On-site rapid detection technology of arsenic concentration in drinking water

Water is closely related to human production and life. The content of various substances in drinking water directly affects human life and health. Ensuring the safety of drinking water quality is a key point of public health protection. However, in many places in China, especially in rural areas, the centralized water supply system has not been fully established. There are many regional water quality problems in the water sources of local decentralized water intake. At the same time, the excessive activities of various industrial and mining enterprises have introduced a large amount of man-made pollution, which has further deteriorated the problem of rural drinking water quality safety. Among them, high arsenic water is a major water quality safety problem that is prevalent in large areas of China. It involves a wide range and a large number of victims, and the negative impact it brings cannot be ignored.

This article will briefly introduce the current status of high arsenic problems in China and the hazards they cause, focusing on the on-site rapid detection methods for these two types of water quality problems. It is hoped that these technologies can provide strong support for the investigation and survey of rural drinking water safety issues, emergency warnings, and acceptance and supervision of water treatment projects.

1 Distribution and causes of drinking water with high arsenic content in rural China

The World Health Organization (WHO) has lowered the standard for arsenic in drinking water to 0.01 mg/L in 1993. The U.S. Environmental Protection Agency (USEPA) lowered the standard for arsenic in drinking water from 0.05 mg/L to 0.01 mg/L in January 2006. The European Union has set the standard for arsenic in drinking water at 0.02 mg/L, while the standard for arsenic in drinking water in developing countries is generally 0.05 mg/L. In the drinking water hygiene standards issued by the Ministry of Health and the National Standardization Administration of China in 2006, the maximum limit of arsenic content was set at 0.01 mg/L, which is consistent with the WHO standard.

According to the statistical results of arsenic poisoning cases around the world, the arsenic content in drinking water reaches 0.05 mg/L, which is a mild arsenic poisoning area. According to this standard, the survey work of the "Eleventh Five-Year Plan" for rural drinking water safety projects in China showed that by the end of 2005, the number of people drinking high-arsenic water (arsenic concentration> 0.05mg/L) in rural areas across the country was 2.89 million, accounting for 1.3% of the population with unsafe water quality, distributed in 23 provinces (autonomous regions, municipalities directly under the central government). In the screening of high-deep water sources in Henan Province from 2003 to 2007, a total of 18 provincial cities and more than 2,000 villages were surveyed, and 30,000 water samples were tested. A total of 28 villages with high-arsenic drinking water were found, covering a population of nearly 50,000 people, which also reflects a certain high-arsenic water problem, especially in Lankao and Huaxian, which has a concentrated distribution trend. It is a regional water quality problem worthy of attention.

The infiltration of arsenic in drinking water is mainly due to the fact that in the rock-forming and mineralization structures, non-ferrous metal ore bodies are generally associated with arsenic elements. After the weathering of the minerals, they enter the surface water with precipitation, penetrate into the ground, and are enriched in low places, causing the arsenic in the water to exceed the standard. In addition to this natural source, human factors such as mining, mineral processing and smelting industrial wastewater and waste residue pollution are also an important cause of excessive arsenic in drinking water.

2 The harm of arsenic to human health

has been identified as a Class I carcinogen by the US Centers for Disease Control (CDC) and the International Agency for Research on Cancer (LARC). Residents have long consumed water and food containing excessive arsenic, and trace amounts of arsenic have accumulated in the body for a long time, causing long-term chronic damage to the body. We define this damage as "arsenic poisoning", such as skin cancer, blackfoot disease, neuralgia, vascular damage and gangrene, and increased incidence of heart disease. The main clinical manifestations are darkening of the skin color, keratinization, hypertrophy and rubbery, cracked ulcers, excessive keratinization and desquamation of the palms and soles, joint and muscle pain, etc. In severe cases, it can cause cancer and neonatal malformations, and these diseases can develop chronically and occur after many years. At present, reports of arsenic poisoning can be seen all over the world, especially in third world countries, such as India, Bangladesh and other countries are severely affected by arsenic poisoning. In recent years, there have also been reports of arsenic poisoning in Inner Mongolia, Shanxi, Hebei and other regions in China.

