-10m2/s?,indicated adsorption rate control steps through intraparticle diffusion.We used the PDM to predict Li breakthrough curve and confirmed the kf and Dp value that was calculated.The PDM predictions for the SBA were excellent?R2? 0.99?.Further,higher temperatures increased the number of BVs that may be treated.The calculated value of Bip was 29.8 ? 20,which implies that intraparticle diffusion controls the overall mass transport of the syste.The result further demonstrate that the Li?I?can be adsorbed in three-dimensional?1 × 3?tunnels,rather than on the surface sites.Using PDM can well predict desorption curve for Li?I?,the high enrichment factors arefavorable for Li recovery.IX can accurately predicts for multiple ions breakthrough.Hence,the models can reduce time consuming and cost,which provide specific guidance for engineering application.