Author: danwalls (Page 7 of 10)

Today, Dan started a set of experiments to determine the soil texture (percentage of sand, silt, and clay) of the Troy soil samples. Sand, silt, and clay are the main nonliving solid components of soils. The primary difference between sand, silt, and clay particles is their size. The Soil Science Society of America created the graphic below to demonstrate the relative size of sand (largest), silt (middle), and clay (smallest) particles:

Soil texture is a common way to classify soils. It is also relevant for human health if the soil contains lead or other metals. Smaller particles are more likely to stick to our hands when we touch soil, and they are also more likely to be kicked up as dust in the air.

One way to measure soil texture is with a sedimentation experiment. Individual soil particles settle in water at different speeds based on their size. By mixing a soil sample in a column of water and then measuring how it settles over time, the particle sizes in the soil can be calculated. Below is a picture of the sedimentation columns from one set of experiments with the Troy soils:

Another way to measure soil texture is by feel. The USDA created a flow chart so that texture could be determined through a series of steps where soils are rubbed between your fingers. The flow chart can be found here.

Dan is continuing to work on the Community Soil Study Toolkit in the Ramírez-Andreotta laboratory with the field test for measuring copper in soil. This test is based on a method developed by our collaborators on the Nuestros Suelos project in Chile. Dan has completed measurements for the soil samples used with the soil lead and arsenic field tests performed earlier in the month: samples from public spaces in and around Troy, NY, and several samples archived in the Ramírez-Andreotta laboratory. He is now in the process of organizing and comparing the field results with the laboratory methods used to measure copper in soils. By comparing the field and laboratory results, a color scale can be established so that the color response can be quantified as the amount of copper in the soil.

The picture below shows Dan’s setup in the laboratory for conducting the copper field test: (1) soil samples are first added to vials and mixed with acidic solutions to extract copper from the soils; (2) after thirty minutes of the soil soaking in the acidic solution, a filed nail is added to the vial; (3) copper ions in solution undergo an electrochemical reduction-oxidation reaction and deposit on the nail; and (4) the intensity of the copper color on the nail corresponds to the amount of copper present in the soil.

Dan is continuing to work on the Community Soil Study Toolkit in the Ramírez-Andreotta laboratory, now looking at a field test for measuring arsenic in soil. This test is based on a method under development in the Ramírez-Andreotta laboratory. He has completed measurements for the soil samples used with the soil lead field test earlier in the month: samples from public spaces in and around Troy, NY, and several samples archived in the Ramírez-Andreotta laboratory. He is now in the process of organizing and comparing the field results with the laboratory methods used to measure arsenic in soils.

The picture below shows Dan’s setup in the laboratory for conducting the arsenic field test:

Dan is working in the Ramírez-Andreotta laboratory with the field test for measuring lead in soil reported by Landes et al (2019) [open access article here]. He has completed measurements for the soil samples from public spaces in and around Troy, NY, as well as several soil samples archived in the Ramírez-Andreotta laboratory. He is now in the process of organizing and reviewing the resultant data to compare with the laboratory analyses of the soils in order to confirm that our use of the field kit compares favorably to the findings reported by Landes et al.

The picture below shows Dan’s setup in the laboratory for conducting the field test: (1) soil samples are first added to vials and mixed with acidic solutions to extract lead from the soils; (2) after an hour of the soil soaking in the acidic solution, a syringe and a filter is used to transfer just the solution to a second vial; (3) a gelatin capsule containing sodium rhodizonate is added to the solution in the second vial; (4) sodium rhodizonate turns purple in the presence of lead, with more lead leading to a more intense purple; and (5) the intensity of the color response reveals the amount of lead present in the soil.

Dan used a portable x-ray fluorescence (XRF) analyzer to measure lead, arsenic, and copper in the soil samples from public spaces in and around Troy, NY, as well as soil samples archived in the Ramírez-Andreotta laboratory. The results will be compared to another laboratory technique, inductively coupled plasma mass spectrometry (ICP-MS), as well as the field tests for lead, arsenic, and copper in the Community Soil Study Toolkit. The XRF analyzer is portable and can be brought into the field, but it is a costly piece of equipment. It is able to measure the concentrations of elements like lead, arsenic, and copper in soil through the property of fluorescence.

Fluorescence occurs when an object or material absorbs light and then subsequently re-emits light. A vibrant example is when an object absorbs ultraviolet light (which our eyes cannot see) and re-emits visible light, such as in the image below of various fluorescent minerals taken by Hannes Grobe and shared under a Creative Commons license:

Similarly, materials can exhibit x-ray fluorescence by absorbing and re-emitting x-rays, although we cannot see this with our eyes. The XRF analyzer produces x-rays that are absorbed by a material and then measures the re-emitted x-rays. The individual atoms in a material are what absorb and re-emit the x-rays, and different chemical elements re-emit x-rays with different energy signatures. So by measuring the energies of the re-emitted x-rays, the concentrations of elements like lead, arsenic, and copper in a material can be determined. In the image below, the XRF analyzer is measuring the elemental concentrations of a soil sample through a plastic bag.

Mónica’s interview with Dan aired on Tucson community radio station KXCI 91.3 as part of its Thesis Thursday program. Mónica and Dan discuss the Our Soil project that brought him to the Ramírez-Andreotta laboratory, what he will be doing there to support the project, and music for long road trips. You can listen to the interview on KXCI’s website here.