3 On-site rapid detection technology of arsenic concentration - DigiPAsS

In most areas affected by high arsenic water, there are practical difficulties such as complex terrain, remote areas, and relatively backward economic conditions. In these areas, it is very necessary to find safe water sources through field investigations; in addition, when abnormal external arsenic pollution sources suddenly appear, it is very important to be able to quickly grasp the degree of pollution and monitor the spread of pollutants in real time. These tasks require mature and reliable on-site rapid detection technology to support them. In order to meet such practical needs, the British company Behringer introduced the latest DigiPAsS digital rapid arsenic detection technology to China, which can complete the detection of arsenic concentration in water within 20 minutes. It is a very effective technology.

3.1 Working Principle

The basic principle of DigiPAsS is based on the well-developed Gutzeit principle, i.e., the arsenic spot method. When arsenic hydrogen gas comes into contact with mercuric bromide, a bright yellow intermediate (see the formula below) is generated, which further reacts with arsenic hydrogen to form a brown product. The depth of the brown spot is proportional to the arsenic concentration in contact.

AsH3 + HgBr2 > H2As – HgBr + HBr

In natural water bodies and drinking water, arsenic usually exists in the form of arsenate (pentavalent) and arsenite (trivalent), both of which are biologically toxic. In the actual use of DigiPAsS, pentavalent arsenic is first reduced to trivalent arsenic using NaSCN under acidic conditions to ensure that all arsenic concentrations in water can be detected; then trivalent arsenic is reduced to arsenic hydrogen gas using zinc reducing agent, and finally it comes into contact with test paper soaked in mercuric bromide to present brown spots. The depth of the spots is automatically compared using a photometer to complete the detection. The entire color development process only takes about 20 minutes. The photometer detection effectively avoids the human error caused by visual estimation. Its accurate range can reach 2-100ppb, and the resolution within this range is 1ppb.

Usually, water samples contain organic matter, and hydrogen sulfide gas may be produced during the reduction process. It can also react with mercuric bromide to form a brown complex and interfere with the detection. DigiPAsS is designed with a unique three-channel filter device, which is capped on the conical flask where the reduction reaction is carried out. The bottom of the filter device is equipped with a cotton ball soaked in lead acetate, which can specifically remove hydrogen sulfide gas; the purified hydrogen arsenide gas further contacts the test paper soaked in mercuric bromide for color development; finally, all residual gases pass through filter paper soaked in potassium iodide, which can completely absorb excess hydrogen arsenide tail gas, prevent leakage, and ensure the safety of users and the environment. [page]

3.2 Verification of DigiPAsS detection accuracy

Since its introduction, DigiPAsS arsenic concentration rapid detection technology has been involved in a number of projects funded by UNICEF and carried out in third world countries to reduce the toxicity of high arsenic in drinking water as a simple and fast on-site analysis method. In these projects, several third-party organizations have verified the detection accuracy, reproducibility and other indicators of this technology, and compared them with the standard hydride generation graphite furnace atomic absorption spectrometry (GF-AAS). The following will quote the verification experiment conducted by the Shriram Industrial Research Institute in Delhi, India (Experiment 1) and the World Children's Fund Water Sanitation Working Group in Yangon, Myanmar (Experiment 2). The experimental results show that DigiPAsS is a technology with high detection accuracy and stable and reliable detection results.

