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David Yafté
David Yafté

From lab curiosity to suitable environment-healthcare for the world and human beings.

By David Yafté Díaz–Sánchez

Waste, a very popular word nowadays, commonly refers to a human-made product with no significant value, which must be eliminated. When I was younger, I studied in a technical college in the south of Mexico City, and I used to ask my teachers what happened to chemical waste generated at the clinical laboratory. Today, I am studying at university to be a metallurgical chemical engineer, but I cannot forget my old questions about chemical waste, because some chemical wastes have  properties that make them hazardous to health and the environment.

When working at the clinical laboratory, some hazardous waste is often produced. It has to be reduced and, if it cannot be, we should learn what to do with this waste to minimise its potential risk or its impact on  the environment and our health. There are principally two kinds of dangerous waste at a clinical laboratory: chemical and biological waste. This  article focuses only on  chemical waste.

Chemical wastes could be hazardous in different ways(UNAM, 2007; Yale, 2015): some could be corrosive, like HNO3, some others could be toxic, like NaCN, or some could be hazardous to the environment, like CuSO4. There are different types of chemical waste, and we must learn how to treat them properly. Sometimes a neutralisation reaction is enough to make waste less dangerous, but at times more than that is necessary. Here we refer  to some examples:


 1: Spectrophotometric determination of chloride ion in blood

The principle of the method is the quantitative displacement of thiocyanate by chloride from mercuric thiocyanate, and subsequent formation of a red ferric thiocyanate complex is measured colourimetrically. Here the intensity of the colour formed is proportional to the chloride ion concentration in the sample:

2Cl-(aq) + Hg(SCN)2(aq) → HgCl2(aq) + 2SCN-(aq)

SCN-(aq) + Fe3+(aq) → FeSCN2+(aq)

     In this case the reagent used to measure the chloride ion in blood contains mercury compounds, which are highly toxic and can be accumulated in  the human body. The mercury concentration in this reagent is 6 mmol L-1 , and usually about 0.250L is used. This makes it necessary   to treat  this waste properly. A possible treatment is to add thioacetamide, which releases hydrogen sulphide  while heating in aqueous solution, permitting the precipitation of the very insoluble mercury (II) sulphide without obnoxious odours.

C2H5NS(aq) + H2O(l) → H2S(aq) + C2H5NO(aq)

HgCl2(aq) + H2S(aq) → HgS(s) +2HCl(aq)

Ksp(HgS) = 4.0 × 10-54

     Although both thioacetamide and hydrogen sulphide are toxic, only quantities in the order of micrograms are needed, and mercury (II) sulphide  can be recycled to obtain mercury, just like a metallurgical engineer does with the mineral cinnabar.

HgS(s) + O2(g) → Hg(l) + SO2(g)

     Going beyond mercury, on  an industrial scale, this can be used to obtain the always useful H2SO4 from the SO2.

2SO2(g) + O2(g) → + 2SO3(g)

SO3(g) + H2SO4(l) → + H2S2O7(l)

H2S2O7(l) + H2O(l) → + 2H2SO4(l)


2: Haemoglobin determination by Drabkin’s reagent

Haemoglobin, the oxygen–carrying protein of erythrocytes (red blood cells) is oxidised by hexacyanoferrate (III) ion to methemoglobin and cyanide complex, to get cyanmethemoglobin:

Hb(FeII)(aq) + Fe[(CN)6]3-(aq) → Hb(FeIII)(aq) + Fe[(CN)6]4-(aq)

Hb(FeIII)(aq) + CN-(aq) → HbCN(FeIII)(aq)

     Drabkin’s reagent contains cyanide ions, extremely poisonous. This time, the cyanide concentration in this reagent is 77 mmol L-1 and, like the first example, usually about 0.250L is used. Fortunately, we can find  different ways to treat this waste. One of them is the destruction of cyanide by oxidation with  H2O2 using CuSO4 as catalyst (Khodadadi, et al, 2005):

CN-(aq) + H2O2(aq) → CNO-(aq) + H2O(l)

     Produced cyanate reacts with water at pH<7 to produce ammonium and carbonate ions:

CNO-(aq) + 2H2O(l) → CO32-(aq) + NH4+(aq)

     It is essential to do  this treatment “one step at a time”; If we venture  to make this reaction in just one step, adding H2O2 and CuSO4 to cyanide and immediately adding an acid to bring  down the pH, we could die, because cyanide reacts with acids to produce the even more poisonous gas hydrogen cyanide.

However, not everything  is bad: we can even find gold if we know where to look.


