Online Water Contamination Analyzing

Online Water Contamination Analyzing
The Crisis
Arsenic is the 20th most abundant element in the earth’s crust, and there are numerous natural sources, as well as human activities, that can introduce it into drinking water. Primary natural sources include mineral deposits, hydro-geothermal activity, and volcanism. Major anthropogenic sources of arsenic include WOOD PRESERVATIVES, PESTICIDES, INDUSTRIAL USES and MINING AND SMELTING. The most toxic form of arsenic is ARSINE GAS followed by ARSENITE and ARSENATE which are the species normally found in groundwater. Chronic exposure to low concentrations of arsenic in drinking water can lead to skin, bladder, lung, and prostate Cancer. Non-Cancer effects of ingesting arsenic at low levels include cardiovascular disease, diabetes, and anemia, as well as reproductive and developmental, immunological, and neurological effects.
The public health concern for environmental exposure to arsenic has been widely recognized for decades. In fact, arsenic exposure is a worldwide problem rather than national since chronic arsenic poisoning is a serious issue for China, Taiwan, Thailand, Mexico, Chile, India, and Bangladesh where the sources of arsenic exposure vary from burning arsenic-rich coal (China) and mining activities (Malaysia, Japan) to the ingestion of arsenic-contaminated drinking water (Taiwan, Inner Mongolia, China, India, Bangladesh).
The following may be the prime sources that releases Arsenic,
1. Naturally Occuring
2. Semiconductor
3. Coal based Poer Plant - Ash and Vapour
4. Volcanic Actvities
5. Metal Alloy Industry
6. Mining and Smelting
7. Pesticides
8. CCA ( Chromated Copper Arsenate) - a Wood Preservative
9. Chemicals used in Defence Application
Because of its toxicity, the US EPA and other world wide regulatory authorities considers arsenic to be a Contaminant of Primary Concern (CPC). The target concentration of arsenic in drinking water is 0 ppb and recently California Office of Environmental Health Hazard Assessment set a Public Health Goal of 4 parts-per-trillion (ppt). At the moment, as a compromise between technical feasibility, economic impact, analytical capabilities and toxicological concerns, the US EPA has lowered the drinking water criterion for arsenic from 50 to 10 parts-per-billion (ppb w/v). Additionally, the new rule requires that arsenic sampling be reported to the nearest 1 ppb and the systems sample at every point to the distribution system and also increase the monitoring frequency for arsenic.
Arsenic Analyzing is not a new concept today and there are several methods already available in the Water Industry. Present popular methods for the determination of arsenic use Atomic Optical Absorption Spectrometry or Plasma-Assisted Mass-Spectroscopy and these techniques provide the required detection limits. Unfortunately, techniques such as Inductively Coupled Plasma (ICP) with Atomic Emission Spectrometric Detection (ICP-AES), or Mass Spectrometric Detection (ICP-MS), Graphite Furnace Atomic Absorption Spectrometry (GFAAS), and Hydride Generation Combined with Flame Atomic Absorption Spectrometry (HG-AAS) require instruments that cannot be modified or miniaturized and/or automated towards on-line monitoring. At present, verification of water quality entails samples packed and shipped to off-site laboratories for analysis, resulting in poor control of the arsenic treatment process.
System Requirement
It’s very unfortunate that today lots of public water-supply systems are presently contaminated with lethal Arsenic. Even though arsenic in natural waters is predominantly present in the pentavalent form (Arsenate),if the waters are Oxic (e.g., surface waters). However, ground waters are often anoxic and contain significant amounts of the trivalent form (Arsenite).
Further, wells with water of this type are known for a variable As (III)/As(V) ratio that can change in a matter of days. This is particularly important as most treatments will not remove As (III) and require the addition of oxidants (chlorine) to convert all arsenic present to the treatable form: As(V). It is important to minimize the addition of chlorine so as it is highly carcinogenic in nature as well as to preserve taste of drinking water.
