Environmental Chemistry - Water, Sewage, and Climate Guide

Introduction to Environmental Chemistry and Water Resources

• Global Abundance and Necessity: Water is the most abundant compound on Earth and is essential for all living beings. Human activities, including domestic, industrial, and agricultural sectors, depend heavily on water supplies.

• Solvency and Contamination: Because water acts as a universal solvent, it is highly susceptible to contamination.

• Availability: Less than $1\%$ of the world’s total water resources are available for ready use. This scarcity necessitates careful and economical usage of available water.

• Natural Sources of Water:     * Surface Water: Includes rain, rivers, lakes, and seas.     * Underground Water: Includes wells and springs.

Specifications for Industrial and Domestic Water Use

• Industrial Specifications:     * Textile Industry: Water must be soft and entirely free from organic matter.     * Laundry: Requires soft water that is free from color, turbidity, Iron ($Fe$), and Manganese ($Mn$).     * Boilers: Require water of zero hardness. Failure to use zero-hardness water leads to scale and sludge formation, and in extreme cases, boiler explosions.     * Paper Industry: Requires water free from Silicon Dioxide ($SiO_2$), turbidity, alkalinity, and hardness.     * Beverages: Require water that is not alkaline.     * Sugar Industry: Requires water free from hardness to prevent difficulties during the crystallization of sugar.

• Domestic and Drinking Water Specifications:     * Condition: Must be clear, odorless, and pleasant in taste.     * Turbidity: Must not exceed $10\,ppm$.     * Chemical Purity: Free from gases like hydrogen sulfide ($H_2S$) and ammonia ($NH_3$). Free from Phosphorus ($P$), Arsenic ($As$), and Chromium ($Cr$).     * Chloride and Fluoride: Chloride content should be less than $250\,ppm$ and the water must be fluoride-free.     * pH Level: Should be approximately $8$.     * Hardness: Should be reasonably soft, with hardness levels less than $400\,ppm$.     * Total Dissolved Solids (TDS): Must be less than $500\,ppm$.     * Biological Safety: Must be free from disease-producing microorganisms.

Water Analysis: Hardness

• Definition: Hardness is the characteristic of water that prevents the lathering of soap.     * Hard Water: Water that does not produce lather with soap.     * Soft Water: Water that easily produces lather with soap.

• Cause of Hardness: Caused by the presence of dissolved salts of Calcium ($Ca$), Magnesium ($Mg$), and certain other heavy metals.

• Mechanism of Soap Interaction:     * Soap consists of sodium or potassium salts of higher fatty acids, such as stearic acid or palmitic acid (e.g., sodium stearate: $C_{17}H_{35}COONa$).     * When introduced to hard water, $Ca^{2+}$ or $Mg^{2+}$ ions replace the $Na^+$ or $K^+$ ions in the soap.     * This results in the formation of insoluble calcium or magnesium soaps, which appear as a precipitate or "scum."     * Chemical Reaction Example (Sodium Stearate with Calcium): $2C_{17}H_{35}COONa + Ca^{2+} \rightarrow (C_{17}H_{35}COO)<em>2Ca \downarrow + 2Na^+$     * Chemical Reaction Example (Sodium Stearate with Magnesium Sulfate): $2C</em>{17}H_{35}COONa + MgSO_4 \rightarrow (C_{17}H_{35}COO)_2Mg \downarrow + Na_2SO_4$

Types of Hardness

• Temporary (Carbonate) Hardness:     * Caused by the presence of bicarbonates of Calcium or Magnesium.     * It can be removed by simple boiling, which decomposes the bicarbonates into insoluble carbonates or hydroxides.     * Reaction for Calcium: $Ca(HCO_3)_2 \xrightarrow{\text{boil}} CaCO_3 \downarrow + CO_2 \uparrow + H_2O$     * Reaction for Magnesium: $Mg(HCO_3)_2 \xrightarrow{\text{boil}} Mg(OH)_2 \downarrow + 2CO_2 \uparrow$

• Permanent (Non-carbonate) Hardness:     * Caused by the presence of chlorides and sulfates of Calcium and Magnesium (or other soluble salts).     * It cannot be removed by boiling.     * Removal methods include the lime-soda process, zeolite process, and ion exchange process.

• Total Hardness: The sum of temporary and permanent hardness.     * $\text{Total Hardness} = \text{Temporary Hardness} + \text{Permanent Hardness}$

Calcium Carbonate ($CaCO_3$) as Reference

• Reasons for Use:     1. It is stable, non-hygroscopic, and available in pure form, making it ideal for preparing standard hard water solutions.     2. It is insoluble in water, allowing it to be easily precipitated during water treatments.     3. Mathematical simplicity: The molecular weight is exactly $100$.

• Hardness Units:     * Parts Per Million (ppm): The number of parts of hardness-producing substance per $10^6$ parts of water.     * Milligrams per litre (mg/L): The number of milligrams of hardness-producing substance per litre of water.     * Equivalence: $1\,mg/L = 1\,ppm$.

