Mohammad Taghi Kouhiyan Afzal; Ahmad Farrokhian Firouzi; Mehdi Taghavi
Abstract
Introduction: Nowadays pollution of sand columns and water resources with hexavalent chromium is enhancing due to the increase of industrial and agricultural activities. In recent years nanoscale zerovalent iron particles (nZVIP) have been used according to special properties comprising of high surface ...
Read More
Introduction: Nowadays pollution of sand columns and water resources with hexavalent chromium is enhancing due to the increase of industrial and agricultural activities. In recent years nanoscale zerovalent iron particles (nZVIP) have been used according to special properties comprising of high surface area, high reaction sites, non-toxic, non-expensive and high potential for removal of pollutants such as hexavalent chromium from sand columns and water. The size range of Zerovalent iron nanoparticles is less than 100 nm. So they could potentially be transported into the subsurface and finally be mixed with the target pollutants. Chromium is one of the steely-grey, lustrous and toxic heavy metals with high toxicity potential. Upon chromium (VI) toxicity this element is classified as a primary contaminant. Chromium (III) compounds is not toxic and hazardous and are grouped as one of the beneficial elements for human and other animals, while the toxicity and carcinogenic properties of hexavalent chromium have been realized for a long time in all over the world. Application of polymers as nZVIP stabilizers diminishes flocculation and sedimentation of nanoparticles. So usage of such polymers may lead to decreasing of particle size, enhancing reactivity and increasing particle transport in column and continuous medium studies. The objectives of this study were: (1) synthesis and characterization of different surface modified nZVIP with some polymers including Polyacrylamide (PAM) and Guar gum (GG), (2) the removal of hexavalent chromium ions from sand columns by application of different stabilized nZVIP, (3) investigation the impacts of different experimental situations on hexavalent chromium removal from sand columns including primary nZVIP dosages and primary hexavalent chromium dosages, and (4) evaluation nZVIP transportation in sand columns.
Material and Methods: In this research, nZVIP were synthesized using chemical reduction of ferrous sulfate by sodium borohydride. Guar gum (GG) as a green and environmentally friendly coating and polyacrylamide (PAM) as an anionic and biodegradable polymer were applied for stabilizing of nanoparticles. Zeta potential values were determined by a ZetaPlus zeta-potential analyzer from Brookhaven Instruments Corporation and the measured values were gathered from a suspension containing 5 mg zero valent iron nanoparticles in 100 mL of 1 mM NaCl solution, at room temperature. The hydrodynamic diameter of nanoparticles was determined using a ZetaPlus zeta potential analyzer and the Brownian movement of particles was related to nanoscale zerovalent iron particles hydrodynamic diameter. The morphology of zerovalent iron nanoparticles was determined by scanning electron microscope (SEM). Furthermore, the size of the synthesized nanoparticles was considered using a transmission electron microscope (TEM) via image measuring software.
Results and Discussions: The results showed by increasing of nanoparticle and hexavalent chromium dosage the removal efficiency of chromium increased and decreased from sand columns, respectively. Increasing of hexavalent chromium dosage from 40 to 80 mg/L in sand columns lead to more excessive chromium ions at sand columns and diminishing of hexavalent chromium removal efficient from sand particles. When the dosage of nanoscale zerovalent iron particles raised from 1 to 3 g/L and while the concentration of Cr (VI) was on a constant value of 100 mg/L, the effective reaction sites for hexavalent chromium removal would increase and so the removal performance would enhance according to the nanoparticle dosages. The findings of the current study also revealed when ionic strength and nZVIP dosage enhanced, the transportation of nZVIP decreased in sand columns. By enhancing the concentration of nZVIP, the surface reaction sites of nZVIP increased and hence the efficiency of chromium removal raised from sand columns.
Zeta potential is a good parameter for evaluating the colloidal stability. This parameter is an index of stability that reflects the electrostatic repulsion forces between charged particles. By shifting the zeta potential values to the higher records (more negative) the magnitude of repulsion forces among the particles will increase and the stability of them will enhance as a result. In this research the achieved zeta potential records for synthesized nZVIP showed that PAM-stabilized nZVIP and non-stabilized nZVIP had the most and the least stability values respectively.