The Solution
According to Soumen Kumar - a SCADA, Instrumentation, Automation Consultant specially in field of Water and Power industry Online System for Arsenic Analyzer/Monitor (OAA) with Nano-Band Technology (Developed at University of Washington, USA) results stable and dynamic performance which is also better than popular methods in all the aspects along with the cost of Analyzing is very minimum then conventional methods. This Online System is presently configured to measure total arsenic in the 1 - 50 ppb range (for drinking water applications) or in the 50 - 500 ppb range for industrial Waste water applications. Water utilities can use this monitor to ensure compliance, to adjust their treatment chemistry, to prevent “break-through” in the treatment system, and to monitor the ion-exchange regeneration system.
This instrument combines a novel sample pretreatment system with Sequential Injection Analysis (SIA) and Anodic Stripping Voltammetry (ASV) performed with patented Nano-Band™ Electrode Sensor. This is a unique technology developed by TraceDetect and used in a successful line of manual analysis instruments.
Although all the systems are automated, a single measurement proceeds as follows - which may help its working principal. The sample is loaded into Preparation Module and acidified. If required, the sample is also diluted at this point. If a measurement of As(III) is desired, the sample is moved immediately to the Test Cell and measured. The measurement produces a peak for arsenic in a voltammogram: peak height is proportional to the As(III) concentration. After the initial measurement, standard additions are made to the test cell and the measurement is repeated after each addition. A calibration curve is constructed and used to calculate the concentration of the original sample. Matrix effects and sensor drift are automatically accounted for in this method.
This unit can be adapted to other sample streams. If organic arsenic is present, the pretreatment module can be programmed to inject oxidizer into the sample. This frees the organically bound arsenic and converts it to As(V). Subsequent addition of the reducing agent then converts the arsenic to the measurable form,As(III), prior to analysis. If the sample stream contains a significant amount of copper (not typical in ground water but a common contaminant due to plumbing fixtures), a ion-selective adsorption cartridge can be added that removes 99% of the copper, but passes the As(III) and As(V).
As(III), prior to analysis. If the sample stream contains a significant amount of copper (not typical in ground water but a common contaminant due to plumbing fixtures), a ion-selective adsorption cartridge can be added that removes 99% of the copper, but passes the As(III) and As(V).
Stripping methods use two key measurement steps,
1. Pre Concentration (Plating)
2. Stripping ( Oxidation)
Anodic Stripping Voltammetry and Nano-Band Technology
Anodic Stripping Voltammetry thus ASV is an electrochemical technique for performing ultra-trace measurements of selected metals and semi-metals such as arsenic. The technique relies on the two-step process of (1) electrochemical plating (metal pre-concentration) and (2) electrochemical stripping. During the plating phase, the electrode is held at a negative voltage and arsenic is plated onto the tip of our Nano-Band electrode: the longer the plating time, the more sensitive the measurement. Only 0.1 pg of material is required for a good measurement.
Anodic Stripping Voltammetry is a voltammetric method for quantitative determination of specific ionic species. The analyte of interest is electroplated on the working electrode during a deposition step, and oxidized from the electrode during the stripping step. The current is measured during the stripping step. The oxidation of species is registered as a peak in the current signal at the potential at which the species begins to be oxidized. The stripping step can be either linear, staircase, squarewave, or pulse.
During the stripping phase, the electrode voltage is ramped to a positive voltage greater than the oxidation-reduction potential for metallic arsenic. When present, arsenic is stripped from the electrode and a characteristic current peak is measured. This process converts solid As(0) to dissolved As(III) and required three electrons per arsenic atom. The height (or area) of the resulting current peak is used to make a calibrated measurement. Peak parameters are linear with concentration and plating time. Typical calibrations result in a linear fit factor (r-squared) better than 0.995 (r2 = 1.000 implies a perfect linear fit).
Soumen Kumar says that the advantage of using this method of standard additions is that changes in measurement sensitivity can automatically be accounted for, sensor performance can be monitored and statistics can be collected with each measurement that can be used to qualify each result. If the response of the sensor to the standard additions is significantly non-linear, the software alerts the user to clean the sensor. This significantly reduces the occurrence of false positive and false negative results.

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