Reduction of arsenic mobilization in soil by application of hematite nanoparticles and acrylic polymers

Background and objectives: Arsenic (As) is one of heavy metals that because of its toxicity and carcinogenicity, attention to the environmental remediation of this element is growing. It is the forms of chemical associations of As with various soil solid phases rather than its total concentration that affects its mobility, bioavailability and toxicity to the biosphere. Chemical stabilization technique is based on reduction the mobility and availability of toxic metals by different additives (adsorbents). This study was conducted to evaluate and compare the effectiveness of hematite nanoparticles and acrylic polymers in As immobilization in soils contaminated with different levels of As and find appropriate application dosage of the adsorbents.
Materials and methods: A Factorial experiment was conducted using a completely randomized design and three replications. The experimental factors were types and dosages of adsorbents (Four types of adsorbents including hematite nanoparticles, acrylamide - acrylic acid copolymer (A), Maleic anhydride- Styrene - acrylic acid copolymers (B and C) in four levels including 0, 0.05, 0.1 and 0.2 percent) and As levels (0, 6, 12, 24, 48 and 96 mg/kg). Soil samples were incubated for five months, then adsorbents were added to them and after three months the concentrations of soil available As and specifically sorbed As were determined using 0.05 M of ammonium sulfate and 0.05 M of ammonium phosphate respectively. The concentration of sum of soil available and specifically sorbed As was calculated. The effectiveness of the adsorbents in reduction the concentrations of soil As fractions were discussed.
Results: SEM and TEM images showed that the average diameter of hematite nanoparticles was 32.69 nm and the morphology was spherical. The results showed that the concentrations of soil available, specifically sorbed and sum of soil available As and specifically sorbed As concentrations increased as the levels of soil As increased. Application of adsorbents decreased the concentrations of soil available, specifically sorbed and sum of soil available As and specifically sorbed As and with increasing the application dosage of adsorbents, the rate of reduction of concentrations of soil available, specifically sorbed and sum of them increased, so that the lowest concentrations of these As fractions obtained from application of 0.2 % of adsorbents. The application of hematite nanoparticles decreased the concentrations of soil As fractions more than other adsorbents, and C, A and B acrylic copolymers stand in next places respectively. At all levels of soil As, the minimum concentration of soil sorbed As was related to soil treated with 0.2% of hematite nanoparticles and the maximum concentration of it was related to control treatment (not treated with adsorbent). The rates of reduction of soil available As concentrations by application of 0.2 % of hematite nanoparticles, C, A and B acrylic copolymers were 60.80%, 56.82%, 37.87%, 26.57% respectively. At the same types and dosageof adsorbents, the rates of reduction of soil specifically sorbed As concentrations were 27.56%, 22.72%, 14.88% and 10%. Finally the rate of reduction of soil total sorbed As concentrations, at the same types and dosage of adsorbent, were 39.39%, 34.84%, 23.05%, 15.88% respectively. The nature and then number of metal chelating ligands determined polymers adsorption capacity. At higher levels of soil As, adsorbents were more effective in reducing the concentrations of soil As. Also their effectiveness in reducing the concentration of soil available As was higher than the soil specifically sorbed As.
Conclusion: Hematite nanoparticles and C, A and B acrylic copolymers, especially hematite nanoparticles, can be used as effective adsorbents in reducing the concentrations of soil available As, specifically sorbed As.