**Abstract**

Adsorption processes are extensively used in wastewater treatment for heavy metal removal. The most widely used adsorbent is activated carbon giving the best of results but it's high cost limits its use. It has a high cost of production and regeneration. As the world today faces a shortage of freshwater resources, it is inevitable to look for alternatives that lessen the burden on existing resources. Also, heavy metals are toxic even in trace concentrations, so an environmentally safe method of their removal necessitated the requirement of low cost adsorbents. Adsorption is a cost-effective technique and gained recognition due to its minimum waste disposal advantage. This chapter focuses on the process of adsorption and the types of adsorbent available today. It also encompasses the low-cost adsorbents ranging from agricultural waste to industrial waste explaining the adsorption reaction condition. The cost-effectiveness, technical applicability and easy availability of raw material with low negative impact on the system are the precursors in selecting the adsorbents. The novelty of the chapter lies in covering a wide range of adsorbents with their efficiency in removal of heavy metals from wastewater.

**Keywords:** adsorption, low-cost adsorbent, isotherm, wastewater, heavy metals

## **1. Introduction**

Heavy metals are toxic elements having specific gravity greater than 5g/cm3 e.g. Zn, Fe, Cu, Cr, Hg, Pb, Ni, Co, etc. [1]. The main natural sources of heavy metals include volcanic processes, weathering of rocks and soil erosion. While the anthropogenic sources include mineral processing, fuel combustion and industrial activities like mining, metal processing, chemical fertilizers and dye manufacturing etc. Heavy metals are non-biodegradable, recalcitrant and have high mobility in aqueous media, so they tend to accumulate in soils and living organisms leading to environmental repercussions. Heavy metals are taken up by plants which biomagnify through food chains in animals and humans causing serious negative health effects due to their carcinogenicity [2–5]. **Table 1** gives the maximum contaminant level (MCL) in drinking water given by USEPA [7] along with their harmful effects.

Heavy metals have a high tendency to form complexes, are highly reactive and have increased biochemical activity which makes them very persistent in the environment. They are transported through aqueous medium and can concentrate in soil and water resources. This makes them extremely dangerous to all kinds of life forms and the environment. Hence, it is necessary to remove these toxic metals


#### **Table 1.** *Harmful Effects of Heavy Metals [6]*

from wastewater before discharge to prevent further detrimental consequences. Conventional methods like membrane filtration [8–12], chemical precipitation [13–17], ion exchange [17–22], etc. have been used to remove of heavy metals from wastewater. However, these methods suffer from some disadvantages like low efficiency, high energy requirement, precipitation of toxic substances, cost ineffectiveness, etc. [3, 23]. To get past these demerits, processes like adsorption are investigated, since it greatly impacts the bioavailability and transport of toxic metals. It is low-cost and efficient technique for remediation of heavy metals from wastewater. Adsorption process is often reversible in many cases, so the adsorbent can be regenerated back adding another advantage to this process [24, 25]. Many factors such as temperature, pH, initial concentration, contact time and rotation speed affect the efficacy of adsorbents [23, 25].