3.2.1 Comparison of the results of the detection of standard samples with known concentrations with the GF-AAS method

In Experiment 1, 26 groups of standard samples with known concentrations were detected using the GF-AAS method and DigiPAsS, respectively. The test results are shown in Table 1. Based on this, the correlation and standard deviation of the detection results of the two methods are calculated as shown in Table 2:

Table 1 Comparison of test results of two methods for water samples with known concentration (unit: ppb)

Sample No.	GF-AAS	DigiPAsS	Sample No.	GF-AAS	DigiPAsS
1	5	5	14	32	30
2	10	9	15	37	40
3	20	19	16	44	46
4	25	24	17	50	51
5	30	32	18	56	56
6	40	41	19	66	63
7	50	48	20	76	73
8	60	62	21	80	82
9	70	73	22	95	92
10	80	78	23	103	97
11	90	85	24	109	104
12	100	94	25	123	118
13	26	24	26	134	128

Table 2 Correlation and standard variance analysis

Mean value of the test results obtained by GF-AAS (ppb)	61.96
Average test results obtained by DigiPAsS (ppb)	60.54
Number of samples n	26
Standard Deviation	0.9472
Correlation coefficient r	0.998

From the analysis results, it can be seen that the two methods in this experiment showed an excellent match. DigiPAsS has the same detection accuracy as AAS for standard samples in the concentration range of 5-100ppb.

3.2.2 Comparison of the detection results of the two methods on actual water samples from different regions

In experiment 2, researchers collected actual water samples from six different regions, tested them on-site with DigiPAsS and in the laboratory with GF-AAS, and compared the test results. Each sample was tested 5 times in parallel using DigiPAsS. The comparison results are shown in Figure 1:

Figure 1 Comparison of detection results of DigiPAsS and GF-AAS for standard samples

In the experiment, abnormal discrete values ​​appeared in group C. By reviewing the detection operation process, it was found that the reason for the abnormally low value was that after the zinc reducing agent was added to cause the reduction reaction, the filter was not quickly and timely covered on the conical bottle mouth, resulting in the escape of the generated hydrogen arsenide, which made the measured value low.

Except for this abnormal value, the other experimental groups showed excellent matching with the GF-AAS method, and the 5 parallel test results in the same group also showed good reproducibility, especially in the precise range of 2-100ppb, the parallel error was controlled within 10%.

3.3 Conclusion

DigiPAsS digital arsenic detection technology is a development and application of the principle of traditional arsenic spot method, which successfully solves the defects of poor stability of detection data, high detection limit and poor detection accuracy of the principle, and optimizes both the reaction principle and the detection method. By comparing with the laboratory standard GF-AAS detection method, it can be seen that DigiPAsS can accurately detect arsenic concentrations in the range of 2-100ppb, with a detection accuracy of 1ppb and a detection time of only 20 minutes. This technology can effectively cover the detection range and accuracy requirements of WHO and my country for arsenic concentration in drinking water. It is easy and fast to operate and is particularly suitable for rapid use of arsenic concentration in the field.

References:
1. National Development and Reform Commission, Ministry of Water Resources, Ministry of Health, National Rural Drinking Water Safety Project "Eleventh Five-Year Plan", 2005;
2. Sun Guifan, Research progress on the pathogenesis of drinking arsenic poisoning, Journal of Medical Research, 2007.9;
3. Sun Tianzhi, Wu Kegong, Xing Chunmao, Epidemiological survey of endemic arsenic poisoning in Inner Mongolia, Chinese Journal of Endemiology, 1995.9;
4. Cheng Jinshan, Mierfang, Zhang Qingxi
, et al., Preliminary report on epidemiological survey of endemic arsenic poisoning in Shanxi Province, Shanxi Preventive Medicine, 1994.3 (3); 5. Niu Caixiang, Luo Kunli, Li Huijie, et al., Preliminary study on arsenic content and source in drinking water around Xi'an, Northwest Geology, 2008. 41(3);
6. Peter Swash, Field Evaluation of the Palintest DigiPAsS, report to UNICEF Water & Sanitation Team in Yangon, 2003.10;
7. Shriram Institute for Industrial Research, Evaluation of Palintest DigiPAsS, UNICEF, 2006.3.(end)