• Calculation Formulas:     * $\text{Equivalent Hardness as } CaCO_3 = \frac{\text{Mass of substance} \times 100}{\text{Molecular weight of substance}}$     * $\text{Equivalent Hardness as } CaCO_3 = \frac{\text{Mass of substance} \times 50}{\text{Equivalent weight of substance}}$

Disadvantages of Hard Water

• Domestic Disadvantages:     * Soap Wastage: Sticky precipitates form until all calcium and magnesium salts are exhausted, consuming large quantities of soap.     * Cooking: Hard water has a higher boiling point, leading to increased fuel consumption.

• Industrial Disadvantages:     * Concrete Mixing: Chlorides and sulfates affect the curing process and reduce the final strength of concrete.     * Dyeing: Insoluble soaps adhere to fabrics, changing the exact shades of colors.     * Sugar Industry: Interferes with the crystallization process of sugar.     * Paper Industry: Negatively impacts the smoothness, glossiness, and color of the paper.     * Boiler Trouble: Causes scale and sludge formation, boiler corrosion, priming, foaming, and caustic embrittlement.

Softening of Water: Ion Exchange Method (Demineralization)

• Principle: Water is passed through ion-exchange materials containing replaceable ions ($H^+$ or $Na^+$). These are exchanged for hardness-producing cations ($Ca^{2+}, Mg^{2+}$) and other anions.

• Result: Produces demineralized or deionized water, free from both cations and anions.

• Ion Exchange Resins: Insoluble, cross-linked, organic polymers with microporous structures and functional groups.     * Cation Exchange Resins ($RH$): Contain acidic groups ($-COOH, -SO_3H$). They exchange $H^+$ for cations like $Ca^{2+}, Mg^{2+},$ and $Na^+$.         * Reaction: $2RH + Ca^{2+} \rightarrow R_2Ca + 2H^+$         * Examples: Sulphonated coals and polystyrenes.     * Anion Exchange Resins ($ROH$): Contain basic groups ($-OH, -NH_2OH$). They exchange $OH^-$ for anions like $Cl^-$ and $SO_4^{2-}$.         * Reaction: $ROH + Cl^- \rightarrow RCl + OH^-$         * Examples: Urea formaldehyde resin, cross-linked quaternary ammonium salts.

• The Process:     1. Hard water passes through the cation exchange column (removes cations, releases $H^+$).     2. Water then passes through the anion exchange column (removes anions, releases $OH^-$).     3. Replacement ions combine: $H^+ + OH^- \rightarrow H_2O$.

• Regeneration:     * Cation Resins: Regenerated using dilute $HCl$ or $H_2SO_4$.     * Anion Resins: Regenerated using dilute $NaOH$.

• Pros/Cons:     * Advantages: Achieves very low hardness ($2\,ppm$); suitable for highly acidic or alkaline water.     * Disadvantages: Expensive; cannot remove dissolved non-ionic salts or organic substances.

Disinfection of Water

• Definition: The process of destroying harmful bacteria and microorganisms to make water safe for use (disinfectants).

• Ozone ($O_3$) Method: Ozone is unstable and produces nascent oxygen ($O$).     * Reaction: $O_3 \rightarrow O_2 + O$     * Pros: No residue; removes color, odor, and taste.     * Cons: Expensive.

• UV Light Method:     * Mechanism: Short wavelength UV ($250 - 270\,nm$), known as "germicidal UV," destroys molecular bonds in microorganism DNA.     * Equipment: Low-pressure mercury vapor lamp inside quartz sleeves in a stainless steel chamber.     * Effectiveness: 20,000 times more effective than boiling; independent of pH and temperature.     * Suitability: Not suitable for turbid water or water with high suspended solids.

• Chlorination:     1. Chlorine Gas: $Cl_2 + H_2O \rightarrow HOCl + HCl$     2. Chloramine: $ClNH_2 + H_2O \rightarrow HOCl + NH_3$     3. Bleaching Powder: $CaOCl_2 + H_2O \rightarrow Ca(OH)_2 + Cl_2$ followed by $Cl_2 + H_2O \rightarrow HOCl + HCl$.     * Germicidal Agent: Hypochlorous acid ($HOCl$) is the active disinfectant.

Break Point Chlorination

• Definition: The specific amount of chlorine added where free residual chlorine begins to appear, signaling complete oxidation of impurities.

• Stages:     * Stage I: No residual chlorine; all added chlorine is consumed for initial oxidation.     * Stage II: Residual chlorine increases; chloro-organics and chloramines are formed.     * Stage III: Residual chlorine decreases as it is consumed to oxidize the previously formed chloro-organics and chloramines.     * Stage IV: Residual chlorine increases linearly with the applied dose; disinfection is complete.

• Advantages: Destroys organic compounds and bacteria; removes color, odor, and taste; prevents weed growth.

Dissolved Oxygen (DO) and Winkler's Method

• Significance: Necessary for aquatic life; healthy ecosystems require $5-6\,ppm$.

• Influencing Factors: Solubility decreases as temperature increases; increases with pressure; decreases with biodegradable waste (oxygen-demanding waste).

