Hydroxymethylated humic acid magnetic nanoparticles for the selective removal of Pb2+ and Hg2+ in water

Because of the non-degradation of heavy metals (HMs),the control on the pollution transfer of HMs in water treatment,and their recycling utilization would be an effective way to achieve a win-win situation of envi-ronmental and economic benefits.In this case,the adsorbent with a selectivity for the target heavy metals is required.Therefore,the density of oxygen-containing functional groups on the surface of humic acid (HA) was improved by hydroxymethylation modification to promote the formation of multi-coordination structure,which had a high adsorption stability and affinity towards certain HMs.Finally,the hydroxymethylated HA magnetic nanoparticles (OHA-MNPs) with high selectivity and easy solid-liquid separation were prepared by co-precipitation,which was used for the deep removal of heavy metals in wastewater.The characterization of adsorbent,adsorption kinetics,adsorption thermodynamics,adsorption mechanism,adsorption selectivity for Pb2+ and Hg2+ under competitive adsorption,and the regeneration of adsorbent were studied.The results show that the content of HA attached on the surface of OHA-MNPs is about 9.81%.The particle size of adsorbent is mainly distributed in 7-11 nm,which shows good dispersion.The pseudo-second order kinetics and Langmuir adsorption model could well describe the adsorption kinetics and thermodynamics of Pb2+ or Hg2+ on OHA-MNPs.The adsorption of OHA-MNPs towards Pb2+ or Hg2+ is mainly attributed to the coordination of oxygen-containing functional groups,accompanied by ion exchange and electrostatic adsorption.In the solution with mixed metals,OHA-MNPs has a high selectivity for Pb2+ and Hg2+ and the selective adsorption sequence is Hg2+> Pb2+ Cu2+ Ni2+/Cd2+.About 93.76% of Pb2+ can be selectively removed in acid mine wastewater with total cation concentration 44.78 times that of Pb2+.Besides,OHA-MNPs also has good regeneration ability.Therefore,OHA-MNPs is a kind of efficient,green and low-cost adsorbent for the removal of heavy metals in wastewater.