Removal or Immobilization of Arsenic by Stabilized Magnetite Nanoparticles in Water, Soil, and Poultry Litter

Abstract

Stabilized magnetite (Fe3O4) nanoparticles were synthesized and extensively tested for enhanced removal of arsenate, As(V), from water, and for in situ immobilization of arsenic in soil and poultry litter. Two low-cost and “green” polysaccharides, a water-soluble starch and sodium carboxymethyl cellulose (CMC), were used as stabilizers to facilitate particle size control and enhance adsorption capacity. Results indicated that particle size, morphology, soil mobility and arsenic sorption capacity can be controlled by manipulating the stabilizer type and concentration. Starch at ≥0.04 wt.% or CMC at ≥0.005 wt.% can stabilize 0.1 g/L (as Fe) of the nanoparticles. While CMC-stabilized magnetite displays a highly negative zeta (ζ) potential, starch-stabilized magnetite shows a nearly neutral surface. Starch-stabilized magnetite offers much faster sorption rate and greater capacity than CMC-stabilized magnetite. Increasing starch concentration from 0 to 0.04 wt.% doubles arsenate uptake, yet the nanoparticles remain settleable by gravity. Further increasing starch concentration to 0.1 wt.% results in fully dispersed nanoparticles and an increase in arsenate uptake by 14%. The sorption kinetics can be modeled using an intraparticle-diffusion model. The sorption capacity increases with decreasing pH. Dissolved organic matter at 20 mg/L as TOC decreases arsenate uptake by 19%. When aged for >1.5 years, the nanoparticles did not show any arsenate leaching or particle dissolution. As(V) immobilization in soil using starch-stabilized magnetite nanoparticles was investigated through a series of batch and column experiments. Batch sorption tests showed that the nanoparticles could effectively immobilize As(V) in As(V)-laden sandy soil with As distribution coefficient for nanoparticles of 9999.5 L/g, which is >5 orders of magnitude than for the sandy soil. As leachability based on TCLP (Toxicity Characteristic Leaching Procedure) was reduced by 80% upon the nanoparticle treatment. Column tests showed that water-leachable As(V) from the sandy soil containing 31.45 mg/kg As was reduced by ~93% and the TCLP leachability by >83% when the soil was treated with 34 pore volumes (PVs) of a 0.1 g/L Fe3O4 nanoparticle suspension. The starch stabilizer was able to facilitate the soil deliverability of the nanoparticles, and the effective travel distance of the stabilized nanoparticles can be manipulated by controlling the injection flow rate. Once delivered, the Fe3O4 nanoparticles are retained by the soil matrix within a limited distance (<10 cm) under natural groundwater flow conditions (velocity ≤ 2.2×10-7 cm/s). Magnetite and Fe-Mn binary oxide nanoparticles were prepared and tested for reducing arsenic (As) leaching from poultry litter (PL). At a magnetite dosage of 2.5 g/L as Fe, or Fe-Mn dose of 0.5 g/L as Fe, >93% of ~150 mg/L soluble As in PL leachates was removed from the aqueous phrase within 24 hours. The Fe-Mn binary oxide particles, which act as an oxidizing agent and an high capacity adsorbent, showed 3.8 times higher arsenic sorption capacity than the magnetite particles based on batch tests. The arsenic sorption for both magnetite and Fe-Mn oxide particles was pH-dependent. 18% and 12% more of As was removed by magnetite and Fe-Mn, respectively, from aqueous phrase when pH was dropped from 10 to 4. The use of polysaccharide (a water-soluble starch and carboxymethyl cellulose) enhanced arsenic sorption at different extent. Column tests that simulate field application of PL suggested that amending PL with the nanoparticles reduced the total leachable arsenic by more than 91% compared to untreated PL. The results indicated that amending PL using low-cost and “green” particles may greatly mitigate adverse environmental impacts associated with land application of PL.