Example 3: Pregnancy test and the rainbow’s end

Most  pregnancy tests are based on the detection of the human chorionic gonadotropin (hCG) in urine and serum. This is because hCG is a hormone produced by the placenta and other tissues when a woman gets pregnant, making hCG in urine and serum an early indicator of pregnancy. Commonly, a pregnancy test is a one-step lateral flow chromatographic immunoassay. The test strip in the device consists of a conjugate pad containing mouse monoclonal anti-hCG antibody conjugated to colloidal gold, and a nitrocellulose membrane strip containing two lines, test line and a control line.

And the magic word here is gold. Just like the so-called e-waste, here  is another example of urban mining, where we can find gold in higher concentrations than in the ores from mining (Dupont, 2014). But  contrary to e-waste, a pregnancy test only contains gold, and its extraction process could be hydrometallurgical or pyrometallurgical. Even in the hydrometallurgical method we could try to use an alternative lixiviant, as the amino acids (Feng, et al, 2011) or thiosulphate  (Heath, et al, 2008) based solutions. Recently, these last systems have been identified as potential industrial competitors to cyanide based systems.

But pregnancy tests are just an example; we can find gold, and other precious metals in a large number of clinical tests and studies. A well-established source of silver (3-6 g m-2) is X-ray films.  Approximately 2 billion X-rays  per year are taken around the world (Khunprasert, et al, 2008) and are nowadays  collected to recover silver, which  has  insufficient world production. Other tests contain precious metals, for example plasmonic ELISA tests, that contain colloidal gold, or the surface plasmon resonance, that uses thin layers of gold or silver and is often used in immunology. All we need is to search in these and in other tests to find the pot at the rainbow’s end.

Technical and economic studies need to be done to determine the most inexpensive and less toxic methods to get precious metals from those sources efficiently and economically. This last example gives us a revolution in the way we see what we call “waste”. Each day we can find that more and more things contain silver, gold, palladium and others.


Some useful thoughts  about waste management and treatment

Now I have been writing a Standard Operating Procedure (SOP) manual where I clearly say what and how to do, step by step, reactions to reduce potential risk from chemical waste potential.  My technical college has recently improved that manual and together, we are working to evaluate the document. A few  people are interested in this project, and it is about changing people’s way to think. At school, industries or research laboratories and everybody who produces chemical waste must know why and how to treat them[HM1] . It is not just a teachers’ issue: auxiliary laboratory workers and students have to learn about chemical waste management. Each action plan needs a person to  supervise that everything is going on correctly. This person must know about chemistry, chemical waste and health care and must be qualified to evaluate  any way to minimise chemical waste, helped by chemists, medics and biologists.

About chemical waste itself, safety data sheets are good resources to get information about chemical substances. Generally a small number of substances of daily use have  properties comparable to cyanide or mercury (II), but remember, waste could be reactive, corrosive, explosive, oxidant, toxic, poisonous, flammable and/or hazardous to the environment. Personnel  must be under constant training,  taking discussion sessions with experts and caring about themselves and their safety.

Only if we are concerned about treating chemical clinical waste, could we  guarantee  integral health care for  every person who goes to a clinical laboratory. Otherwise we  shall quickly see how the green hills around our technical college becoming  grey and the sown fields near to us turning into sterile loads of soil. And I am sure this is not only happening here in Mexico.



  • Facultad de Química–UNAM (2007). Reglamento para el manejo, tratamiento y minimización de residuos generados en la Facultad de Química de la UNAM. México.


  • Yale Environmental Health & Safety Environmental Affairs Section (2015, February 6). Management of Hazardous Waste A Policy and Procedures Manual. Retrieved from
    • Khodadadi, A., Abdolahi, M. & Teimoury, P. (2005). Detoxification of cyanide in gold processing wastewater by hydrogen peroxide. Journal of Environmental Health Science and Engineering, 2(3), 177-182.
    • Dupont, D. (2014). Let’s not waste our e-waste. Chemistry International, 36(4), 10-11.
    • Feng. D., Van Deventer, J.S.J. (2011). The role of ammino acids in the thiosulphate leaching of gold. Miner. Eng. 22(9), 1022-1024.
    • Heath, J.A., Jeffrey, M.I., Zhang, H.G., Rumball, J.A. (2008). Anaerobic thiosulphate leaching: development of in situ gold leaching systems. Miner. Eng. 21(6), 424-433.
    • Khunprasert, P., Grisdanurak, N., Thaveesri, J., Danutra, V., Puttitavorn, W. (2008). Radiographic film waste management in Thailand and cleaner technology for silver leaching. Journal of Cleaner Production 16(1), 28-36.

 [HM1]Poorly constructed sentence; meaning not clear