• Estimation (Winkler’s Method):     1. Oxygen is made to react with manganese hydroxide ($Mn(OH)_2$), formed by adding $MnSO_4$ and alkaline $KI$.     2. Equations:         * $MnSO_4 + 2KOH \rightarrow Mn(OH)_2 + K_2SO_4$         * $2Mn(OH)_2 + O_2 \rightarrow 2MnO(OH)_2$ (Basic manganese oxide/brown precipitate)     3. On acidification with $H_2SO_4$, iodine ($I_2$) is liberated in proportion to the oxygen content.         * $MnO(OH)_2 + H_2SO_4 \rightarrow MnSO_4 + 2H_2O + O$         * $2KI + H_2SO_4 + O \rightarrow K_2SO_4 + H_2O + I_2$     4. Titration: Liberated $I_2$ is titrated against standard sodium thiosulphate ($Na_2S_2O_3$) with starch as the indicator.         * $I_2 + 2Na_2S_2O_3 \rightarrow Na_2S_4O_6 + 2NaI$

BOD and COD (Oxygen Demands)

• Biological Oxygen Demand (BOD): Amount of oxygen required by aerobic bacteria to oxidize organic matter at $20^\circ C$ for 5 days (or $27^\circ C$ for 3 days).     * Formula: $\text{BOD} = (D_1 - D_2) \times \frac{\text{Volume after dilution}}{\text{Volume before dilution}}$     * $D_1$: Initial DO; $D_2$: Final DO after incubation.

• Chemical Oxygen Demand (COD): Amount of oxygen required for chemical oxidation of all oxidisable matter (organic and inorganic) using acidified $K_2Cr_2O_7$.     * Catalyst: Silver sulfate ($Ag_2SO_4$).     * Estimation: Refluxing sewage with excess $K_2Cr_2O_7$, ثم titrating unreacted dichromate against Mohr’s salt solution.     * Formula: $\text{COD (mg/L)} = \frac{(V_2 - V_1) \times N \times 8 \times 1000}{X}$     * $V_2$: Blank titre; $V_1$: Sample titre; $N$: Normality; $X$: Sample volume.

• Comparison: COD is always higher than BOD. COD takes 2-3 hours; BOD takes 5 days.

Sewage Water Treatment

• Objectives: Neutralize harmful compounds, eliminate smell and solids, destroy pathogens.

• Steps:     1. Preliminary: Screening to remove coarse solids.     2. Primary: Sedimentation using coagulants (e.g., alum, aluminum sulfate) to remove suspended solids.     3. Secondary (Biological): Aerobic degradation using methods like the Trickling Filter (bio-film process) or Activated Sludge.         * Trickling Filter: Sewage sprayed over a bed of gravel/rock/coal; aerobic bacteria on the bed oxidize organic matter.     4. Anaerobic Treatment (UASB):         * Upflow Anaerobic Sludge Blanket process.         * Wastewater moves upward through a blanket of sludge granules ($1 - 5\,mm$ diameter).         * Produced biogas ($CH_4 + CO_2$) is collected at the top. Effluent can be used in agriculture.     5. Tertiary: Precipitation (using lime for phosphates), Nitrogen Stripping (removing $NH_3$ via baffle plates), and Chlorination.

E-Waste Management

• Definition: Waste Electrical and Electronic Equipment (WEEE).

• Components: Glass ($30\%$), Plastics ($30\%$), and Metals ($40\%$ in PCBs).     * Precious Metals: $Au, Ag, Cu, Pt, Rh, Ru$.     * Base Metals: $Al, Zn$.     * Critical Materials: $Co, Pd, In, Sb$.

• Environmental Hazards:     * CRT Glass: Contains up to $4\,kg$ of lead ($Pb$), plus phosphorus and barium.     * PCBs: Contain polymers like ABS, epoxy resin, PVC, and brominated flame retardants.     * Others: Mercury ($Hg$), Beryllium oxide, Polychlorinated Biphenyls (PCBs).

• Management Strategies (The 4 R's):     1. Reduce: Minimize unnecessary purchases.     2. Reuse: Direct second-hand use or use after modification.     3. Recycle: Dismantling, separation of plastics/metals.     4. Recovery: Extraction of valuable materials through shredding and unit operations.

Chemistry of Climate Change and Ozone Depletion

• Greenhouse Effect: Natural process where GHGs trap thermal radiation emitted by Earth. Without it, Earth's temperature would be $0^\circ F$ ($-18^\circ C$) instead of $57^\circ F$ ($15^\circ C$).

• Primary GHGs: Carbon dioxide ($CO_2$) and Methane ($CH_4$).

• Ozone Layer ($O_3$): Forms in the stratosphere and absorbs UVB radiation.

• Depletion by CFCs (Freons):     * UV radiation breaks CFCs to release chlorine radicals ($Cl^<em>$).      Reaction Chain:         * $CF_2Cl_2 \xrightarrow{h\nu} CF_2Cl^* + Cl^<em>$          $Cl^* + O_3 \rightarrow O_2 + ClO^<em>$          $ClO^* + O_3 \rightarrow 2O_2 + Cl^<em>$      One chlorine atom can destroy $10^5$ ozone molecules.

• Montreal Protocol (1987): International agreement to phase out ozone-depleting substances. Ozone Day is celebrated on September 16.