Taming the Dragons

Stosh Kozimor's Quest to Unlock Actinide Secrets

"If the periodic table is an atlas, the actinides should be labeled 'Here be dragons.'"

Stosh Kozimor, Los Alamos National Laboratory

Introduction: Where Few Scientists Dare to Tread

Nestled in the high desert of New Mexico, Los Alamos National Laboratory has long been a crucible for scientific breakthroughs that reshape humanity's understanding of matter and energy. Here, Stosh Kozimor leads a fearless team exploring chemistry's final frontier: the radioactive actinides.

These elusive elements—including plutonium, uranium, and synthetic heavyweights like einsteinium—are notorious for their scarcity, radioactivity, and bewildering reactivity. Yet mastering their behavior is critical for solving 21st-century challenges, from clean energy production to cancer therapeutics. Kozimor's work bridges fundamental science and lifesaving applications, proving that even the most fearsome "dragons" can be tamed for human benefit ² ⁸ .

The Actinide Enigma: Why These Elements Matter

Actinides occupy the bottom row of the periodic table, a realm where elements defy textbook rules. Their unique electron configurations enable unpredictable redox chemistry and complex bonding behaviors. Kozimor's research focuses on two pillars:

Environmental and Energy Security

Nuclear waste repositories and fuel reprocessing demand precise control over actinide solubility and reactivity. Plutonium, for example, can exist in four oxidation states simultaneously in water, triggering chaotic disproportionation reactions ¹ .
Kozimor's team revealed how plutonium speciation shifts abruptly in hydrochloric acid (HCl). Below 4.5 M HCl, water-dominated complexes stabilize Pu(III). Above 8 M HCl, chloride ligands dominate, favoring Pu(IV). This ionic "switch" explains erratic plutonium behavior during nuclear waste processing ¹ .

Medical Therapeutics

Alpha-emitting actinides like actinium-225 can obliterate tumors with minimal collateral damage. Kozimor notes: "You get four alpha particles for every actinium atom—a lot of bang for your buck" ⁸ .
But first, scientists must design molecular "cages" (chelators) to securely deliver isotopes to cancer cells. This requires decoding actinides' covalent bonding preferences—a core focus of Kozimor's work ⁵ ⁸ .

Experiment Spotlight: Plutonium's Ionic Tug-of-War in HCl

Objective: Decipher how hydrochloric acid concentration controls plutonium oxidation state stability—a critical variable for nuclear waste separation ¹ .

Methodology: Precision in a Radioactive Realm

1. Sample Preparation

Chemically pure Pu(IV) stock solution (0.02 M in 5.5–6 M HCl) was evaporated under gentle heat. Residues were dissolved in HCl concentrations spanning 1 M to 11 M.
Safety protocol: All manipulations used paired researchers, "muscle memory" drills with non-radioactive surrogates, and splash-free techniques ² ⁸ .

2. Multi-Technique Interrogation

X-ray Absorption Spectroscopy (XANES/EXAFS): Measured Pu oxidation states and Pu–Cl/Pu–O bond distances at the Pu L₃-edge.
UV-Vis-NIR Spectroscopy: Tracked electronic transitions signaling speciation changes.
Cyclic Voltammetry: Quantified the ease of Pu(IV) → Pu(III) reduction via half-wave potentials (E½) ¹ .

Results & Analysis: A Dramatic Speciation Shift

Table 1: Plutonium Speciation vs. HCl Concentration

HCl Concentration (M)	Dominant Species	Oxidation State Stability
≤ 3 M	Pu(H₂O)ₓ⁴⁺	Pu(III) favored
4.5–8 M	Mixed PuClᵧ(H₂O)ₓ⁴⁻ʸ	Transition zone
> 8 M	PuClᵧ⁴⁻ʸ	Pu(IV) favored

Table 2: Electrochemical Shifts in Pu(IV)/Pu(III) Reduction

HCl Concentration (M)	Half-Wave Potential (E½, V)	Stabilized State
1 M	0.744	Pu(III)
5.5 M	0.658 (intermediate)	Mixed
11 M	0.572	Pu(IV)

Key Insight

Neutral water ligands stabilize electron-rich Pu(III), while anionic chlorides stabilize electron-deficient Pu(IV). The transition between regimes is abrupt—occurring over just 3.5 M HCl—explaining why minor acid fluctuations cause major processing issues ¹ .

The Scientist's Toolkit: Essential Reagents for Actinide Research

Reagent/Equipment	Function	Challenge
Macrocyclic Chelators	Custom organic molecules designed to "hug" large actinide ions (e.g., Ac³⁺)	Actinium's size (coordination number = 10.9 ± 0.5) demands oversized ligands
HCl Solutions	Modulate ligand fields to control redox behavior	High concentrations required for chloride coordination (>8 M)
XANES/EXAFS	Probe bond distances/oxidation states in aqueous media	Requires synchrotron access; radioactive sample constraints
Alpha-Particle Emitters (²²⁵Ac, ²³⁰U)	Targeted cancer radiotherapy	10-day half-life of ²²⁵Ac demands rapid synthesis and delivery

From Dragons to Cures: Actinides in Cancer Therapy

Kozimor's foundational work on actinium coordination chemistry is revolutionizing oncology:

Actinium-225

Emits four alpha particles in its decay chain, delivering lethal radiation to tumors within 50–100 μm. But its large ionic radius (2.63 Å Ac–O bonds) and high coordination number (~11 water molecules) require novel chelators ⁸ .

Designing the Perfect "Cage"

Kozimor's team uses computational models to craft macrocycles that encapsulate actinium, preventing leakage of toxic isotopes into healthy tissue. As he states: "Researchers have chelators for small lanthanides like gadolinium, but they're no match for actinium" ⁵ ⁸ .

Conclusion: The Alchemist of the Atomic Age

Stosh Kozimor embodies a rare blend of curiosity and practicality. His childhood near Los Alamos—where Cold War history cast a shadow over nuclear science—fueled a determination to transform actinides from weapons of destruction into tools for healing. By decoding the bonding secrets of plutonium in HCl tanks and optimizing actinium chelators for cancer trials, his work epitomizes science's highest calling: harnessing nature's complexity for human good. As he grills in his backyard while watching his children play, Kozimor remains grounded in what matters most: "Everything outdoors with my family" ² ⁸ . In the lab and beyond, he navigates the dragons' lair with equal parts rigor and wonder.