How To Measure Heavy Metals In Water

Heavy metal water refers to the abnormal concentration of metal elements and their compounds with a relative density of more than 4.5 in water, resulting in water quality. Heavy metals with relative density above 4.5 include copper, lead, zinc, nickel, chromium, cadmium, mercury and non-metallic arsenic.

The determination methods of heavy metals in water include flame atomic absorption spectrophotometry, graphite furnace atomic absorption spectrophotometry, atomic fluorescence spectrometry, inductively coupled plasma emission spectrometry and inductively coupled plasma mass spectrometry.

Determination of Heavy metals in water

The electrochemical analysis is a method to determine the composition and concentration of substances according to their electrochemical properties and changes in the solution. Voltammetry, polarography and ion-selective electrode are used for determining heavy metals by electrochemical analysis methods.

The measurement parameter of electrochemical analysis is electrical signals such as conductivity, potential, current and electric quantity.

Simple device

Easy to automate and continuously analyze.

Fast and high sensitivity, and accurate in micro and trace analysis.

Low detection limit at 10-12

Although voltammetry and polarography have low detection limits and harsh and complex operation conditions, they are not used much in practical detection. Taking polarography as an example, lead exists in the form of ionic after digestion.

In an acidic medium, the [Pbi4] 2-complex ion formed by Pb2 + and I – have electrical activity and generate a reduction current on the dropping mercury electrode. The peak current is a linear relationship with lead content and determines the volume compared to the standard series.

The peak current of lead was recorded at the peak potential of – 470 MV with an Oscillopolarograph. Calculate the content of lead in the sample by the standard curve method. The determination limit of polarography is 0.085mg/kg.

Ion-selective electrode method is a kind of electrochemical method to measure ion activity by measuring electrode potential.

2) Atomic spectrometry

Atomic spectrometry is a critical determination method for trace element analysis, which includes nuclear absorption spectrometry, atomic emission spectrometry and atomic fluorescence spectrometry.

Pros：

• Low detection limit and high sensitivity.
• Fast analysis speed, minor interference in the determination of high concentration elements
• Stable signal

Cons：

• The same element’s light source is needed to determine an element.
• Not easy to determine multiple elements at the same time.
• Low sensitivity of some elements determination for complex samples.

flame atomic absorption spectrometry

The sensitivity of flame atomic absorption spectrometry in determining lead is low when it is directly used to determine trace lead in samples. To improve the sensitivity of flame atomic absorption spectrometry, it commonly uses separation and enrichment technology to pretreat the samples.

Some researchers added sensitizer Tween-80 to simplify the pretreatment, eliminate the interference of Fe, Ca, Al and other elements, and decrease the detection limit. Microwave digestion can simplify the pretreatment and reduce the detection limit.

Pros:

Simple operation and fast test speed

Cons:

High detection limit. It can only be used to determine samples with high lead content.

Graphite furnace atomic absorption spectrophotometer

It can analyze trace lead in water, food and plastic products and is suitable for determining lead in different samples. However, if the lead content in the model is too low and lead is volatile at low temperature, it is affected by matrix interference.

So, we should treat the sample before determining lead by graphite furnace atomic absorption spectrometry. The relative standard deviation was 5.4%, and the detection limit was 0.9 ngl-1 if the lead was separated and enriched by floating organic microextraction and determined by graphite furnace atomic absorption spectrometry.

Pros：

Low detection limit

Cons：

high price and slow analysis speed, but its detection limit are low.

It can be used to determine tap water, healthy water, river water and seawater.

Atomic emission spectrometry-ICP-AES

It determines the characteristic radiation emitted by gaseous atoms when excited by heat or electricity.

Cons:

High equipment and operation cost

Atomic fluorescence spectrometry (AFS)

It is an analytical method to determine the element by measuring the emission intensity of the fluorescence generated by the atomic vapour of the element under the excitation of radiation energy.

Pros：

High sensitivity, strong selectivity, a small number of samples and a simple method.

Cons：

With a wide linear range and limited application elements,

High operation costs due to many substances, including metals, do not produce fluorescence, and it needs to add some reagent to make fluorescence analysis, and the fluorescent reagent itself is more expensive.

Ultraviolet-visible spectrophotometrySpectrophotometry is a method for qualitative and quantitative analysis of the substance by measuring the light absorption at a specific wavelength or within a certain wavelength range. The basic principle of quantitative determination by UV visible spectrophotometry is the Beer-Lambert Law (a= ε bc）.

Pros：

Simple operation and relatively cheap operation

Most ions in water samples can be determined by UV visible spectrophotometry.

Very low detection limit.

Cons:

Serious spectral interference and lack of selectivity;

Analytical substances must add chromogenic agents.

Dithizone spectrophotometry

It is a good method for the determination of lead. It is suitable for the determination of trace lead in natural water and wastewater. It needs to be extracted with chloroform and measured at the maximum absorption wavelength of 510nm. The lead concentration is between 0.01 ~ 0.30mg/l. The molar absorption coefficient is 6.7 × 104 l/mol.cm, minimum detection concentration 0.01mg/l.

Due to the use of highly toxic reagents, potassium cyanide and chloroform and cumbersome extraction operations, it is resulting in the application of the dithizone method.

Using the Tween-20 micelle dithizone chromogenic system to determine trace lead makes the operation simple and avoids the use of potassium cyanide and chloroform.

The linear range is 0.06-60mg/l, and the detection limit can reach 10ug / L.

1. based on QCM Technology

Quartz crystal microbalance is a highly sensitive mass sensor based on piezoelectric (sensitivity up to ng level). It has been widely used in biochemical sensing detection due to its simple device and convenient operation. The large cluster mass of gold nanoparticles provides a highly sensitive material basis for the mass-sensitive sensor represented by quartz crystal microbalance.