Conclusion: The final results of this study revealed that increasing dosages of synthesized zerovalent iron nanoparticles enhanced the removal efficiency of nitrate and hexavalent chromium from sand columns. When dosages of Cr (VI) increased the removal efficiency of current pollutants decreased. TEM results showed the order of particle sizes were upon to the following trend: PAM-nZVIP < GG- nZVIP < Bare- nZVIP. So PAM stabilized zerovalent iron nanoparticles and the bare nanoparticles were the smallest and largest sizes of all. The efficiency of hexavalent chromium removal and nanoparticle transormation were according to the following trend: polyacrylamide (PAM)- nZVIP> Guar gum (GG)- nZVIP> Bare- nZVIP
S. Akbari; Ali reza Karimi; A. Lakzian; A. Fotovat
Abstract
Introduction: Parent materials as one of the main soil formation factors have a great impact on the concentration of heavy metals in the soil. Heavy metals are released to the soil during weathering and pedogenic processes. Ultrabasic rocks are known as the potential natural source of heavy metals, especially ...
Read More
Introduction: Parent materials as one of the main soil formation factors have a great impact on the concentration of heavy metals in the soil. Heavy metals are released to the soil during weathering and pedogenic processes. Ultrabasic rocks are known as the potential natural source of heavy metals, especially Ni, Cr and Mn in the soil. Average concentrations of Ni and Cr in the soils are 84 and 34 mg kg-1, respectively; while, in soil derived from ultrabasic parent material, the concentration of these elements may reach up to 100000 mg kg-1. Binaloud zone in northeastern composed of different geological materials. There is a narrow band of ophiolitic rocks in this zone that located along Mashhad city. The geochemical behavior of ultrabsic rocks and the associated soil have been frequently studied mostly in humid regions. But, there are a few research works done in arid environments. The objective of this study was to investigate the physical and chemical properties and concentrations of Ni, Cr and Mn in soils formed along a toposequence of ultrabasic rocks in western Mashhad.
Materials and Methods: The study area is located in the hilly land landscape of Binaloud zone in the Western part of Mashhad. Mean annual precipitation and temperature is 260 mm and 13.7 oC, respectively. Soil temperature and moisture regimes are thermic and aridic boarder on mesic, respectively. Studied soils developed on hornblendite rocks that are ultrabasic rocks with SiO2 less than 45% and contain ferromagnesian minerals. A toposequence was selected and, three soil profiles on shoulder, backslope and footslope geomorphic positions were described acoording to key to soil taxonmy 2014 and the soil horizons were sampled. Air-dried samples were passed through 2 mm sieve and were used for laboratory analysis. Pseudo-total concentrations of Ni, Cr and Mn were extracted by aqua regia digestion procedure. Free iron oxides (Fed) and amorphous iron oxides (Feo) were extracted by citrate-bicarbonate-dithionite (CBD) and oxalic acid methods, respectively and were measured by atomic absorption spectroscopy. The soil was extracted by ammonium acetar 1N and concentration of Ca and Mg were measured by EDTA titrimetric method. Calcium carbonate equivalent, gypsum, pH, Sand, silt and clay fractions and soil organic materials were measured using custom laboratory methods.
Results and Discussion: Solum thickness of the studied soils is less than 45 cm. Calcification and gypsification are the two main soil formation processes leading to formation of calcic (Bk) and gypsic (By) horizons. Calcium carbonate equivalent and gypsum contents in the studied soils varied from 5.1 to 30 and 5.9 to 40.1 %, respectively. Regarding the type of parent material, presence of large amounts of gypsum and carbonates can be attributed to aeolian addition to the soil system. The presence of discontinuous and thin loess deposits in the study area confirms the dustfall deposition. High amount of these minerals cause Ca/Mg ratio is up to 33.3. Concentration of Fed and Feo were less than 6.8 and 0.2 g kg-1 reflecting weak wethering state of the soils. Morphological characteristics are the indications of weak soil development and weathering. Concentrations of Ni, Cr and Mn varied from 52.6 to 312.5, 35.2 to 135.3 and 375.3 to 628.9 mg kg-1 that are low values in comparison to soils in humid regions due to weak soil weathering and eolian addition of materials containing gypsum and carbonates. The Ni and Cr contents increase from shoulder to foot slope. Direct and concordant variations of Ni with Cr and Mn with Fed indicate the similar mineralogy and trend of weathering of these elements. Regarding the high concentration of Ni and Cr in the studied soils, the bioaccessibility of these elements should be investigated.
Conclusion: Results of this study indicated the weak development of soil formed on ultabasic rocks in the western Mashhad that was expected regarding the arid climate of the study area. Because of the low weathering status of the soil, the concentration of Ni, Cr and Mn were less than that of similar soils in humid areas. Also aeolian addition of carbonates and gypsum to the soil system dilutes the concentration of these elements. To evaluate risk assessment of Ni, Cr and Mn in the studied soils, successive extraction and pot experiments are suggested.
