Study On The Effect Of MgO/Fe₃O₄ Modified PSB On Microplastics PS Transport In Saturated Porous Media

In recent years,microplastics(MPs),as an emerging pollutant with small particle size and high mobility,have attracted widespread attention at home and abroad.As mobile colloidal particles in the soil,microplastics are capable of loading heavy metals and organic pollutants and transport through the food chain,the problem of microplastic pollution is considered as a global threat to natural ecosystems and human health.To date,more studies have focused on the sources,distribution and physiological toxicity of microplastics in the environment,but fewer studies have been conducted on the transport and deposition release behavior of microplastics in soil,and the impact mechanisms are still unclear.Biochar is widely used as an excellent soil amendment for remediation of heavy metal and organic matter contamination in soil,and it is urgent to study whether its application to soil affects the transport process of soil microplastics.Based on this,this paper used polystyrene microplastics as the research object and used peanut shell waste as the biomass raw material to prepare peanut shell biochar(PSB),Mg O-modified peanut shell biochar(Mg O-PSB)and Fe₃O₄-modified peanut shell biochar(Fe₃O₄-PSB),respectively,and the three types of biochars were mixed with quartz sand at 0.5%mass ratio to fill the plexiglas columns to simulate the soil.the effects of three typical environmental factors(humic acid concentration,ionic strength,and cation type)on the transport and retention of microplastics in saturated porous media were investigated,and the transport mechanisms of microplastics under the conditions of different soil factors were explored by combining the two-site kinetic retention model and DLVO theory.The main findings are as follows:(1)The characterization results of biochars showed that the pH of all three biochars was greater than 7,indicating that all three biochars were alkaline.Compared with PSB,the specific surface area and total pore volume of Mg O-PSB and Fe₃O₄-PSB were significantly increased,with the specific surface area increased by 12.5 times and 20.04times,and the total pore volume increased by 4.57 times and 14.48 times,respectively.FTIR analysis showed that the oxygen-containing functional groups on the surface of the original biochar were higher than those of the two modified biochars,mainly due to the complexation between the metal oxides and the oxygen-containing functional groups,which led to a decrease in the content of the oxygen-containing functional groups.In addition,SEM analysis revealed that the surface of PSB was smoother and had no obvious pore structure,but the surfaces of Mg O-PSB and Fe₃O₄-PSB were rougher,with more mesopores and macropores,and the surfaces were loaded with more Mg O particles and Fe₃O₄ particles,respectively.(2)The results of the mechanism study on the effect of PSB and MgO-PSB on the transport of microplastics in soil showed that the addition of PSB and Mg O-PSB significantly inhibited the transport of MPs in saturated porous media under the conditions of Na⁺solution when the humic acid(HA)concentration was 0 mg/L.For example,when the ionic strength(IS)was 1 m M Na Cl,the effluent recovery of MPs decreased from 65.85%to 40.96%and 24.47%,respectively.However,the inhibitory effect of PSB and Mg O-PSB on the transport of MPs was significantly attenuated when HA was present,and the transport ability of MPs always increased gradually with the decrease of IS and the increase of HA concentration.Under Ca²⁺solution conditions,the retention of MPs in the filled media was greater than that under Na⁺solution conditions when the HA concentration was 0 mg/L.However,when HA was present,the effluent recovery of MPs was significantly enhanced with increasing HA concentration under low IS(1 m M Ca Cl₂)conditions,and the effluent recovery of MPs was instead greater than that in Na⁺solution.However,the effluent recovery of MPs significantly decreased with increasing HA concentration at higher IS(10,100 m M Ca Cl₂)in Ca²⁺solutions,and the effluent recovery was lower in the Mg O-PSB added filled media.The results of the two-site kinetic retention equation fitting revealed that the addition of PSB and Mg O-PSB added irreversible attachment sites for the retention of MPs in the filled media.Under Na⁺solution conditions,both the decrease in IS and the increase in HA concentration decreased the retention coefficient(k₂)of MPs at kinetic site 2.However,under the condition of Ca²⁺solution with high IS,the value of k₂ gradually increased with the increase of HA concentration.DLVO theoretical calculations showed that the repulsive potential of interaction energy between microplastics and quartz sand gradually increased with the decrease of IS and the increase of HA concentration,indicating that the decrease of IS and the increase of HA concentration led to the more difficult deposition of microplastics on the surface of the filling medium.Moreover,the repulsive potential barrier between microplastics and Mg O-PSB was significantly lower than that between MPs and PSB,further proving that microplastics had lower effluent recovery in the Mg O-PSB added filling media.(3)The results of the mechanism study on the effect of Fe₃O₄-PSB on the transport of microplastics in soil showed that the addition of Fe₃O₄-PSB to quartz sand significantly inhibited the transport of microplastics in the filled system when HA was not present,and the ability of Fe₃O₄-PSB to inhibit the transport of microplastics was greater than that of PSB and Mg O-PSB under all experimental conditions.For example,when the IS was 1 m M Na Cl,the effluent recovery of microplastics in the filled medium in the presence of Fe₃O₄-PSB was only 10.08%,which was mainly attributed to the less negative charge on the surface of Fe₃O₄-PSB.Under the conditions of Na⁺solution and Ca²⁺solution with low ionic strength,the effluent recovery of microplastics always increased with the increase of HA concentration.However,under the conditions of Ca²⁺solution with high IS,the effluent recovery of microplastics significantly decreased with increasing HA concentration.The fitting results of the two-site kinetic retention model showed that the addition of Fe₃O₄-PSB enhanced the irreversible retention of microplastics at kinetic site 2,and the value of k₂ was always larger than that of k₂ in the filled column with PSB/Mg O-PSB added.the analysis of DLVO calculations showed that the interaction between Fe₃O₄-PSB and microplastics exclusion barrier is always lower than that between PSB and microplastics.The repulsive potential barrier of the interaction energy between microplastics and Fe₃O₄-PSB always increases gradually both with the decrease of IS and the increase of HA concentration.However,under the condition of Ca²⁺solution with high IS,the repulsive potential barrier between microplastics and Fe₃O₄-PSB almost did not exist even when the HA concentration was 20 mg/L.The non-recoverability of MPs in Fe₃O₄-PSB added filled media under this condition was as high as 40.43%,indicating that HA,microplastics and Fe₃O₄-PSB easily formed larger aggregates deposited in porous media and are more difficult to be separated by external forces.