Pros：

High sensitivity, good selectivity, simple, fast, low cost, and easy on-site analysis.

1. Inductively coupled plasma atomic emission spectrometry.

(ICP) has absorption at the characteristic spectral line of lead. Its absorption value is directly proportional to the lead content within a certain concentration range. The lead content in the sample is determined by the standard curve method. The detection limit of the ICP method can reach 0.1 ~ 1 μ g/g。

Pros:

It can quickly analyze multiple elements at the same time

Low detection limit, the wide linear range of standard curve, up to 4 ~ 6 orders of magnitude

Less sample consumption.

Cons:

• High equipment cost
• Complex sample preparation
• It consumes a lot of argon

1. Mass spectrometry

It is to convert the molecules of the substance into charged particles and use a stable magnetic field (or alternating electric field) to separate the charged particles according to the order of mass charge ratio and produce a detectable spectrum.

In the determination of heavy metals, plasma mass spectrometry (ICP-MS) is commonly used. Combined with mass spectrometry, the inductively coupled plasma is used to vaporize the sample and separate the metal to pass the mass spectrometry for determination.

ICP-MS can make a qualitative and quantitative analysis of inorganic elements through ion charge mass ratio. Combined with high-performance liquid chromatography, gas chromatography, capillary electrophoresis and other sampling or separation technologies, which have these advantages:

Pros：

Significantly lower detection limit than the atomic absorption method

High sensitivity, good precision, very low detection limit (up to PPT or PPQ level),

The wider linear range of analysis curves can be used for the determination of most heavy metals except mercury.

Cons:

High price and easy pollution

1. X-ray fluorescence spectrometry

It is a method to determine the composition of samples qualitatively or quantitatively by using the change of X-ray absorption of samples with the composition and its change.

The X-ray fluorescence spectrometer is mainly composed of a light source, dispersion, detection, spectrometer control and data processing. There is no significant difference between the results of elemental analysis by X-ray fluorescence spectrometry, inductively coupled plasma mass spectrometry and emission spectrometry.

Pros:

Low detection limit, high accuracy and precision

Determination method of heavy metal cadmium in water

The heavy metal cadmium in water mainly comes from different mining minerals, such as coal, aluminium, and copper. In practical application, the determination methods of cadmium common is atomic absorption spectrometry, such as direct inhalation flame atomic absorption spectrometry, Extraction Flame atomic absorption spectrometry and graphite furnace atomic absorption spectrometry.

Direct inhalation flame atomic absorption spectrometry

The principle:

Water samples or digested samples are directly sucked into the flame, and the atomic vapour formed in the flame absorbs the characteristic electromagnetic radiation emitted by the light source. Compare the absorbance of the sample with that of the standard solution, and then determine the content of cadmium in the water.

Instruments and reagents

1. Atomic absorption spectrophotometer

2. Background correction device

3. Element lamp

4. Nitric acid

5. Perchloric acid

6. Auxiliary gas

7. Purity of acetylene

8. Metal standard stock solution

a. Cadmium and zinc standard stock solution

weigh 0.5000g of spectral pure metal cadmium, and metal zinc washed and dried with dilute acid accurately. Dissolve them with 50ml nitric acid (1 + 1), and heat them if necessary until they are completely dissolved. Dilute to 500.0ml with tertiary reagent water.

b. Copper standard stock solution

weigh 0.2500g of spectral pure metal copper washed and dried with dilute acid, dissolve it with 50ml of (1 + 1) nitric acid, heat it if necessary until it is completely dissolved, and dilute it to 100.0ml with tertiary reagent water.

c. Lead standard stock solution

weigh 0.5000g of pure spectral metal lead washed and dried with dilute acid Accurately, dissolve it with 50ml of (1 + 1) nitric acid, heat it if necessary until it is completely dissolved, and dilute it to 100.0ml with tertiary reagent water.

9. Mixed standard solution

Take 5ml of cadmium and zinc standard stock solution and 10ml of copper and lead standard stock solution into the same 500ml volumetric flask and dilute to the scale with 0.2% nitric acid. The mixed standard solution contains 10.0ug, 50.0ug, 100.0ug and 10.0ug of cadmium, copper, lead and zinc, respectively.

Detection steps of cadmium in water

1. Pretreatment of water quality samples

Put 100ml water sample into a 200ml beaker, add 5ml nitric acid, heat it on an electric heating plate for digestion (do not boil), steam it to about 10ml, add 5ml nitric acid and 2ml perchloric acid, and continue digestion until about 1ml. If the digestion is not complete, add 5ml nitric acid and 2ml perchloric acid and steam again to about 1ml. Remove the cooling, add tertiary reagent water to dissolve the residue, and fix the volume to 100ml.

Take 100ml of nitric acid as the blank and operate according to the above procedure.

Water sample detection

Select the analysis line and adjust the flame according to relevant parameters. Zero the instrument with 0.2% nitric acid, inhale blank samples and samples, and measure their absorbance. After deducting the absorbance of the blank sample, find out the metal cadmium concentration in the sample from the calibration curve. Directly read out the metal cadmium concentration in the sample from the instrument if possible.

3. Calibration curve

Draw the mixed standard solution according to the standard concentration configuration table, put it into six 100ml volumetric flasks, respectively, and dilute it to volume with 0.2% nitric acid. Then measure the absorbance according to the steps of sample determination, plot the corresponding concentration with the absorbance of each standard corrected by blank, draw the calibration curve, and finally calculate the content of measured metal cadmium according to the corresponding formula.