Water Chemistry: Inorganic Ions

Chemical Characteristics of Water: Inorganic Matter

• Chemical measures of water quality involve analyzing the presence of specific ions such as calcium, magnesium, and lead.
• Gross chemical measures like alkalinity and hardness are also used to define water quality.

Major Ionic Species in Water

• Cations include Calcium (Ca+2), Magnesium (Mg+2), Sodium (Na+), and Potassium (K+).
• Anions include Bicarbonate (HCO3-), Sulfate (SO4-2), Chloride (Cl-), and Nitrate (NO3-).
• Calcium bicarbonate [Ca(HCO3)2] comes from the dissolution of limestone, marble, chalk, calcite, dolomite, and other minerals containing calcium carbonate.
• Magnesium bicarbonate [Mg(HCO3)2] comes from the dissolution of magnesite, dolomite, dolomitic limestone, and other minerals containing magnesium carbonate.
• Sodium bicarbonate [Na(HCO3)] is the white salt known as baking soda, typically a manufactured product but also present in natural waters.
• Calcium sulfate (CaSO4) includes minerals such as gypsum, alabaster, and selenite.
• Magnesium sulfate (MgSO4) includes the heptahydrate form, commonly known as Epsom salt, or as epsomite when found in salt beds or mines.
• Sodium sulfate (Na2SO4) forms in salt lakes, salt beds, and caverns etc; decahydrate form (Na₂SO₄ 10H2O) is known as Glauber's salt.
• Calcium chloride (CaCl2) comes from natural brines, salt beds, and is a byproduct of the chemical industry.
• Magnesium chloride (MgCl2) comprises anhydrous forms found in natural brines and salt beds.
• Sodium chloride (NaCl) comes from salt beds, salt lakes, connate waters, and other natural brines.

Major Ionic Species in Wastewater

• Tests for Calcium, Magnesium, and Sodium (Ca+2, Mg+2, Na+) are used to determine the sodium adsorption ratio for agricultural reuse.
• Chloride (Cl) is tested to assess the suitability of wastewater for agricultural reuse.
• Nitrogen in forms such as Organic Nitrogen (Org N), Free Ammonia (NH3), Nitrites (NO2), Nitrates (NO3), and Total Kjeldahl Nitrogen (TKN), are used to measure nutrients present and the degree of decomposition in wastewater.
• Phosphorus, including Total Phosphorus (TP), Organic Phosphorus (Org P), and Inorganic Phosphorus (Inorg P, principally PO4-3), helps measure nutrients and decomposition in wastewater; oxidized forms indicate the degree of oxidation.
• Sulfate (SO4-2) is used to assess the treatability of waste sludge.
• Alkalinity Σ(HCO3- + CO3-2 + OH-) is used to measure the buffering capacity of the wastewater.
• pH measurements, expressed as pH = log1/[H+], indicate the acidity or basicity of an aqueous solution.
• Testing for heavy metals assesses the suitability of wastewater for reuse and potential toxicity effects in treatment and receiving waters.
• Trace elements are important in biological treatment and agricultural use.

Major Measureable Parameters in Wastewater

• Five-day carbonaceous biochemical oxygen demand BOD5 (CBOD5) indicates the amount of oxygen needed to stabilize waste biologically.
• Ultimate carbonaceous biochemical oxygen demand BODt (CBODt) is tested.
• Nitrogenous oxygen demand (NOD) indicates the oxygen amount to biologically oxidize nitrogen in the wastewater to nitrate.
• Chemical oxygen demand (COD) measures the amount of oxygen needed to stabilize the waste completely.
• Total organic carbon (TOC) is often used as a substitute for the BOD test.
• Testing specific organic compounds determines the presence of pesticides, phenols, surfactants, grease, and other organic materials.
• Oxygen (O2) is measured to assess aeration facilities.
• Hydrogen sulfide (H₂S) is measured to assess the production of odors.
• Total Kjeldahl nitrogen = organic nitrogen + ammonia nitrogen.

Analysis Audit

• Equation |Σanions – Σcations| ≤ (0.1065 + 0.0155Σanions) is used to check water analysis accuracy.
• Following steps are used to prepare a cation-anion balance:
  • Prepare a cation-anion balance using Eq. (2.12).
  • Check accuracy using Eq. (2.20): |Σanions - Σcations|≤ (0.1065 +0.0155Σanions).

Minor Inorganic Species in Water

• Minor cations include Aluminum (Al+3), Ammonium (NH4+), Arsenic (As+), Barium (Ba+2), Boron as borate (BO3), Copper (Cu+2), Iron as ferrous (Fe+2), Iron as ferric (Fe+3), and Manganese (Mn+2).
• Minor anions include Bisulfate (HSO4), Bisulfite (HSO3), Carbonate (CO3 -2), Fluoride (F-), Hydroxide (OH-), Phosphate mono (H2PO4), Phosphate di (HPO4-2), Phosphate tri (РО4-3), Sulfide (S-2), and Sulfite (SO3-2).

Inorganic Species Added by Humans

• Cations added by humans include Arsenic (As+3), Barium (Ba+2), Cadmium (Cd+2), Chromium (Cr+3), Chromium (Cr+6), Lead (Pb+2), Mercury (Hg+2), Selenium (Se), Silver (Ag+2), and Zinc (Zn+2).
• Anions added include Cyanide (CN-).

Nutrients Added by Humans

• Nitrogen and phosphorus are essential for plant and animal growth.
• These elements are identified as nutrients/biostimulants when discharged in wastewater to surface waters.
• Both organic and inorganic forms of these constituents are important.

Nitrogen

• Nitrogen can exist in seven states of oxidation.
• Nitrogen-containing compounds include organic nitrogen, ammonia (NH3), nitrite (NO2 -), nitrate (NO3 -), urea [CO(NH2)2], and nitrogen gas (N₂).
• Nitrogen is cycled between its organic and inorganic forms in nature.
• Bacteria and plants produce proteins (organic compounds with nitrogen) from inorganic nitrogen forms.
• Animals, including humans, cannot utilize nitrogen from the atmosphere or inorganic compounds to produce proteins.

Phosphorus

• Phosphorus is essential for the growth of algae and aquatic organisms.
• Excess phosphorus in treated effluents can cause serious problems in aquatic environments.
• Phosphorus compounds are used in synthetic detergents and corrosion control in water supply and industrial cooling water systems.
• The discharge of phosphorus to ecologically sensitive waters must be controlled to avoid algal blooms.
• Phosphorus levels should be no more than 30mg/l according to the SSMO.

pH (Hydrogen Ion Concentration)

• When water ionizes, H₂O ⇔ H+ + OH-.
• The equilibrium expression for this reaction is [H+][OH-]/[H2O] = K.
• K is the equilibrium constant, and the ion concentration product for water is defined as [H+][OH-] = Κω.
• The equilibrium constant for water = Kw.
• To satisfy electro-neutrality, positive ions must balance negative ions; thus, [H+]=[OH-].
• Substituting for OH- yields [H+ ]2=Κω.
• Taking the negative logarithm of Eq. yields -log[H+]= -logKw.
• Using the pK notation from chemistry where pK = -log K, pH is defined as pH= -log[H+]=log1/[H+] =1/2pKw.
• In water, the value of Kw at 25°C is 10-14.
• After substituting, you get pH= -log[H+] = 1/2pKw= -1/2 log10-14 =7.0.