Effects of Humic Acid and Water Hardness on Acute Cadmium Toxicity by Using Simocephalus expinosus as a BioindicaterIntroduction

Adsorption of pollutants on dissolved organic materials and suspended particles has been shown to be one of the most important factors controlling the bioavailability of hydrophobic organic contaminants in freshwater.(Livens,1991) (Schlautman&Morgan,1993) (Thomas,1997).
Adsorption define as , the binding of molecules or particles to another molecules or particles without real chemical reaction and the adsorption process is generally classified as physisorption (characteristic of weak van der Waals forces) or chemisorption (characteristic of covalent bonding).(Aiguo &Richard ,2000)
Dissolved organic matter (DOM) is present in all natural waters ; the concentration varies greatly between location depending on the geochemistry , season ,type of water ,etc . Several studies have shown that in most cases DOM either decreases or not affect the bioavailability certain compounds(Akkanen ,2002).

Cadmium is a natural, usually minor constituent of surface and groundwater. It may exist in water as hydrated ion , inorganic complexes such as carbonates, hydroxides, chlorides or sulphates , or as organic complexes with humic acids (Ravevra,1984).
Cadmium may enter aquatic systems through weathering and erosion of soils and bedrock , atmospheric deposition from direct industrial discharge , leakage from contaminated sites, and the dispersive use and fertilizers in agriculture.(Calamari ;et al 1980) . Much of the cadmium entering fresh waters from industrial sources may be rapidly adsorbed by particulate matter or dissolved humic materials, and thus sediment may be a significant sink for cadmium emitted to the aquatic environment (Hartung,1973).
Partitioning of cadmium between the adsorbed-in-humic acid, sediment state and dissolved-in-water state is therefore an important factor in whether cadmium emitted to waters is or is not available to enter the food chain and affect human health.(Wang,1987).
Adsorption of cadmium onto water humic acid is generally considered one of the most important processes that controls cadmium behavior, performances, and fate in water, including mobility, transport, accumulation ,bioavailability and toxicity, and the assessment of their potential environmental hzard.(Loffredo&Nicola,2006)
Toxicity of pollutants affected by chemical and physical characteristic of water, temperature , pH , salinity ,and hardness which consider among the well-known factors that modify the toxicity of trace elements on many species of fish and invertebrate (Pascoe et al, 1986).
Zooplankton plays an important role in the functioning of aquatic ecosystem by making part of the production of phytoplankton available to higher trophic levels .Crustaceans ,which often dominate the zooplankton are the major herbivores in many aquatic communities and are the main food for bigger crustaceans ,various species of fish and representative from many other taxa .(Ghazy et al ,2009)

In this study two factors that may influence the toxicity of cadmium were investigated water hardness and humic acid by using fresh water invertebrate Simocephalus exspinosus( Branchiopoda: Cladocera) as a bioindecater .

Materials & Methods

A- Experimental animal
A freshwater invertebrates Simocephalus expinosus were obtained from the fisheries in Baghdad city they were kept in glass containers (40L) filled with local tap water, which was aerated vigorously for about three days before use .Prior to performing each experiment, they were acclimated to laboratory conditions for at least 10 days. During the acclimatization period, they were feed 3times a week with(150ML) Alfa alfa jues .
Gravid females were transferred at regular intervals to 1- L glass beakers and were checked daily for release of newborn (<24h old) of Simocephalus exspinosus to be used in experiments.

Simocephalus expinosus become a standard test organism as it is easy to culture , handle in the laboratory, and it is sensitive to chemicals and environmental changes(Leblanc,2007)

B-Experimental solution
1-The experiments were carried out in 250-ml glass beakers contained de - chlorinated tap water and inoculated with 10 newborn of S. exspinosus <24h old .
2- Cadmium used in these experiments was of a reagent grade CdCl2.H2O. A solution of cadmium was freshly prepared by dilution in distilled water for each experiment according to the concentrations needed .

3- Humic acid, obtained from shinyang humate Chemical Company, was prepared by dissolving it in distilled water and the concentration was expressed as ash-free dry weight. The stock solution of humic acid was then filtered through a 1.2 membrane filter to remove remaining particulates before use (Brockaert et al, 1989). The final humic acid concentrations used were 50,100, and150 mg/l.
4- Two different levels of water hardness , with the average total hardness concentrations of 50, and100 mg/L as CaCO3 , were used. The hardness concentrations were tested by the EDTA titration method (APHA, 1995).
* Control solution was without either humic acid or hardener’s added.
* The experimental water temperature was maintained
at around 22 throughout the study, pH=7.6 , salinity = 0.179‰ ,hardness =50 mg/L CaCO3 .
C- Cadmium toxicity
The acute toxicity of cadmium was determined by median lethal concentration (LC50) bioassays using S. expinosus for each experiment, a preliminary assay consisting of eight concentrations ( 1.0, ,2.0 , 3.0,4.0,5.0, 6.0, 7.0 and 8.0) mg Cd/L was first conducted to establish the critical range of cadmium toxicity, then, the definitive assay was performed to determine the concentration at which a particular end point occurred.
After acclimatization, ten S. expinosus age (24h) randomly selected were simultaneously exposed to each of eight cadmium concentrations
(1.0 ,2.0 ,3.0 ,4.0 ,5.0 ,6.0 7.0 , 8.0 mg\l )and a control for 48 hours without aeration or feeding.
In the second series of experiments, the effects of water hardness concentrations on the acute cadmium toxicity were evaluated. Two water hardness concentrations (moderately and hard) were prepared by adding the hardness ions as described previously.
In the third series of experiments, the influence of humic acid on acute cadmium toxicity was assessed using three humic acid concentrations (50,75, and100 mg/L).

Data analysis :-
The acute toxicity of cadmium was determined as the median lethal concentration (LC50), with the 95% confidence intervals, using the Probit Analysis Program (Finney ,1971).