Resource Recycling of Red Soil to Synthesize Fe₂O₃/FAU-type Zeolite Composite Material for Heavy Metal Removal

A green chemistry approach has been developed to convert the widely-distributed red soil into the iron oxide FAU zeolite composite material. This material is capable for remove the hair metal irons in water and restore the water health. This protocol demonstrates the strategy to directly use soil as raw material for eco-material sensors, and providing insight into resource utilization of four typical soils based on the elemental cycling philosophy.

The present study can be informative for the related researchers who are interested in soil resource utilization. Now that can be material synthesis and water mediation. To begin, remove the 30 centimeter top layer of the soil containing plants and residual organic matter at the collection site, and collect the red soil and air dry it at room temperature.

After proper drying, filter the soil through a 30-mesh sieve. Weigh five grams of this pretreated red soil, one gram of silicon dioxide, and 7.63 grams of sodium hydroxide, and add them to a natural agate mortar. Next, grind them for two to three minutes into a fine powder.

For alkali-activation, transfer this alkaline mixture into a 100 milliliter Teflon reactor liner without the stainless steel outer covering, and heat it in the 200 degree Celsius oven for one hour. Then add 60 milliliters of deionized water into the Teflon reactor liner containing the activated alkaline mixture. Add a stir bar of the appropriate size and stir the mixture at 600 RPM on the magnetic stirrer for three hours at 25 degrees Celsius.

For the crystallization process, transfer the homogenous gel into a 100 milliliter stainless steel autoclave and heat the gel in a 100 degree Celsius oven for 12 hours. Next, wash the obtained zeolite with deionized water several times until the solution pH is close to seven. Use a centrifuge to separate the solid and liquid and collect the solid at the bottom of the 50 milliliter centrifuge tube.

Finally, dry the obtained product for eight hours in an 80 degree Celsius oven, followed by grinding it into a fine powder for subsequent characterization. Prepare 50 milliliters of 1000 parts per million aqueous solutions of bivalent copper ion, trivalent chromium ion, hexavalent chromium ion, trivalent arsenic ion, bivalent cadmium ion, bivalent lead ion, bivalent zinc ion, and bivalent nickel ion. And note the pH of each solution.

Next, add 50 milligrams of zeolite to each heavy metalloid solution. Finally, adjust the pH of the solution with 0.1 molar hydrochloric acid or 0.1 molar sodium hydroxide, and stir the mixture at 600 revolutions per minute for 48 hours at 25 degrees Celsius. Then, filter the mixed solutions through 0.22 micron membranes.

Dilute them to 1000 x by adding 2%nitric acid solution and measure the residual heavy metalloid concentrations with inductively-coupled plasma mass spectrometry with a testing range of 0.001 parts per million to one part per million. The crystal structure of the FAU-type zeolite framework and ferric oxide shows that the FAU zeolite is composed of three-dimensional, 12-membered rings. It belongs to the cubic crystal system.

The space group is FD 3M, and the unit cell parameter is 24.3450 angstrom. The powder X-ray defraction pattern of the iron oxide FAU-type zeolite composite material revealed a great match of this sample with the simulated standard materials, suggesting the success of the synthesis. The scanning electron microscopy image clarified that the iron oxide FAU-type zeolite composite material shows needle-like morphology with high purity.

Energy dispersive x-ray spectroscopy mapping show that the typical zeolite composition elements like silicon, aluminum, sodium, and oxygen were evenly distributed on the material, and iron was distributed discreetly in the composite material. The absorption capacity of iron oxide FAU-type zeolite composite material for eight typical heavy metal solutions showed a fascinatingly high capacity for bivalent lead ion and bivalent cadmium ion absorption. The most crucial steps concerning valid counted material synthesis include alkali-activation, precursor preparation, and crystallization.

This work paves the way for the direct utilization of soil as raw materials for your mentor material synthesis, which can be further expanded to other type of soils for broad environmental engineering applications.