Magnetite nanoparticles were synthesized by a simple and ecofriendly method using onion peel (MNp-OP) and corn silk extract (MNp-CS), in order to develop new low-cost adsorbents for arsenic removal from groundwater. As a point of comparison, magnetite nanoparticles were also synthesized with a conventional chemical process (MNp-CO). The antioxidant potential of onion peel and corn silk extracts was determined using ferric reducing antioxidant power (FRAP) and free radical (DPPH) scavenging assays, including the total phenolics, flavonoids and tannins contents. The synthesized magnetite nanoparticles were characterised using different techniques (Scanning electron microscope/Energy dispersive spectroscopy (SEM/EDS), X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR) and Brunauer-Emmett-Teller (BET) surface area analyzer). The adsorption capacity of MNp-OP and MNp-CS and the arsenic removal mechanism of these novel adsorbents was investigated through kinetic and equilibrium experiments and their corresponding mathematical models. Characterisation of MNp-OP and MNp-CS shows high BET specific surface areas of 243 m²/g and 261 m²/g, respectively. XRD and FTIR analysis confirmed the formation and presence of magnetite nanoparticles. The arsenic adsorption mechanism on MNp-OP, MNp-CS and MNp-CO involves chemisorption, intraparticle and external diffusion. Maximal adsorption capacities of MNp-OP, MNp-CS and MNp-CO were 1.86, 2.79, and 1.30 mg/g respectively. The green synthesis applied using onion peel and corn silk extracts was cost effective and environmentally friendly, and results in adsorbents with a high capacity for arsenic and magnetic properties, making them a very promising alternative approach in the treatment of arsenic contaminated groundwater.

Functionalized multiwalled carbon nanotubes (MWCNTs) have drawn wide attention in recent years as novel materials for the removal of heavy metals from the aquatic media. This paper investigates the effect that the functionalization (oxidation) process duration time (3 h or 6 h) has on the ability of MWCNTs to treat water contaminated with low levels of Cu(II), Ni(II) and Cr(VI) (initial concentrations 0.5–5 mg L⁻¹) and elucidates the adsorption mechanisms involved. Adsorbent characterization showed that the molar ratio of C and O in these materials was slightly lower for the oxMWCNT6h, due to the higher degree of oxidation, but the specific surface areas and mesopore volumes of these materials were very similar, suggesting that prolonging the functionalization duration had an insignificant effect on the physical characteristics of oxidized multiwalled carbon nanotubes (oxMWCNTs). Increasing the Ph of the solutions from Ph 2 to Ph 8 had a large positive impact on the removal of Cu(II) and Ni(II) by oxMWCNT, but reduced the adsorption of Cr(VI). However, the ionic strength of the solutions had far less pronounced effects. Coupled with the results of fitting the kinetics data to the Elowich and Weber–Morris models, we conclude that adsorption of Cu(II) and Ni(II) is largely driven by electrostatic interactions and surface complexation at the interface of the adsorbate/adsorbent system, whereas the slower adsorption of Cr(VI) on the oxMWCNTs investigated is controlled by an additional chemisorption step where Cr(VI) is reduced to Cr(III). Both oxMWCNT3h and oxMWCNT6h have high adsorption affinities for the heavy metals investigated, with adsorption capacities (expressed by the Freundlich coefficient K_F) ranging from 1.24 to 13.2 (mg g⁻¹)/(mg l⁻¹)ⁿ, highlighting the great potential such adsorbents have in the removal of heavy metals from aqueous solutions.

Granular activated carbon (GAC) was modified with Fe–Mn binary oxide to produce a novel effective hybrid adsorbent (GAC–FeMn) for simultaneous removal of As(III) and As(V) from water. After characterization (including BET, SEM/EDS and XRD analyses) of the raw and modified GAC, FTIR analysis before and after As removal showed that ligand exchange was the major mechanism for As removal on GAC–FeMn. Sorption kinetics followed pseudo-second order kinetics for both As(III) and As(V) and were not controlled by intraparticle diffusion. Batch equilibrium experiments yielded adsorption capacities for As(III) and As(V) of 2.87 and 2.30 mg/g, and demonstrated that better sorption was achieved at low pH. Of the competitive anions investigated (PO₄³⁻, SiO₃²⁻, CO₃²⁻, SO₄²⁻, NO₃⁻, Cl⁻), phosphate had the greatest negative effect on As(III) and As(V) adsorption. Three sorption/desorption cycles were conducted in continuous column tests with a real arsenic contaminated groundwater, with subsequent TCLP leaching tests confirming the stability of the spent sorbent. In the column tests, breakthrough curves were also obtained for phosphates, which were present at a relatively high concentration (1.33 mg/L) in the investigated groundwater. The phosphates limited the effective operational bed life of GAC–FeMn for arsenic removal. Nonetheless, the maximum arsenic adsorption capacities for GAC–FeMn obtained by the Thomas model during the three sorption cycles were high, ranging from 18.8 to 29.8 mg/g, demonstrating that even under high phosphate loads, with further process improvements, GAC–FeMn may provide an excellent solution for the economic removal of arsenic from real groundwaters.

Arsenic contamination of drinking water sources is a widespread global problem. Of the As species commonly found in groundwater, As(III) is generally more mobile and toxic than As(V). In this work, magnetic nanoparticles (MNp) modified with Fe-Mn binary oxide (MNp-FeMn) were synthesized in order to develop a low cost adsorbent with high removal efficiency for both arsenic species which can be readily separated from water using a magnetic field. MNp-FeMn were characterized using different techniques including SEM/EDS, XRD and BET analysis. Adsorption of As(III) and As(V) on MNp-FeMn was studied as a function of initial arsenic concentration, contact time, pH, and coexisting anions. The BET specific surface area of MNp-FeMn was 109 m²/g and maghemite (γ-Fe₂O₃) was the dominant precipitated phase. The adsorption rate of As(III) and As(V) on MNp-FeMn was controlled by surface diffusion. FTIR analysis confirms that surface complexation through ligand exchange was the main mechanism for As(III) and As(V) removal on MNp-FeMn, with As(III) conversion to As(V) occurring on the adsorbent surface. The maximal adsorption capacity q_max of MNp for As(III) (26 mg/g) was significantly improved after modification with Fe-Mn binary oxide (56 mg/g), while q_max for As(V) was 51 and 54 mg/g, respectively. ${\mathrm{PO}}_4^{3-}$, ${\mathrm{SiO}}_3^{2-}$ and ${\mathrm{CO}}_3^{2-}$ reduced As(III) and As(V) uptake at higher concentrations. MNp-FeMn can be easily regenerated and reused with only a slight reduction in adsorption capacity. The high oxidation and sorption capacity of MNp-FeMn, magnetic properties and reusability, suggest this material is a highly promising adsorbent for treatment of arsenic contaminated groundwater. GRAPHICAL ABSTRACT